Modeling soil acidification in typical Chinese cropping systems

Impact of short-term soil disturbance on cadmium remobilization and associated risk in vulnerable regions

A comprehensive understanding of cadmium (Cd) migration in soils near contaminated hotspots is crucial for optimizing remediation efforts and ensuring crop health. This study investigates agricultural soils from four sites in mining and sewage-irrigation areas, assessing the impact of inorganic and organic fertilizer application on soil Cd remobilization. Results revealed that fertilization, particularly with mineral phosphorus, disrupts soil stability, substantially increases short-term Cd mobility in vulnerable regions. Random Forest analysis identified elevated dissolved organic matter and pH changes as key drivers of Cd remobilization. Monte Carlo simulation, integrating Michaelis-Menten reaction kinetics model, further accessed the potential risk of Cd remobilization. The model predicted that the probabilities of grain exceeding Cd thresholds ranged from 021.6 % for rice, 13.8 %100 % for wheat, and 084.2 % for maize in the absence of fertilizer use. Fertilization significantly increased these exceedance probabilities by 6.1 %87.4 %, with the highest risks observed in irrigation-contaminated soils, particularly under mineral phosphorus fertilization. Nevertheless, it recommended that while fertilization can elevate Cd remobilization risk in hotspots, remediation strategies might not always be necessary. This study highlights the potential of hybrid data-driven approaches, combining machine learning, mechanistic model and stochastic prediction to simplify the complex environmental process, allowing for integrated risk evaluations.

Insights into soil autotrophic ammonium oxidization under microplastics stress: Crossroads of nitrification, comammox, anammox and Feammox

Microplastics (MPs) are widespread in agroecosystems and profoundly impact soil microbiome and nutrient cycling. However, the effects of MPs on soil autotrophic ammonium oxidization processes, including nitrification, complete ammonium oxidation (comammox), anaerobic ammonium oxidation (anammox), and anaerobic ammonium oxidation coupled to iron reduction (Feammox), remain unclear. These processes are the rate-limiting steps of nitrogen cycling in agroecosystems. Here, our work unveiled that exposures of polyethylene (PE), polypropylene (PP), polylactic acid (PLA), and polybutylene adipate terephthalate (PBAT) MPs significantly modulated ammonium oxidization pathways with distinct type- and dose-dependent effects. Nitