Cellular Cd2+ fluxes in roots confirm increased Cd availability to rice (Oryza sativa L.) induced by soil acidifications

Soil acidifications become one of the main causes restricting the sustainable development of agriculture and causing issues of agricultural product safety. In order to explore the effect of different acidification on soil cadmium (Cd) availability, soil pot culture and hydroponic (soil potting solution extraction) were applied, and non-invasive micro-test technique (NMT) was combined. Here three different soil acidification processes were simulated, including direct acidification by adding sulfuric acid (AP1), acid rain acidification (AP2) by adding artificial simulated acid rain and excessive fertilization acidification by adding (NH4)2SO4 (AP3). The results showed that for direct acidification (AP1), DTPA-Cd concentration in field soils in Liaoning (S1) and Zhejiang (S2) increased by 0.167 - 0.217 mg/kg and 0.181 - 0.346 mg/kg, respectively, compared with control group. When soil pH decreased by 0.45 units in S1, the Cd content of rice stems, leaves and roots increased by 0.48 to 6.04 mg/kg and 2.58 to 12.84 mg/kg, respectively, When the pH value of soil S1 and S2 decreased by 0.20 units, the average velocity of Cd2+ at 200 µm increased by 10.03 - 33.11 pmol/cm2/sec and 21.33 -52.86 pmol/cm2/sec, respectively, and followed the order of AP3 > AP2 > AP1. In summary, different acidification measures would improve the effectiveness of Cd, under the same pH reduction condition, fertilization acidification increased Cd availability most significantly.

Calcium-based polymers for suppression of soil acidification by improving acid-buffering capacity and inhibiting nitrification

Soil acidification is a major threat to agricultural sustainability in tropical and subtropical regions. Biodegradable and environmentally friendly materials, such as calcium lignosulfonate (CaLS), calcium poly(aspartic acid) (PASP-Ca), and calcium poly γ-glutamic acid (γ-PGA-Ca), are known to effectively ameliorate soil acidity. However, their effectiveness in inhibiting soil acidification has not been studied. This study aimed to evaluate the effect of CaLS, PASP-Ca, and γ-PGA-Ca on the resistance of soil toward acidification as directly and indirectly (i.e., via nitrification) caused by the application of HNO3 and urea, respectively. For comparison, Ca(OH)2 and lignin were used as the inorganic and organic controls, respectively. Among the materials, γ-PGA-Ca drove the substantial improvements in the pH buffering capacity (pHBC) of the soil and exhibited the greatest potential in inhibiting HNO3-induced soil acidification via protonation of carboxyl, complexing with Al3+, and cation exchange processes. Under acidification induced by urea, CaLS was the optimal one in inhibiting acidification and increasing exchangeable acidity during incubation. Furthermore, the sharp reduction in the population sizes of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) confirmed the inhibition of nitrification via CaLS application. Therefore, compared to improving soil pHBC, CaLS may play a more important role in suppressing indirect acidification. Overall, γ-PGA-Ca was superior to PASP-Ca and CaLS in enhancing the soil pHBC and the its resistance to acidification induced by HNO3 addition, whereas CaLS was the best at suppressing urea-driven soil acidification by inhibiting nitrification. In conclusion, these results provide a reference for inhibiting soil re-acidification in intensive agricultural systems.

Changes in microbial composition explain the contrasting responses of glucose and lignin decomposition to soil acidification in an alpine grassland

Soil acidification often suppresses microbial growth and activities, resulting in a negative impact on soil organic carbon (C) decomposition. While the detrimental effects of acidification on soil and plant properties have been extensively studied, less attention has been paid on the shifts in soil microbial communities and their influences of the decomposition of organic C with different chemical complexities. Taking advantage of an acid addition experiment in a Tibetan alpine meadow, here we examined the response of soil microbial communities to soil acidification and microbial effect on the decomposition of organic C with different chemical complexities (i.e., glucose and lignin, representing labile and recalcitrant C respectively). We found that soil acidification had no impact on microbial respiration and microbial abundance even though it decreased bacterial diversity significantly. Soil acidification increased the relative abundance of some microbial taxa, like Alphaproteobacteria and Acidobacteriia in bacteria increased by 36 %, 284 %, and Eurotiomycetes, Sordariomycetes and Leotiomycetes in fungi increased by 145 %, 279 % and 12.7-fold, but decreased the relative abundance of Acidimicrobiia by 33 % in highest acid addition treatment. Changes in microbial communities (bacterial and fungal community composition, the diversity of bacterial community and the ratio of fungi to bacteria) are significantly related to the decomposition of glucose and lignin. More specifically, soil acidification decreased the decomposition of glucose but increased the decomposition of lignin, indicating a trade-off between the decomposition of labile and recalcitrant soil organic C under soil acidification. Overall, shifts in microbial communities under soil acidification might be accompanied by an increased ability to break down more recalcitrant C. This trade-off between the decomposition of labile and recalcitrant C may change soil C quality under future acid deposition scenarios.

Wood ash application for crop production, amelioration of soil acidity and contaminated environments

Agriculture is vital to human life and economic development even though it may have a detrimental influence on soil quality. Agricultural activities can deteriorate the soil quality, endangers the ecosystem health and functioning, food safety, and human health. To resolve the problem of soil degradation, alternative soil conditioners such as wood ash are being explored for their potential to improve soil-plant systems. This study provides an overview of the production, properties, and effects of wood ash on soil properties, crop productivity, and environmental remediation. A comprehensive search of relevant databases was conducted in order to locate and assess original research publications on the use of wood ash in agricultural and environmental management. According to the findings, wood ash, a byproduct of burning wood, may improve the structure, water-holding capacity, nutrient availability, and buffering capacity of soil as well as other physico-chemical, and biological attributes of soil. Wood ash has also been shown to increase agricultural crop yields and help with the remediation of polluted regions. Wood ash treatment, however, has been linked to several adverse effects, such as increased trace element concentrations and altered microbial activity. The examination found that wood ash could be a promising material to be used as soil conditioner and an alternative supply of nutrients for agricultural soils, while, wood ash contributes to soil improvement and environmental remediation, highlighting its potential as a sustainable solution for addressing soil degradation and promoting environmental sustainability in agricultural systems.

Major drivers of soil acidification over 30 years differ in paddy and upland soils in China

Elevated nitrogen (N) fertilization has largely increased crop production in China, but also increased acidification risks, thereby threatening crop yields. However, natural soil acidification due to bicarbonate (HCO3) leaching and base cation (BC) removal by crop harvest also affect soil acidity whereas the input of HCO3 and BC via fertilizers and manure counteract soil acidification. Insights in rates and drivers of soil acidification in different land use types is too limited to support crop- and site-specific mitigation strategies. In this study, we assessed the historical changes in cropland acidification rates and their drivers for the period 1985-2019 at 151 sites in a typical Chinese county with the combined nutrient and soil acidification model VSD+. VSD+ could well reproduce long-term changes in pH and in the BC concentrations of calcium, magnesium and potassium between 1985 and 2019 in non-calcareous soils. In paddy soils, the acidity production rate decreased from 1985 onwards, mainly driven by a pH-induced reduction in HCO3 leaching and N transformations. In upland soils, however, acidity production was mainly driven by N transformations and hardly changed over time. Crop BC removal by harvesting played a minor role in both paddy and upland soils, but its relative importance increased in paddy soils. The acidity input was partly neutralized by HCO3 input from fertilizers and manure, which decreased over time due to a change from ammonia bicarbonate to urea. Soil buffering by both BC and aluminium release decreased in paddy soils due to a reduction in net acidity production, while it stayed relatively constant in upland soils. We conclude that acidification management in paddy soils requires a focus on avoiding high HCO3 leaching whereas the management in upland soils should focus on balancing N with recycling organic manure and crop residues.

Effect of carbon and nitrogen mineralization of chitosan and its composites with hematite/gibbsite on soil acidification of an Ultisol induced by urea

Chitosan is a biodegradable polymer with a vast range of applications. Along with its metal composites, chitosan has been applied in the remediation of polluted soils as well as a biofertilizer. However, little attention has been given to the degradation of chitosan composites in soil and how they affect soil respiration rate and other physicochemical parameters. In this study, the degradation of chitosan and its composites with gibbsite and hematite in an acidic Ultisol and the effect on urea (200 mg N kg-1) transformation were investigated in a 70-d incubation experiment. The results showed that the change trends of soil pH, N forms, and CO2 emissions were similar for chitosan and its composites when applied at rates <5 g C kg-1. At a rate of 5 g C kg-1, the C and N mineralization trends suggested that the chitosan-gibbsite composite was more stable in soil and this stability was owed to the formation of a new chemical bond (CH-N-Al-Gibb) as observed in the Fourier-transform infrared spectrum at 1644 cm-1. The mineralization of the added materials significantly increased soil pH and decreased soil exchangeable acidity (P < 0.01). This played an important role in decreasing the amount of H+ produced during urea transformation in the soil. The soil's initial pH was an important factor influencing C and N mineralization trends. For instance, increasing the initial soil pH significantly increased the nitrification rate and chitosan decomposition trend (P < 0.01) and thus, the contribution of chitosan and its composites to increase soil pH and inhibit soil acidification during urea transformation was significantly decreased (P < 0.01). These findings suggest that to achieve long-term effects of chitosan in soils, applying it as a chitosan-gibbsite complex is a better option.

Improving food security and farmland carbon sequestration in China through enhanced rock weathering: Field evidence and potential assessment in different humid regions

Enhanced rock weathering (ERW) in farmland is an emerging carbon dioxide removal technology with crushed silicate rocks for soil improvement. However, due to climatic variability and field data limitations, uncertainties remain regarding the influence of ERW on food security and soil carbon pools in temperate regions. This study focused to evaluate the crop productivity and carbon sequestration potential of farmland ERW in China by conducting field monitoring in different humid regions and ERW performance model. Additionally, the contribution of climate, soil, and management factors to ERW-mediated yield and carbon sequestration changes was explored using random forest and correlation networks. Field monitoring indicated that farmland ERW significantly improved crop yield in humid region (13.5 ± 5.2 %), along with notable improvements in soil pH and available nutrients. Precipitation (10.4-16.7 %) and soil pH (9.7-16.8 %) had the highest contribution on ERW mediated yield and carbon sequestration changes, but the contribution of management factors (24-26.2 %), especially N input (2.7-7.0 %), should not be disregarded. The model evaluation demonstrated that the carbon sequestration rate of farmland ERW in China can reach 0.28-0.40 Gt yr-1, thereby presenting an opportunity to expand and accelerate the nationally determined contributions of China. The mean sequestration cost of farmland ERW was 633 ± 161 CNY ¥ t-CO2-1, which was an attractive sequestration price considering the positive benefits of rock powder on soil pH and nutrients. Deploying ERW in acidified and mineral nutrient deficient regions was able to serve as an alternative to lime and part chemical fertilizers to improve yield and maximize agricultural sustainability and resource co-benefits. Farmland ERW also has the potential to resource silicate waste to assist traditional, difficult-to-decarbonize industries to reduce carbon emissions. As a result, a comprehensive assessment of existing artificial silicate waste materials could further expand the application of farmland ERW.

Isolation of plant growth-promoting rhizobacteria from the agricultural fields of Tattiannaram, Telangana

BACKGROUND

Plant probiotics bacteria are live microbes that promote soil health and plant growth and build the stress-tolerant capacity to the plants. They benefit the plants by increasing nutrient absorption and release of stress-related phytohormones. These plant probiotic bacteria serve a better purpose to the plant when compared to chemical fertilizers. Use of chemical fertilizers such as arsenic and cadmium can lead to soil acidification and even release of harmful gases such as methane which further pollutes the environment.

RESULTS

Different bacterial species were isolated from the agricultural fields of Tattiannaram, Telangana, and identified as the efficient rhizosphere bacteria with the essential qualities of plant growth promotion by evaluating the nitrogen-fixing ability on a selective media and various other methods. Upon the molecular characterization of the isolates, they were identified as Corynebacterium spp., Bacillus spp., Lactobacillus spp., and Cytobacillus spp. The results were also examined using various bioinformatics tools for accuracy in their phylogenetic pattern.

CONCLUSION

The recognized species of plant probiotics have established roles in promoting plant growth and strengthening plant immunity. This research introduces an innovative methodology for evaluating and investigating recently identified bacterial isolates, focusing on their distinctive plant probiotic attributes. Through harnessing the potential of advantageous microorganisms and comprehending their interaction with plants and soil, our objective is to formulate inventive approaches to elevate crop productivity, enhance soil richness, and foster environmentally sustainable and robust agricultural methodologies. These characteristics exhibit promising potential for future incorporation into plant systems, fortifying growth and development, and underscoring their distinctive significance within the realm of agriculture.

Nitrogen addition increased soil particulate organic carbon via plant carbon input whereas reduced mineral-associated organic carbon through attenuating mineral protection in agroecosystem

Nitrogen (N) addition can have substantial impacts on both aboveground and belowground processes such as plant productivity, microbial activity, and soil properties, which in turn alters the fate of soil organic carbon (SOC). However, how N addition affects various SOC fractions such as particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), particularly in agroecosystem, and the underlying mechanisms remain unclear. In this study, plant biomass (grain yield, straw biomass, and root biomass), soil chemical properties (pH, N availability, exchangeable cations and amorphous Al/Fe - (hydr) oxides) and microbial characteristics (biomass and functional genes) in response to a N addition experiment (0, 150, 225, 300, and 375 kg ha-1) in paddy soil were investigated to explore the predominant controls of POC and MAOC. Our results showed that POC significantly increased, while MAOC decreased under N addition (p < 0.05). Correlation analysis and PLSPM results suggested that increased C input, as indicated by root biomass, predominated the increase in POC. The declined MAOC was not mainly dominated by microbial control, but was strongly associated with the attenuated mineral protection (especially Ca2+) induced by soil acidification under N addition. Collectively, our results emphasized the importance of combining C input and soil chemistry in predicting soil C dynamics and thereby determining soil organic C storage in response to N addition in rice agroecosystem.

Effects of soil amendments on soil acidity and crop yields in acidic soils: A world-wide meta-analysis

Soil amendments, including lime, biochar, industrial by-products, manure, and straw are used to alleviate soil acidification and improve crop productivity. Quantitative insight in the effect of these amendments on soil pH is limited, hampering their appropriate use. Until now, there is no comprehensive evaluation of the effects of soil amendments on soil acidity and yield, accounting for differences in soil properties. We synthesized 832 observations from 142 papers to explore the impact of these amendments on crop yield, soil pH and soil properties, focusing on acidic soils with a pH value below 6.5. Application of lime, biochar, by-products, manure, straw and combinations of them significantly increased soil pH by 15%, 12%, 15%, 13%, 5% and 17%, and increased crop yield by 29%, 57%, 50%, 55%, 9%, and 52%, respectively. The increase of soil pH was positively correlated with the increase in crop yield, but the relationship varied among crop types. The most substantial increases in soil pH and yield in response to soil amendments were found under long-term applications (>6 year) in strongly acidic (pH < 5.0) sandy soils with a low cation exchange capacity (CEC, <100 mmolc kg-1) and low soil organic matter content (SOM, <12 g kg-1). Most amendments increased soil CEC, SOM and base saturation (BS) and decreased soil bulk density (BD), but lime application increased soil BD (1%) induced by soil compaction. Soil pH and yield were positively correlated with CEC, SOM and BS, while yield declined when soils became compacted. Considering the impact of the amendments on soil pH, soil properties and crop yield as well as their costs, the addition of lime, manure and straw seem most appropriate in acidic soils with an initial pH range from <5.0, 5.0-6.0 and 6.0-6.5, respectively.

Response and driving factors of soil enzyme activity related to acid rain: a meta-analysis

As a global pollution, acid rain can significantly alter soil physicochemical and biochemical processes, but our knowledge of how acid rain affects soil enzyme activity is still limited. To quantify the overall magnitude and direction of the response of soil enzyme activity to acid rain, we conducted a linear mixed model-based meta-analysis of 40 articles. Our analysis revealed that acid rain decreased enzyme activity by an average of 4.87%. Soil dehydrogenase and protease activities were particularly sensitive to acid rain, with significant inhibitions observed. The effect of acid rain was moderated by acid rain intensity (i.e., H+ addition rate, total H+ added, and acid rain pH) and soil fraction (i.e., rhizosphere and bulk soil). Structural equation modelling further revealed that acid rain suppressed soil microbial biomass by acidifying the soil and that the reduction in microbial biomass directly led to the inhibition of enzyme activity in bulk soil. However, the enzyme activity in the rhizosphere soil was not affected by acid rain due to the rhizosphere effect, which was also not impacted by the decreased soil pH induced by acid rain in rhizosphere. Our study gives an insight into how bulk soil enzyme activity is impacted by acid rain and highlights the need to incorporate rhizosphere processes into acid rain-terrestrial ecosystem models.

Acidification of previously limed upland pastures - An overlooked flood risk factor?

In low-lying land, the impact of agriculture on flooding has focussed on soil compaction, whilst in the uplands there has been more interest in the influence of afforestation. The potential effect of acidification of previously limed upland grassland soils on this risk has been overlooked. The marginal economics of upland farms has led to inadequate lime application on these grasslands. In Wales, UK, agronomic improvement of upland acid grasslands with liming was widespread in the last century. The extent and topographical distribution of this land use in Wales was estimated and these characteristics were mapped in four catchments studied in more detail. Then 41 sites on improved pastures within the catchments were sampled, where lime had not been applied for periods of between two and 30 years; unimproved acid pastures adjacent to five of these sites were also sampled. Soil pH, organic matter, infiltration rates and earthworm populations were recorded. Grasslands at risk of acidification without maintenance liming were estimated to cover almost 20 % of upland Wales. The majority of these grasslands were located on steeper slopes (gradients >7^o) where any reduction in infiltration would promote surface runoff and limit rainwater retention. The extent of these pastures varied markedly between the four study catchments. There was a 6-fold reduction in infiltration rates between high and low pH soils, and this trend was correlated with reductions in anecic earthworm abundance. The vertical burrows of these earthworms are important for infiltration and no such earthworms were present in the most acidic soils. Recently limed soils had infiltration rates similar to those of unimproved acid pastures. Soil acidification has the potential to exacerbate flood risk but further research is needed to assess the extent of any impact. Modelling of catchment specific flood risk should include the extent of upland soil acidification as an additional land use factor.

Dissolved biochar fractions and solid biochar particles inhibit soil acidification induced by nitrification through different mechanisms

Biochar can inhibit soil acidification by decreasing the H⁺ input from nitrification and improving soil pH buffering capacity (pHBC). However, biochar is a complex material and the roles of its different components in inhibiting soil acidification induced by nitrification remain unclear. To address this knowledge gap, dissolved biochar fractions (DBC) and solid biochar particles (SBC) were separated and mixed thoroughly with an amended Ultisol. Following a urea addition, the soils were subjected to an incubation study. The results showed that both the DBC and SBC inhibited soil acidification by nitrification. The DBC inhibited soil acidification by decreasing the H⁺ input from nitrification, while SBC enhanced the soil pHBC. The DBC from peanut straw biochar (PBC) and rice straw biochar (RBC) decreased the H⁺ release by 16 % and 18 % at the end of incubation. The decrease in H⁺ release was attributed to the inhibition of soil nitrification and net mineralization caused by the toxicity of the phenols in DBC to soil bacteria. The abundance of ammonia-oxidizing bacteria (AOB) and total bacteria decreased by >60 % in the treatments with DBC. The opposite effects were observed in the treatments with SBC. Soil pHBC increased by 7 % and 19 % after the application of solid RBC and PBC particles, respectively. The abundance of carboxyl on the surface of SBC was mainly responsible for the increase in soil pHBC. Generally, the mixed application of DBC and SBC was more effective at inhibiting soil acidification than their individual applications. The negative impacts of dissolved biochar components on soil microorganisms need to be closely monitored.

Nitrogen addition increases topsoil carbon stock in an alpine meadow of the Qinghai-Tibet Plateau

Soil carbon (C) sequestration plays a critical role in mitigating climate change. Nitrogen (N) deposition greatly affects soil C dynamics by altering C input and output. However, how soil C stocks respond to various forms of N input is not well clear. This study aimed to explore the impact of N addition on soil C stock and to elucidate the underlying mechanisms in an alpine meadow on the eastern Qinghai-Tibet Plateau. The field experiment involved three N application rates and three N forms, using a non-N treatment as a control. After six years of N addition, the total C (TC) stocks in the topsoil (0-15 cm) were markedly increased by an average of 12.1 %, with a mean annual rate of 20.1 ‰, and no difference was found between the N forms. Irrespective of rate or form, N addition significantly increased the topsoil microbial biomass C (MBC) content, which was positively correlated with mineral-associated and particulate organic C content and was identified as the most important factor that affecting the topsoil TC. Meanwhile, N addition significantly increased the aboveground biomass in the years with moderate precipitation and relatively high temperature, which leads to higher C input into soils. Owing to decreased pH and/or activities of β-1,4-glucosidase (βG) and cellobiohydrolase (CBH) in the topsoil, organic matter decomposition was most likely inhibited by N addition, and this inhibiting effect varied under different N forms. Additionally, TC content in the topsoil and subsoil (15-30 cm) exhibited parabolic and positive linear relationship with the topsoil dissolved organic C (DOC) content, respectively, indicating that DOC leaching might be an important influencing factor for soil C accumulation. These findings improve our understanding of how N enrichment affects C cycles in alpine grassland ecosystems and suggest that soil C sequestration in alpine meadows probably increases with N deposition.

Biochar enhancement of nitrification processes varies with soil conditions

Application of inorganic nitrogen (N) fertilizers in agriculture can increase emissions of nitrous oxide, a potent greenhouse gas, leaching of nitrate (NO₃^-), a groundwater contaminant hazardous to human health, and soil acidification. Soil amendment with biochar potentially mitigates these losses and undesirable outcomes. However, there have been considerable inconsistencies in reported impacts, likely owing to variable physiochemical characteristics of the biochar materials and/or the soil environment. This study methodically evaluated the impact of biochar soil incorporation on N transformation and underlying microbial processes using soils with varying biochar types, soil texture, soil moisture, and manure compost co- amendments. Laboratory incubations were conducted to monitor the fate of urea fertilizer N spiked in biochar amended and unamended soils by assaying soil ammonium (NH₄⁺), nitrite (NO₂^-), and NO₃^- concentrations, pH, and abundances of soil nitrifiers; ammonia oxidizing bacteria and archaea (AOB and AOA) and Nitrospira with the capacity to perform complete ammonia oxidation (comammox). Soil moisture was a critical factor affecting N transformation processes, more so than biochar, but biochar did result in significantly different concentrations of N species in response to urea application. Biochar enhanced nitrification, more significantly in drier conditions and in sandy soil. Biochar offered some buffering potential in the neutral-alkaline, unsaturated soils, preventing >1 unit drop in pH compared to unamended soils. Co-application of biochar with manure composts enhanced nitrification slightly, which was evidenced by higher abundances of some soil nitrifiers at 4 weeks, although increases in nitrification rates were not statistically significant. Soil nitrifier populations tended to increase in response to a pinewood biochar, but trends differed for saturated soil, in soils of differing textures, or when different biochar materials were evaluated. Thus, when evaluating implications of biochar on the fate of mineral N fertilizer, soil moisture and other environment conditions should be considered.

Competitive and synergistic effects of phosphate and cadmium coadsorbed on goethite

Soil acidification in southern China increases the environmental risk posed by heavy metals, especially cadmium (Cd), and has been accelerated by anthropic activities. Long-term fertilizer use increases the quantity of phosphate (P) in soils, which affects the biogeochemistry of heavy metals in soil environments. In this study, various analytical methods were used to investigate the critical mechanisms controlling P(V)-Cd(II) interactions with goethite (α-FeOOH, a typical iron oxide widely distributed in red soil areas) with added P(V)/Cd(II) in different molar ratios (to simulate the annual P fertilizer surplus). After excluding pH interference by using organic buffers, Cd(II) adsorption processes on α-FeOOH were found to be enhanced by the presence of P(V), whereas the Cd(II) adsorption type changed from single inner-sphere complexation to both inner- and outer-sphere complexation. The results of a zeta potential analysis, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) showed different dominant mechanisms underlying the promotion of Cd(II) adsorption by P(V) at different P(V)/Cd(II) molar ratios. Electrostatic adsorption and the formation of ternary complexes were proven to be the critical mechanisms controlling the reactivity of Cd(II), where coprecipitation occurs at high P(V)/Cd(II) molar ratios and considerably enhances the fixation of Cd(II) and P(V). The mechanism for Cd(II) and P(V) coadsorption onto α-FeOOH under stable pH conditions was found to include both competitive and synergistic effects, and the results in this study show that more attention should be given to the impact of anthropic activities on the reactivity of heavy metals in the soil environment, especially farmland.

Effect of phosphate on cadmium immobilized by microbial-induced carbonate precipitation: Mobilization or immobilization?

Microbial-induced carbonate precipitation (MICP) is a promising technology to immobilize/remediate heavy metals (HMs) like cadmium (Cd). However, the long-term stability of MICP-immobilized HMs is unclear, especially in farmland where chemical fertilization is necessary. Therefore, we performed MICP treatment on soils contaminated with various Cd compounds (CdCO₃, CdS, and CdCl₂) and added diammonium phosphate (DAP) to explore the impact of phosphate on the MICP-immobilized Cd. The results showed that MICP treatment was practical to immobilize the exchangeable Cd but to mobilize the carbonate and Fe/Mn oxide-bound Cd. After applying DAP, soil pH declined due to ammonium nitrification. At high P/Ca molar ratios (1/2 and 1), partial previously immobilized Cd was released due to the carbonate dissolution. Contrarily, exchangeable Cd transformed to less mobilizable Fe/Mn oxide-bound at low P/Ca molar ratios (1/4 and 1/8). Meanwhile, other treatments were also helpful in avoiding the release of immobilized Cd, such as applying non-ammonium phosphate and adding lime material after soil acidification. Our investigation suggested that the long-term stability of HMs in remediated sites should be carefully evaluated, especially in agricultural areas with phosphate and nitrogen fertilizer input.

The important role of surface hydroxyl groups in aluminum activation during phyllosilicate mineral acidification

Phyllosilicate minerals are the important components in soils and an important source of activated aluminum (Al) during soil acidification. However, the mechanisms for Al activation in phyllosilicate minerals were not understood well. In this paper, the effect of phyllosilicate surface hydroxyl groups on Al activation during acidification was studied after the minerals were modified with inorganic and organic materials. After modification of kaolinite, montmorillonite, and illite with fulvic acid (FA-), iron oxide (Fe-), Fe combined with FA (Fe-FA-), and siloxane (Si-O-), the interlayer spaces were altered. For instance, when modified with Fe, Fe entered the interlayer spaces of kaolinite and montmorillonite and changed the interlayer spaces of both minerals but did not affect that of illite. Also, the other modification methods had significant effects on the interlayer space of montmorillonite but not on kaolinite and illite. It was observed that all the modification strategies inhibited Al activation during acidification by reducing the number of hydroxyl groups on the mineral surfaces and inhibiting protonation reactions between H⁺ and hydroxyl groups. Nevertheless, the inhibition effect varies with the type of phyllosilicate mineral. For kaolinite (Kao), the inhibition effect of the different modification methods on Al activation during acidification followed: Fe-FA-Kao > Fe-Kao > Si-O-Kao > FA-Kao. Additionally, for montmorillonite (Mon), the inhibition effect was in the order: Si-O-Mon > Fe-Mon > Fe-FA-Mon > FA-Mon, while for illite, it was: Fe-illite > Si-O-illite ≈ Fe-FA-illite > FA-illite. Thus, the hydroxyl groups on the surfaces and edges of phyllosilicate minerals play an important role in the activation of Al from the mineral structure. Also, the protonation of hydroxyl groups may be the first step during Al activation in these minerals. The results of this study can serve as a reference for the development of new technologies to inhibit soil acidification and Al activation.

Soil organic carbon content increase in the east and south of China is accompanied by soil acidification

Increased soil organic carbon (OC) in China has been reported in the past two decades, suggesting the sequestration of atmospheric carbon dioxide into soil, mitigating climate change and improving soil health. On the other hand, soil pH decrease had also been reported nationwide. If the two are related, the strategy of increasing soil OC could negatively affect soil quality for food production and the environment. We investigate this thread based on large-scale soil survey data from two provinces with typical soil and cropping patterns in the east and south of China, Jiangsu (102,600 km²) and Guangdong (177,900 km²). The data include >5000 observations from soil surveys conducted over the past four decades, i.e., the 1980s, 2006-2007, and 2010-2011. Using spatiotemporal modelling, we show that across Jiangsu province, the topsoil OC on average has increased from 8.5 g kg^-1 to 9.9 g kg^-1 from 1980 to 2000 and a further increase to 12.6 g kg^-1 in 2010. This increase was accompanied by a decrease in average pH from 7.63 to 6.90. In Guangdong, there was an overall increase in average topsoil OC content from 14.2 g kg^-1, 16.5 g kg^-1, and 20.2 g kg^-1 with a decrease in average pH from 5.58, 4.90, and 4.98. Based on the spatiotemporal modelling results, the structural equation modelling analysis shows that OC and pH changes were significantly correlated and linked by increased soil N content. On croplands, soil N content was mainly attributed to N fertiliser application. The pH decrease was particularly significant in the east of China where the soils were neutral in pH. We recommend that more revolutionary means be taken to sequestrate atmospheric carbon into soil as the current OC increase due to increasing crop productivity via a high rate of nitrogen application may have a potential acidification effect.

The effects of biochar derived from feedstock with different Si and Al concentration on soil N2O and CO2 emissions

Desilicification and allitization is important characteristic of acidic soil. While decrease in soil silicon (Si) may generate Si limitation, the increase of aluminum (Al) will aggravate soil acidification. Biochar has been used in acid soil improvement, which could mitigate nitrous oxide (N₂O) emissions and alter soil Si and Al concentration. However, the effect of biochar with different Si and Al concentration on greenhouse gas emissions remains unclear. We evaluated the effects of biochar derived from feedstock with different Si (moso bamboo leaves, BL; rice straw, RS) and Al (Camellia oleifera fruit shell, CFS; C. oleifera leaves, CL) concentration on greenhouse gas emissions and soil acidification. Microbial functional gene abundance associated with N₂O emissions were measured to further explore the response of microbiological community. The results showed that BL, RS, CFS and CL significantly increased soil pH (by 19.2%, 16.7%, 18.7% and 24.9%, respectively), decreased soil exchangeable acid and exchangeable Al content, and reduced N₂O emission rate of soil with nitrogen (N) (by 14.2%, 27.3%, 25.6% and 38.7%, respectively), which correlated with increase in soil nosZ abundance. BL, RS, CFS and CL increased soil nirK (by 325.6%, 66.7%, 155.8%, and 253.2%, respectively) and nosZ (by 198.6%, 174.1%, 72.2%, and 152.0%, respectively) abundance with N. Structural equation model showed that Si input via biochar application directly reduced N₂O emissions, and soil acid-extractable Si is inversely proportional to N₂O emission rate. In addition, Si input reduced carbon dioxide (CO₂) emissions via indirect effects. Al input via biochar addition indirectly affected N₂O and CO₂ emissions through mainly indirect effects on other soil factors. In intensive management and production activities, Si-rich biochar can be considered instead of sole addition as fertilizer, which will be beneficial to the sustainable development of agricultural and forestry production in acid soil areas, and mitigation of global change.

Assessing critical loads and exceedances for acidification and eutrophication in the forests of East and Southeast Asia: A comparison with EANET monitoring data

Spatial variations in sulfur (S) and nitrogen (N) deposition have changed in East and Southeast Asia in recent decades. Nevertheless, in this region, including the tropics, regional-scale assessments of the long-term risk of acidification and eutrophication (N saturation) for terrestrial ecosystems using a critical load approach have not been updated since 2001. To evaluate future risks, maps of critical loads and exceedances were updated using recently acquired spatial datasets of soil properties, soil minerals, climate, tree plantations, and the annual S and N depositions estimated using the Community Multiscale Air Quality (CMAQ) model. The resulting maps were verified using data on long-term trends in soil pH and nitrate concentration in surface water acquired by the Acid Deposition Monitoring Network in East Asia (EANET). It was found that N deposition exceeded the critical load for eutrophication not only in East Asia but also in some parts of the tropical monsoon and humid regions in Southeast Asia, whereas S deposition partly exceeded the critical load for soil acidification in China and small parts of the tropical monsoon region. The high-risk areas for eutrophication coincided well with the EANET sites, where the increase in nitrate concentration in the surface water was significant over the last 20 years. Hence, the estimated map of the critical load exceedance for eutrophication is more plausible for assessing the risk in East and Southeast Asia than that for acidification, although the critical load exceedance for acidification would be sufficiently significant as an updated risk map based on the latest input values. This update also suggests that increased N deposition around megacities, water discharge, and tree plantations may play an important role in the spatial variability of eutrophication risks in the tropics of Southeast Asia.

The leaching of antimony and arsenic by simulated acid rain in three soil types from the world's largest antimony mine area

A simulated acid rain (SAR) experiment on leaching of antimony (Sb) and arsenic (As) in three soil types including paddy soils (PS), vegetable soils (VS) and slag based soils (SS) from Xikuangshan (XKS) Sb mine area was conducted. The SAR at pH 2.5, 3.5, 4.5 and 5.6 were sprayed to soil columns with intermittent pattern in a period of 50 days. Through the spraying duration, leaching Sb in PS, VS and SS showed decreasing trends regardless of pH values in SAR and were in the ranges of 0.026-0.064 mg L^-1, 0.19-2.18 mg L^-1 and 11.8-32.4 mg L^-1, respectively. By contrast, leaching As in these three soil types continuously increased at the initial five spraying times and then deeply decreased afterward, with ranges being 0-0.007 mg L^-1, 0.001-0.071 mg L^-1 and 0.17-1.07 mg L^-1, respectively. The leaching Sb in all the three soil types were extremely higher than the reference value in grade IV (0.01 mg L^-1) for groundwater quality of China (GB/T 14,848-2017). For leaching As, peck values in VS and all the values in SS were also greater than the corresponding reference value (0.05 mg L^-1). This indicated that leaching Sb and As could pollute the groundwater in XKS Sb mine area, especially those in slag based soils. The total leaching losses of Sb and As were affected by pH ambiguously, such as SAR at pH 2.5, 5.6 and 2.5 induced the greatest losses of Sb in PS, VS and SS, and pH 3.5, 5.6 and 2.5 resulted in the greatest leaching losses of As in these soils. After SAR treatment, the specific sorbed and Fe/Mn oxide-associated Sb and As significantly decreased. It demonstrated that these two fractions of both Sb and As were involved in leaching losses. The present study also found that the SAR treatment resulted in soil acidification in all the three soil types. In addition, available N, P and K in all the SAR treatments decreased regardless of pH values, except for available N and P in PS.

Biochar increases Panax notoginseng's survival under continuous cropping by improving soil properties and microbial diversity

Replant problem is widespread in agricultural production and causes serious economic losses, which has limited sustainable cultivation of Panax notoginseng (PN), a well-known medicinal plant in Asia. Here we conducted a field experiment to investigate the effectiveness and possible mechanisms of biochar to improve its survival under continuous cropping. Biochar from tobacco stems was applied at 4 rates of 9.0, 12, 15, and 18 t/ha to a soil where PN has been continuously cultivated for 10 years. After 18 months, soil properties, 5 allelochemicals, including p-hydroxybenzoic acid, vanillic acid, syringic acid, p-coumaric acid, and ferulic acid, key pathogen Fusarium oxysporum, microbial community, and PN survival rate were investigated. Our results show that 10 years' continuous PN cropping led to soil acidification, accumulation of NH₄⁺-N and F. oxysporum, and low PN survival rate. However, biochar increased its survival rate from 6.0% in the control to 69.5% under 15 t/ha treatment. Moreover, soil pH, available P and K, organic matter content, and microbial diversity were increased while NH₄⁺-N and allelochemicals vanillic acid and syringic acid contents were decreased under biochar treatment (P<0.05). Soil available K increased from 177 to 283 mg·kg^-1 while NH₄⁺-N decreased from 6.73 to 4.79 mg·kg^-1 under 15 t/ha treatment. Further, soil pH, available P and K, and microbial diversity (bacteria and fungi) were positively correlated with PN survival rate, however, NH₄⁺-N content was negatively correlated (P<0.05). Our study indicates that biochar effectively increased the survival rate of Panax notoginseng under continuous cropping by improving soil properties and microbial diversity.

Factors driving the assembly of prokaryotic communities in bulk soil and rhizosphere of Torreya grandis along a 900-year age gradient

Excessive nutrient inputs imperil the stability of forest ecosystems via modifying the interactions among soil properties, microbes, and plants, particularly in forests composed of cash crops that are under intensive disturbances of agricultural activities, such as Torreya grandis. Understanding the potential drivers of soil microbial community helps scientists develop effective strategies for balancing the protection and productivity of the ancient Torreya forest. Here, we assayed the link between plant and soil parameters and prokaryote communities in bulk soil and T. grandis rhizosphere in 900-year-old stands by detecting plant and soil properties in two independent sites in southeastern China. Our results showed no apparent influence of stand age on the compositions of prokaryote communities in bulk soil and T. grandis rhizosphere. In contrast, soil abiotic factors (i.e., soil pH) overwhelm plant characteristics (i.e., height, plant tissue carbon, nitrogen, and phosphorus content) and contribute most to the shift in prokaryote communities in bulk soil and T. grandis rhizosphere. Soil pH leads to an increase in microbiota alpha diversity in both compartments. With the help of a random forest, we found a critical transition point of pH (pH = 4.9) for the dominance of acidic and near-neutral bacterial groups. Co-occurrence network analysis further revealed a substantially simplified network in plots with a pH of <4.9 versus samples with a pH of ≥4.9, indicating that soil acidification induces biodiversity loss and disrupts potential interactions among soil microbes. Our findings provide empirical evidence that soil abiotic properties nearly completely offset the roles of host plants in the assembly and potential interactions of rhizosphere microorganisms. Hence, reduction in inorganic fertilization and proper liming protocols should be seriously considered by local farmers to protect ancient Torreya forests.

The exacerbation of soil acidification correlates with structural and functional succession of the soil microbiome upon agricultural intensification

Agricultural intensification driven by land-use changes has caused continuous and cumulative soil acidification (SA) throughout the global agroecosystem. Microorganisms mediate acid-generating reactions; however, the microbial mechanisms responsible for exacerbating SA feedback remain largely unknown. To determine the microbial community composition and putative function associated with SA, we conducted a metagenomic analysis of soils across a chronosequence that has elapsed since the conversion of rice-wheat (RW) to rice-vegetable (RV) rotations. Compared to RW rotations, soil pH decreased by 0.50 and 1.56 units (p < 0.05) in response to 10-year and 20-year RV rotations, respectively. Additionally, acid saturation ratios were increased by 7.3% and 36.2% (p < 0.05), respectively. The loss of microbial beta-diversity was a key element that contributed to the exacerbation of SA in the RV. Notably, the 20-year RV-enriched microbial taxa were more hydrogen (H⁺)-, aluminium (Al³⁺)-, and nitrate nitrogen (NO₃^--N) -dependent and contained more genera exhibiting dehydrogenation functions than did RW-enriched taxa. "M00115, M00151, M00417, and M00004" and "M00531 and M00135" that are the "proton-pumping" and "proton-consuming" gene modules, respectively, were linked to the massive recruitment of acid-dependent biomarkers in 20-year RV soils, particularly Rhodanobacter, Gemmatirosa, Sphingomonas, and Streptomyces. Collectively, soils in long-term RV rotations were highly acidified and acid-sensitive, as the enrichment of microbial dehydrogenation genes allowing for soil buffering capacity is more vulnerable to H⁺ loading and consequently promotes the colonization of more acid-tolerant and acidogenic microbes, and ultimately provide new clues for researchers to elucidate the interaction between SA and the soil microbiome.

Effect of biochar and hydrochar from cow manure and reed straw on lettuce growth in an acidified soil

Soil acidification has become a major environmental and economic concern worldwide. Char materials (e.g biochar, hydrochar) have attracted considerable attention as soil amendments to restore degraded soil. However, the comparative study of biochar and hydrochar on plant growth in acidified soil is limited. In this study, a microcosmic experiment was used to compare the effect of biochar and hydrochar from cow manure (CBC, CHC) and reed straw (RBC, RHC) on the growth of lettuce in the acidified soil. CBC and RHC significantly increased and decreased the biomass of lettuce by 18.7% and 32.5% in the acidified soil, respectively. The increase of the lettuce growth by CBC primarily attributed to the enhancement of soil properties (SOM and pH) and soil nutrient content (Olsen-P, K, Ca, Mg, Zn, Fe and Mn). Moreover, CBC enhanced the microbial activity and complexity and increased the abundance of beneficial genera like Gemmatimonas, Ramlibacter and Haliangium, which improved soil health and might indirectly benefited the lettuce growth. Our findings highlighted the priority of char materials feedstock and preparation technology for remediating the acidified soil.

Soil acidification and loss of base cations in a subtropical agricultural watershed

Soil acidification along with base cations loss degrades soil quality and is a major environmental problem, especially in agroecosystems with extensive nitrogen (N) fertilization. So far, the rates of proton (H⁺) production and real soil acidification (loss of base cations) remain unclear in subtropical agricultural watersheds. To assess the current status and future risk of soil acidification in subtropical red soil region of China, a two-year monitoring was conducted in a typical agricultural watershed with upland, paddy fields, and orchards where high N fertilizers are applied (320 kg N ha^-1 yr^-1). H⁺ production, neutralization and base cations losses were quantified based on the inputs (rainwater, inflow of water, and fertilizer) and outputs (outflow of water, groundwater drainage, and plant uptake) of major elements (K⁺, Ca²⁺, Na⁺, Mg²⁺, Al³⁺, NH₄⁺, NO₃^-, SO₄^2-, Cl^-, and H⁺). The result showed that total H⁺ production in the watershed was 5152 mol_c ha^-1 yr^-1. N transformation was the most important H⁺ source (68%), followed by excess plant uptake of cations (25%) and H⁺ deposition (7%). Base cations exchange and weathering of minerals (3842 mol_c ha^-1 yr^-1) dominated H⁺ neutralization, followed by SO₄^2- adsorption (1081 mol_c ha^-1 yr^-1), while H⁺ and Al³⁺ leaching amounted to 431 mol_c ha^-1 yr^-1, only. These results state clearly that despite significant soil acidification, the acidification of surface waters is minor, implying that soils have buffered substantially the net H⁺ addition. As a result of soil buffering, there was abundant loss of base cations, whose rate is significantly higher than the previously reported weathering rate of minerals in red soils (3842 vs 230-1080 mol_c ha^-1 yr^-1). This suggests that the pool of exchangeable base cations is being depleted in the watershed, increasing the vulnerability of the watershed, and posing a serious threat to future recovery of soils from acidification.

Vulnerability and driving factors of soil inorganic carbon stocks in Chinese croplands

The long-term stability of soil inorganic carbon (SIC) and its minimum contribution towards global C cycle has been challenged, as recent studies have showed rapid decreases in SIC stocks in intensive agricultural systems. However, the extent of SIC losses and its driving factors remains unclear. Here, we compared changes in SIC density (SICD) in Chinese croplands between the 1980s and 2010s. The SIC contents in 1980s were obtained from second national soil survey (n = 949) and published studies (n = 47). The SIC contents in 2010s were based on resampling of soil profiles from the same locations during 2019 and 2020 (n = 30), as well as data from published studies and national soil survey (n = 903). We found that Chinese croplands have lost 27-38% of SICD from the 0-40 cm soil layer and that the soil pH has decreased by 0.53 units over the past 30 years. These SIC losses increased with the ratio of precipitation (P) to potential evapotranspiration (PET) and most notably with nitrogen (N) fertilization. The SICD decreased greatly in humid and semiarid regions, and these losses were enhanced by high N fertilization rates; however, the SICD increased in very arid regions. This analysis demonstrates that the water balance and N fertilization are major drivers leading to dramatic losses of SICD in croplands and, consequently, to decreases in soil fertility and functions.

Can acid sulfate soils from the southern Baltic zone be a source of potentially toxic elements (PTEs)?

The mobility of Cr, Cu, Ni, Pb, and Zn in acid sulfate (AS) soils in the reverse delta of the Świna River, NE Poland was studied using extraction of the target metals using the BCR protocol, which targets acid-extractable (F1), reducible (F2), oxidizable (F3) fractions, and aqua regia digestion for residual (F4) fraction. It was assumed that the content of mobile forms determined in air-dried samples during consequent steps of BCR extraction refers to two scenarios of possible release of selected metals from the studied soils: (1) attributed to seasonal soil moisture variation or (2) caused by artificial drainage. The studied AS soils had thin organic layers (muck, peat and mud) overlaying deltaic sands, and contained hypersulfidic material. The field pH was 6.2 ± 0.5 and significantly decreased to 4.3 ± 1.4 after the 8-weeks incubation period. This can be explained by low buffering properties (e.g. lack of carbonates). Total concentrations of metals (Cr 17.9-61.6, Cu 5.7-27.7, Ni 6.2-47.0, Pb 2.2-17.7, and Zn 13.6-130 mg∙kg^-1) in the AS soils were diversified but none of the concentrations exceeded the Polish legal limits. Despite the relative low content of analyzed metals, the studied soils can be a potential source of metal contamination affecting the coastal environment in the southern Baltic Sea region that has been overlooked so far. This is confirmed by the high proportion (44-82%) of Cr, Cu, Ni, and Zn in the F1-F3 fractions which can potentially be released as assumed in two scenarios. The BCR protocol seems to be a useful tool for understanding the chemical behavior and fate of metals in AS soils. Challenges in the assessment of metal mobility in the oxidized and unoxidized zones of individual soil profiles occurred due to their complex morphology resulting from the heterogeneous depositional environment under the human impact.

Soil acidification induced decline disease of Myrica rubra: aluminum toxicity and bacterial community response analyses

The decline disease of Myrica rubra tree is commonly induced by soil acidification, which affects the yield and the quality of fruits. It is hypothesized that aluminum toxicity and microbial community changes caused by soil acidification were the main causes of decline of Myrica rubra tree. In order to explore the decline mechanism of Myrica rubra tree, soils around healthy and decline trees of Myrica rubra were collected to compare the concentrations of different aluminum forms, enzyme activities, and bacterial community structure. In this study, soil samples were collected from the five main production areas of Myrica rubra, Eastern China. The results showed that diseased soils had higher exchangeable aluminum, lower enzyme activities, and lower microbial diversity than healthy soils at various sites. The toxic Al significantly decreased bacterial diversity and altered the bacterial community structure. The diseased soils had significantly lower α-diversity indices (ACE, Chao1, and Shannon) of bacterial community. The Al toxicity deceased the relative abundance of Acidobacteria and Planctomycetes, while enhanced the relative abundance of Cyanobacteria, Bacteroidetes, and Firmicutes in soils. Co-occurrence network analysis indicated that the Al toxicity simplified the bacterial network. The soil ExAl content was significantly and negatively correlated with the nodes (r = -0.69, p < 0.05) and edges (r = -0.77, p < 0.01) of the bacterial network. These results revealed that the Al toxicity altered soil bacterial community structure, resulting in the decline disease of Myrica rubra tree, while highlighted the role of Al forms in the plant growth. This finding is of considerable significance to the better management of acidification-induced soil degradation and the quality of fruits.

Seasonal dynamics of soil pH and N transformation as affected by N fertilization in subtropical China: An in situ 15N labeling study

Nitrogen (N)-induced soil acidification has received much attention worldwide. Nitrification and soil N mineralization are two key N cycle processes that affect soil acidification. However, the seasonal dynamics of soil pH under their combined influence is unclear. We studied the effect of N fertilization on soil pH and N transformations using ¹⁵N tracing in field lysimeters with soils developed from different parent materials (Quaternary red clay, sandstone, and basalt). Maize was planted with 200 kg N ha^-1 yr^-115N-labeled urea addition. During 7-45 days after fertilization, proton (H⁺) production due to nitrification of fertilizer N, nitrate (NO₃^-) leaching, and plant uptake exceeded H⁺ consumption by base cations mobilization and leaching, resulting in a significant soil pH decline. When nitrification activity decreased (after 45 days), due to exhausted ammonium (NH₄⁺) availability, soil pH rose again. During the fallow period, acid neutralization due to base cation mobilization, and ammonification of soil organic N (SON) offset H⁺ production caused by nitrification of mineralized SON, leading to a sustained rise in soil pH. After the one-year experiment, no significant soil pH decrease was observed in any of the soils. Parent material had little effect on the seasonal dynamics of soil acidification, which appeared to be controlled by fertilization, environmental factors (temperature and moisture), and plant uptake. In subtropical regions, monitoring of soil pH on an annual basis may mask the effect of N fertilization on soil acidification.

Calculation of spatially explicit amounts and intervals of agricultural lime applications at county-level in China

Liming is a long-established and widely used agricultural practice to ameliorate soil acidity and improve crop production. Sustainable liming strategies for regional applications require information on both lime requirements and liming intervals given land use and soil dependent acidification rates. We developed a method to optimize lime requirements and liming intervals at regional level. Lime requirements were based on soil pH buffering capacity and liming intervals were estimated by ongoing soil acidity production, derived from major cations and anions balances in cropland systems. About 66% of croplands in Qiyang required liming to raise soil pH to 6.5, with a total lime requirement of 2.4 × 10⁵ t CaCO₃, with an average rate of 2.4 t ha^-1 for paddy soils and 2.6 t ha^-1 for upland soils. The remaining 34% were mainly calcareous soils. Nutrient management practices and crop rotations, affecting N transformation and crop removal, were the main drivers controlling the spatial variation in total acid production in non-calcareous soils, on average contributing 73% and 25%, respectively. Under current soil acidification rates, 33% of Qiyang's croplands would need liming within 30 years after raising the soil pH to 6.5. Averaged liming interval was 20 years, and 6.8 t ha^-1 would be required to maintain soil pH ranges between 5.5 and 6.5. Areas with high soil acidification risk were mostly located in the southeast of Qiyang.

Disentangling drivers of soil microbial nutrient limitation in intensive agricultural and natural ecosystems

Characterized by continuous chemical fertilization, intensive agriculture generally reduces soil ecoenzymatic activities and nutrient mineralization, as well as alters the biomass production and microbial community composition. Soil acidification poses serious threats to the sustainable development of intensive agriculture. However, the mechanism of nutrient cycling and metabolism of soil microorganisms in response to soil acidification in intensive agriculture remains unclear. Herein, we studied the variations in ecoenzymatic stoichiometry of soil β-glucosidase (BG), cellobiohydrolase (CBH), N-acetylglucosaminidase (NAG) and acid phosphatase (AP) under different land use types and pH gradients of tea garden soils. The results revealed that natural forest and cropland soils had significantly higher BG and CBH activities than tea garden soils. Soil BG and CBH activities displayed significant positive correlations with soil pH, total nitrogen (TN) and phosphorus (TP), while soil NAG activity was significantly associated with nitrate nitrogen, total carbon (TC), TN, carbon: phosphorus (C:P) and nitrogen: phosphorus (N:P) ratios. Soil AP activity showed significant negative associations with pH, TP and C:N ratio, but was significantly positively correlated with TC, TN, C:P and N:P ratios. Enzyme vector model revealed that soil microorganisms are limited by P (enzyme vector angle >45°) regardless of land use types. Compared to natural forest soils, the P limitation of microorganisms in tea garden soils became increasingly serious with a decreasing pH gradient, as indicated by the significant increase in enzyme vector angle. Thus, the overall ecoenzymatic stoichiometry was shifted by soil pH. In summary, higher pH increased BG activity and decreased AP activity, but had no significant effect on NAG activity, suggesting co-limitation of soil microorganisms by C and P in this area. This study provides novel insights into the effect of soil acidification on ecoenzymatic stoichiometry, and also highlights the stoichiometric and energy limitations on the metabolism of soil microorganisms in agricultural ecosystems.

The effects of H2O2- and HNO3/H2SO4-modified biochars on the resistance of acid paddy soil to acidification

Biochar was prepared from rice straw and modified with 15% H₂O₂ and 1:1 HNO₃/H₂SO₄, respectively. The unmodified biochars and HCl treated biochars for carbonate removal were used as control. The biochars were added to the acid paddy soil collected from Langxi, Anhui Province, China at the rate of 30 g/kg. The paddy soil was flooded and then air-dried, and soil pH and Eh were measured in situ with pH electrode and platinum electrode during wet-dry alternation. Soil pH buffering capacity (pHBC) was determined by acid-base titration after the wet-dry treatment. Then, the simulated acidification experiments were carried out to study the changing trends of soil pH, base cations and exchangeable acidity. The results showed that soil pHBC was effectively increased and the resistance of the paddy soil to acidification was apparently enhanced with the incorporation of H₂O₂- and HNO₃/H₂SO₄-modified biochars. Surface functional groups on biochars were mainly responsible for enhanced soil resistance to acidification. During soil acidification, the protonation of organic anions generated by dissociation of these functional groups effectively retarded the decline of soil pH. The modification of HNO₃/H₂SO₄ led to greater increase in carboxyl functional groups on the biochars than H₂O₂ modification and thus HNO₃/H₂SO₄-modified biochars showed more enhancement in soil resistance to acidification than H₂O₂-modified biochars. After a wet-dry cycle, the pH of the paddy soil incorporated with HNO₃/H₂SO₄-modified biochar increased apparently. Consequently, the addition of HNO₃/H₂SO₄-modified biochar can be regarded as a new method to alleviate soil acidification. In short, the meaning of this paper is to provide a new method for the amelioration of acid paddy soils.

Soil acidification enhances the mobilization of phosphorus under anoxic conditions in an agricultural soil: Investigating the potential for loss of phosphorus to water and the associated environmental risk

Soil redox potential (E_H) and pH are key parameters regulating the solubility and fate of phosphorus (P). However, the impact of soil acidification on the redox-induced mobilization and speciation of P in soils under a wide range of E_H values has not been extensively studied. Here, we investigated the mobilization and speciation of P in an acidified agricultural soil at two different pH values (e.g., highly acidic soil; pH = 5.6 and slightly acidic soil; pH = 6.1) compared to the un-acidified soil (control soil; pH = 7.3) under a wide range of E_H condition (+459 to -281 mV). The impacts of E_H/pH-dependent changes of Fe-Mn oxides, and dissolved organic (DOC) and inorganic (DIC) carbon on P mobilization and speciation were also investigated using geochemical and spectroscopic (X-ray absorption near edge structure) techniques. The concentrations of dissolved P under anoxic conditions increased up to 69.3% in the highly acidic soil compared with the control soil. The decrease of the Fe-P fraction, the decrease of Ferrihydrite-P_ads speciation, and the strong linear correlation between the dissolved P and Fe²⁺ (R² > 0.85) supports the finding that enhanced P mobilization under anoxic conditions may be attributed to Fe reduction in the highly acidic soil. The concentration of dissolved Fe and P remained low until pH dropped below 6.35 for P and 6.28 for Fe, while a liner increase was found in dissolved Mn accompanying a general trend of pH decrease. This result suggests that the dissolution of reducible Mn under acidic soil conditions was an important factor for enhancing mobilization of dissolved P under anoxic conditions. This trend was due to the low amount of Mn, indirectly speeding up Fe reduction. These results can help to develop management practices to effectively mitigate P export and protect water resources from diffuse P pollution.

Knockdown of Succinate Dehydrogenase Assembly Factor 2 Induces Reactive Oxygen Species-Mediated Auxin Hypersensitivity Causing pH-Dependent Root Elongation

Metabolism, auxin signaling and reactive oxygen species (ROS) all contribute to plant growth, and each is linked to plant mitochondria and the process of respiration. Knockdown of mitochondrial succinate dehydrogenase assembly factor 2 (SDHAF2) in Arabidopsis thaliana lowered succinate dehydrogenase activity and led to pH-inducible root inhibition when the growth medium pH was poised at different points between 7.0 and 5.0, but this phenomenon was not observed in wildtype (WT). Roots of sdhaf2 mutants showed high accumulation of succinate, depletion of citrate and malate and up-regulation of ROS-related and stress-inducible genes at pH 5.5. A change of oxidative status in sdhaf2 roots at low pH was also evidenced by low ROS staining in root tips and altered root sensitivity to H2O2. sdhaf2 had low auxin activity in root tips via DR5-GUS staining but displayed increased indole-3-acetic acid (IAA, auxin) abundance and IAA hypersensitivity, which is most likely caused by the change in ROS levels. On this basis, we conclude that knockdown of SDHAF2 induces pH-related root elongation and auxin hyperaccumulation and hypersensitivity, mediated by altered ROS homeostasis. This observation extends the existing evidence of associations between mitochondrial function and auxin by establishing a cascade of cellular events that link them through ROS formation, metabolism and root growth at different pH values.

Impact of Urea and Ammoniacal Nitrogen Wastewaters on Soil: Field Study in a Fertilizer Industry (Bahía Blanca, Argentina)

Nitrogen compounds in industrial effluents are considered a serious threat to the environment. The aim of this work is to identify the effect produced by nitrogen-rich wastewater on alkaline soils from industrial land. Two plots were irrigated with wastewater as ammoniacal nitrogen (31 to 53 g N m^-2) and urea (167-301 g N m^-2) sources named P1 and P2, respectively. Inorganic nitrogen (N) concentrations (N-NH₃ + N-NH₄, N-NO₂, N-NO₃), soil pH, and N-NH₃ volatilization were monitored during a 2-year period. Variations in the fate of N compounds were distinguished according to the quantity and source of N applied to the soil. A higher N input in the form of urea was related to a greater concentration of nitrates and soil acidification in the topsoil (0-30 cm). Otherwise, ammoniacal N wastewater showed greater relative ammonia losses due to volatilization. Ammonia losses were estimated as 24.2% and 7.43% of the total N applied in P1 and P2, respectively. Besides, in P1 ammoniacal N predominated over nitrate, unlike results obtained in P2. The correct management of nitrogen-rich wastewaters in fertilizer industries could greatly reduce soil and groundwater degradation.

Sugarcane monoculture drives microbial community composition, activity and abundance of agricultural-related microorganisms

Sugarcane monoculture (SM) often leads to soil problems, like soil acidification, degradation, and soil-borne diseases, which ultimately pose a negative impact on agricultural productivity and sustainability. Understanding the change in microbial communities' composition, activities, and functional microbial taxa associated with the plant and soil under SM is unclear. Using multidisciplinary approaches such as Illumina sequencing, measurements of soil properties, and enzyme activities, we analyzed soil samples from three sugarcane fields with different monoculture histories (1-, 2-, and 4-year cultivation times, respectively). We observed that SM induced soil acidity and had adverse effects on soil fertility, i.e., soil organic matter (OM), total nitrogen (TN), total carbon (TC), and available potassium (AK), as well as enzyme activities indicative for carbon, phosphorus, and nitrogen cycles. Non-metric multidimensional scaling (NMDS) analysis showed that SM time greatly affected soil attribute patterns. We observed strong correlation among soil enzymes activities and soil physiochemical properties (soil pH, OM, and TC). Alpha diversity analysis showed a varying response of the microbes to SM time. Bacterial diversity increased with increasing oligotrophs (e.g., Acidobacteria and Chloroflexi), while fungal diversity decreased with reducing copiotrophs (e.g., Ascomycota). β-Diversity analysis showed that SM time had a great influence on soil microbial structure and soil properties, which led to the changes in major components of microbial structure (soil pH, OM, TC, bacteria and soil pH; TC, fungi). Additionally, SM time significantly stimulated (four bacterial and ten fungal) and depleted (12 bacterial and three fungal) agriculturally and ecologically important microbial genera that were strongly and considerably correlated with soil characteristics (soil pH, OM, TC, and AK). In conclusion, SM induces soil acidity, reduces soil fertility, shifts microbial structure, and reduces its activity. Furthermore, most beneficial bacterial genera decreased significantly due to SM, while beneficial fungal genera showed a reverse trend. Therefore, mitigating soil acidity, improving soil fertility, and soil enzymatic activities, including improved microbial structure with beneficial service to plants and soil, can be an effective measure to develop a sustainable sugarcane cropping system.

Study on the new dynamics and driving factors of soil pH in the red soil, hilly region of South China

Soil acidification has always been a substantial eco-environmental problem restricting agricultural development in the red soil region of southern China. It is necessary to determine the dynamic change in soil pH in this area to formulate regional agricultural and environmental management measures. Yujiang County, a typical county with red soil acidification in southern China, was selected as the study area. Based on soil data from 1982, 2007, and 2018, the spatiotemporal variation characteristics and the latest changes in soil pH in the county were analyzed. The results show that the soil pH in Yujiang County decreased from 5.66 to 4.74 and then increased to 4.96 from 1982 to 2018, showing a trend of first decreasing and then increasing. According to the spatial distribution characteristics of soil pH, the low soil pH values in the three periods were mainly distributed in the northern mountainous areas with more forestland and dry land area and some southern hilly areas, while the paddy soil pH values in the middle low hilly areas were relatively higher. The soil pH decreased rapidly from 1982 to 2007, showing a large area of acidification. In 2007, the proportions of acidic (4.5 < pH < 5.5) and strongly acidic (pH < 4.5) soils increased by 67.37% and 10.6%, respectively, compared with that in 1982. However, from 2007 to 2018, the soil pH of the whole county increased, and the acidification trend was alleviated, which is of great significance to the regional red soil ecological environment. Through the analysis of the main factors affecting the change in soil pH, it was found that the sharp decline in soil pH in Yujiang County during 1982-2007 was mainly caused by acid rain and excessive nitrogen application. From 2007 to 2018, no significant reduction in nitrogen fertilizer in this area occurred, and although the increase in soil organic matter contributed to alleviating soil acidification, the analysis showed that the decrease in acid rain was the main reason for the rise in soil pH in Yujiang County. At the same time, notably, there is a large area of soil in the area that is still acidic, and effective control of soil acidification is still an important ecological and environmental issue in this area. In order to further improve the pH value of soil in red soil region, it is suggested that on the basis of continuous improvement of acid rain, in addition to increasing soil organic matter by returning straw to field and other measures, appropriate amount of lime or alkaline biochar can be applied to better improve the soil ecological environment in red soil hilly region.

Soil acidification enhancing the growth and metabolism inhibition of PFOS and Cr(VI) to bacteria involving oxidative stress and cell permeability

Soil acidification is causing more and more attention, not only because of the harm of acidification itself, but also the greater harm to bacteria brought by some pollutants under acidic condition. Therefore, the toxicities of two typical soil pollutants (perfluorooctane sulfonate (PFOS) and chromium (Cr(VI)) to growth and metabolisms of soil bacteria (Bacillus subtilis as modol) were investigated. Under acidic condition of pH = 5, Cr(VI), PFOS and PFOS + Cr(VI) show stronge inhibition to bacteria growth up to 24.3%, 42.3%, 41.6%, respectively, and this inhibition was about 2-3 times of that at pH = 7. Moreover, acid stress reduces the metabolism of bacteria, while PFOS and Cr(VI) pollution futher strengthens this metabolic inhibition involving oxidative stress and cell permeability. The activities of dehydrogenase (DHA) and electron transport system (ETS) at pH = 5 exposed to Cr(VI), PFOS and combined PFOS + Cr(VI) was 21.5%, 16.9%, 23.2% and 8.9%, 32.2%, 19.1% lower than the control, respectively. However, the relative activity of DHA and ETS at pH = 7 are 5-8 and 2-13 times of that at pH = 5, respectively. Isoelectric point, cell surface hydrophobicity and molecular simulation analysis show that the corresponding mechanism is that acidic conditions enhance the interaction between bacteria and PFOS/Cr(VI) through hydrogen bonding, hydrophobic and electrostatic interactions. The results can guide the remediation of acid soil pollution, and provide a reference for the combined toxicity evaluation of heavy metals and micro-pollutants in acid soil.

Effects of straw decayed products of four crops on the amelioration of soil acidity and maize growth in two acidic Ultisols

Variable charge soils have low agricultural productivity associated with low pH, low cation exchange capacity (CEC), and low pH buffering capacity (pHBC). As a result of rapid acidification rates, these soils are prone to infertility resulting from Al phytotoxicity and deficiency of P, Ca, Mg, and K, and thus require amendments that can ameliorate soil acidity and enhance soil CEC and pHBC. A 30-day pot experiment was carried out using a clay Ultisol and a sandy Ultisol amended with straw decayed products (SDPs) of peanut, pea, canola, and rice. The results showed that applying SDPs increased the soil CEC, organic matter content, and exchangeable base cations in the two Ultisols. The ameliorative effects of the SDPs were superior for the sandy Ultisol than for the clay Ultisol. The addition of SDPs significantly increased soil pH and pHBC of the two Ultisols, and simultaneously decreased soil exchangeable Al³⁺. Among them, the greatest effect was found in the treatment with pea straw decayed products (PeaSD). The soil pHs of clay Ultisol and sandy Ultisol treated with PeaSD were respectively 5.70 and 7.37 and were 1.26 and 2.63 pH units higher than those of control. Also, applying SDPs increased maize seedling biomass in both soils and the most significant effect was found in the treatment with PeaSD, which were 0.97 (clay Ultisol) and 2.5 (sandy Ultisol) times higher than in the respective controls. The results of this study demonstrated that carefully selected straws for SDP production can effectively improve soil chemical properties, enhanced soil pHBC, and thus promote agricultural sustainability.

Effects of synthetic nitrogen fertilizer and manure on fungal and bacterial contributions to N2O production along a soil acidity gradient

Reactive nitrogen (Nr) input often induces soil acidification, which may in turn affect bacterial and fungal nitrogen (N) transformations in soil and nitrous oxide (N₂O) emissions. However, the interactive effects of soil acidity and Nr on the contributions of bacteria and fungi to N₂O emissions remain unclear. We conducted a field experiment to assess the effects of anthropogenic Nr forms (i.e., synthetic N fertilizer and manure) on bacterial and fungal N₂O emissions along a soil acidity gradient (soil pH = 6.8, 6.1, 5.2, and 4.2). The abundances and structure of bacterial and fungal communities were analyzed by real-time polymerase chain reaction and high-throughput sequencing techniques, respectively. Soil acidification reduced bacterial but increased fungal contributions to N₂O production, corresponding respectively to changes in bacterial and fungal abundance. It also altered bacterial and fungal community structures and soil chemical properties, such as dissolved organic carbon and ammonia concentrations. Structural equation modeling (SEM) analyses showed that the soil properties and fungal community were the most important factors determining bacterial and fungal contributions to N₂O emissions, respectively. The fertilizer form markedly affected N₂O emissions from bacteria but not from fungi. Compared with synthetic N fertilizer, manure significantly lowered the bacterial contribution to N₂O emissions in the soils with pH of 5.2 and 4.2. The manure application significantly increased soil pH but reduced nitrate concentration. The fertilizer form did not significantly alter the bacterial and fungal community structures. The SEM revealed that the fertilizer form affected the bacterial contribution to N₂O production by changing the soil chemical properties. Together, these results indicated that soil acidification enhanced fungal dominance for N₂O emission, and manure application has limited effects on fungal N₂O emission, highlighting the challenges for mitigation of soil N₂O emissions under future acid deposition and N enrichment scenarios.

Effects of ammonium-based nitrogen addition on soil nitrification and nitrogen gas emissions depend on fertilizer-induced changes in pH in a tea plantation soil

Tea (Camellia sinensis L.) plants have an optimal pH range of 4.5-6.0, and prefer ammonium (NH₄⁺) over nitrate (NO₃^-); strong soil acidification and nitrification are thus detrimental to their growth. Application of NH₄⁺-based fertilizers can enhance nitrification and produce H⁺ that can inhibit nitrification. However, how soil acidification and nitrification are interactively affected by different NH₄⁺-based fertilizers in tea plantations remains unclear. The objective of this research was to evaluate the effect of the application of different forms and rates of NH₄⁺-based fertilizers on pH, net nitrification rates, and N₂O and NO emissions in an acidic tea plantation soil. We conducted a 35-day aerobic incubation experiment using ammonium sulphate, urea and ammonium bicarbonate applied at 0, 100 or 200 mg N kg^-1 soil. Urea and ammonium bicarbonate significantly increased both soil pH and net nitrification rates, while ammonium sulphate did not affect soil pH but reduced net nitrification rates mainly due to the acidic nature of the fertilizer. We found that the effect of different NH₄⁺-based nitrogen on soil nitrification depended on the impact of the fertilizers on soil pH, and nitrification played an important role in NO emissions, but not in N₂O emissions. Overall, urea and ammonium bicarbonate application decoupled crop N preference and the form of N available in spite of increasing soil pH. We thus recommend the co-application of urease and nitrification inhibitors when urea is used as a fertilizer and nitrification inhibitors when ammonium bicarbonate is used as a fertilizer in tea plantations.

Transcriptomic comparison reveals modifications in gene expression, photosynthesis, and cell wall in woody plant as responses to external pH changes

Soil acidification is one of the crucial global environmental problems, affecting sustainable land use, crop yield, and ecosystem stability. Previous research reported the tolerance of crops to acid soil stress. However, the molecular response of woody plant to acid conditions remains largely unclear. Rhododendron L. is a widely distributed woody plant genus and prefers to grow in acidic soils. Herein, weighted gene coexpression network analysis was performed on R. protistum var. giganteum seedlings subjected to five pH treatments (3.5, 4.5, 5.5, 6.0, 7.0), and their ecophysiological characteristics were determined for the identification of their molecular responses to acidic environments. Through pairwise comparison, 855 differentially expressed genes (DEGs) associated with photosynthesis, cell wall, and phenylpropanoid metabolism were identified. Most of the DEGs related to photosynthesis and cell wall were up-regulated after pH 4.5 treatment. Results implied that the species improves its photosynthetic abilities and changes its cell wall characteristics to adapt to acidic conditions. Weighted gene co-expression network analyses showed that most of the hub genes were annotated to the biosynthetic pathways of ribosomal proteins and photosynthesis. Expression pattern analysis showed that genes encoding subunit ribosomal proteins decreased at pH 7.0 treatment, suggesting that pH 7.0 treatment led to cell injury in the seedlings. The species regulates protein synthesis in response to high pH stress (pH 7.0). The present study revealed the molecular response mechanism of woody plant R. protistum var. giganteum to acid environments. These findings can be useful in enriching current knowledge of how woody species adapt to soil acidification under global environmental changes.

Impacts of nitrogen fertilizer type and application rate on soil acidification rate under a wheat-maize double cropping system

Nitrogen (N) fertilizer-induced soil acidification in Chinese croplands is well-known, but insight in the impacts of different N fertilizer management approaches (fertilizer type and rate) on soil acidification rates is very limited. Here, we conducted a field experiment on a moderate acid soil to quantify soil acidification rates in response to N fertilization by different fertilizer types and N rates through monitoring the fate of elements (mainly nutrients) related to H+ production and consumption. Two N fertilizer types (urea and NH4Cl) and three N rates (control, optimized and conventional, 0/120/240 kg N ha-1 for wheat, 0/160/320 kg N ha-1 for maize) were included. Nitrogen addition led to an average H+ production of 4.0, 8.7, 11.4, 29.7 and 52.6 keq ha-1 yr-1, respectively, for the control, optimized urea, conventional urea, optimized NH4Cl and conventional NH4Cl plots. This was accompanied with a decline in soil base saturation of 1-10% and in soil pH of 0.1-0.7 units in the topsoil (0-20 cm). Removal of base cations by crop harvesting and N transformations contributed ~70% and ~20% to the H+ production in the urea treated plots, being ~20% and ~75% in the NH4Cl treated plots, respectively. The large NH4+ input via fertilization in the NH4Cl treated plots strongly enhanced the H+ production induced by N transformations. The low contribution of N transformations to the H+ production in the urea treated plots was due to the limited NO3- leaching, induced by the high N losses to air caused by denitrification. Increased N addition by urea, however, strongly increased H+ production by enhanced plant uptake of base cations, mainly due to a large potassium uptake in straw. Our results highlight the important role of optimizing fertilizer form and N rate as well as straw return to the field in alleviating soil acidification.

Biological mechanisms of cadmium accumulation in edible Amaranth (Amaranthus mangostanus L.) cultivars promoted by salinity: A transcriptome analysis

Strategies to prevent cadmium (Cd) mobilization by crops under salinity conditions differs among distinct genotypes, but the biological mechanisms of Cd accumulation in different genotype crops promoted by salinity have remained scarce. In this study, we investigated the biological mechanisms of Cd accumulation in two quite different amaranth cultivars of low-Cd accumulator Quanhong (QH) and high-Cd accumulator Liuye (LY) in response to salt stress. Transcriptomes analysis was carried out on leaves and roots tissues of LY and QH grown with exchangeable Cd 0.27 mg kg^-1 and salinity 3.0 g kg^-1 treatment or control conditions, respectively. A total of 3224 differentially expressed genes (DEGs) in LY (1119 in roots, 2105 in leaves) and 848 in QH (207 in roots, 641 in leaves) were identified. Almost in each fold change category (2-2⁵, 2⁵-2¹⁰, >2¹⁰), the numbers of DEGs induced by salinity in LY treatments were much more than those in QH treatments, indicating that LY is more salt sensitive. Gene ontology (GO) analysis revealed that salinity stress promoted soil acidification and Cd mobilization in LY treatments through the enhancive expression of genes related to adenine metabolism (84-fold enrichment) and proton pumping ATPase (50-fold enrichment) in roots, and carbohydrate hydrolysis (2.5-fold enrichment) in leaves compared with that of whole genome, respectively. The genes expression of organic acid transporter (ALMT) was promoted by 2.71- to 3.94-fold in roots, facilitating the secretion of organic acids. Salt stress also inhibited the expression of key enzymes related to cell wall biosynthesis in roots, reducing the physical barriers for Cd uptake. All these processes altered in LY were more substantially compared with that of QH, suggesting that salt sensitive cultivars might accumulate more Cd and pose a higher health risk.

Soil acidification in Chinese tea plantations

Soil acidification is a major problem in intensive agricultural systems and is becoming increasingly serious. Most research has reported the soil acidification of cereal crops, forests, and grasslands. However, there is no information about soil acidification under tea cultivation on a national scale. Therefore, we conducted a nationwide survey of soil acidification in the major tea-planting areas of China and used two nationwide surveys in three Chinese counties to evaluate changes in soil acidity over the past 20-30 years. Finally, the acidity of soil from forests and traditional and organic tea plantations was compared to evaluate the effects of agricultural management on soil acidification in tea plantations. Our results show that: (1) the average soil pH was 4.68 nationally and ranged from 3.96 to 5.48 in different provinces. Overall, 46.0% of the soil samples had a pH <4.5, which is too acidic for tea growth and only 43.9% had a soil pH of 4.5-5.5, which is optimal for tea growth. (2) In the past 20-30 years, the greatest soil acidification was observed in tea plantations; the pH decreased by 0.47 to 1.43, which is much greater than the decrease seen in fruit and vegetable systems (0.40 to 1.08) and cereals (0.30 to 0.89). (3) Compared with forests, tea cultivation with chemical fertilizer application caused serious soil acidification, while no significant acidification was observed at organic tea plantations. In conclusion, serious soil acidification occurs nationally in China, and organic management is an adaptive choice for sustainable tea growth.

Modelling long-term impacts of fertilization and liming on soil acidification at Rothamsted experimental station

Liming is widely used to reduce the impacts of soil acidification and optimize soil pH for agricultural production. Whether models can simulate the effect of liming on soil pH, and base saturation (BS), and thereby guide lime application, is still largely unknown. Long-term experimental data from a grassland (Park Grass, 1965-2012) and arable land (Sawyers Field, 1962-1972) at Rothamsted Research, UK, were thus used to assess the ability of the VSD+ model to simulate the effects of long-term fertilization and liming on soil acidification. The VSD+ model was capable of simulating observed soil pH and BS changes over time in the long-term liming experiments, except for a treatment in which sulphur (S) was added. Normalized Mean Absolute Errors (NMAE) and Normalized Root Mean Square Errors (NRMSE) of simulated and observed pH values, averaged over the observation periods varied between 0.02 and 0.08 (NMAE) and 0.01-0.05 (NRMSE). The acidity budget results for Park Grass suggest that nitrogen (N) transformations contributed most to acidity production, causing predominantly aluminium (Al) exchange in the topsoil (0-23 cm) followed by base cation (BC) release, but in the treatment with S addition, BC uptake had a nearly similar effect on acidity production. However, in Sawyers Field, the acidity budget suggested that BC uptake was the dominant cause of soil acidification, while the impacts of N transformations were limited. Liming was found to sufficiently replenish BC and decrease Al exchange in the topsoil layer. Overall, the VSD+ model can adequately reconstruct the impacts of fertilizer and liming applications on acid neutralizing processes and related soil pH and BC changes at the soil exchange complex.

Potential transition in the effects of atmospheric nitrogen deposition in China

Nitrogen (N) deposition in China may increase due to urbanization and economic growth. Current research has considered the ecological significance under the assumption of increasing N deposition. Atmospheric N deposition tending toward levelling or declining has been observed in China. Such potential recovery and responses of high N loads ecosystems under decreasing atmospheric N deposition scenarios have yet to be adequately investigated. This work reviews existing literature to consider possible responses of carbon (C) sequestration, biodiversity and species composition, soil acidification, and greenhouse emissions in ecosystems responding to recent patterns of N deposition. Potential effects of N composition and internal ratios may be further explored through state-of-the-art N addition experiments and model development.

Soil acidification of the soil profile across Chengdu Plain of China from the 1980s to 2010s

Soil acidification is a major environmental issue associated with intensive agricultural land use. Rapid urbanization has inevitably caused great changes in agricultural land use around urban areas. However, the effects of agricultural land-use change and soil parent material on the pH dynamics of the whole soil profile remain poorly understood. Based on a paired soil resampling campaign in the 1980s and 2010s, this study evaluated the effects of agricultural land-use change and parent materials on the pH dynamics of the soil profile across the Chengdu Plain of China. The results showed that soil pH significantly decreased by 1.20, 0.72, 0.66 and 0.68 units at the 0-20, 20-40, 40-60 and 60-100 cm soil depths, respectively. Conversions of traditional rice-wheat/rapeseed rotations to rice-vegetable rotations and afforested land significantly increased the magnitude of pH decline at the 0-60 cm soil depth. Soils formed from Q4 grey-brown alluvium and Q4 grey alluvium, which had a lower soil bulk density (BD) and higher sand content, showed a much higher magnitude of pH decline than soils formed from Q3 (Quaternary Pleistocene) old alluvium, and significant acidification of deep soils only occurred in soils formed from Q4 (Quaternary Holocene) grey-brown alluvium and Q4 grey alluvium. These results suggested that agricultural land-use change aggravated acidification in the soil profile and the soil acidification degrees were parent material-dependent; in particular, significant acidification of deep soils was more inclined to occur in soils with lower soil BD and higher sand content due to their effects on the downward movement of acids and the penetration resistance of plant roots. More attention should be given to minimizing or preventing acidification of both topsoil and deep soils aggravated by agricultural land-use change across urban agricultural areas.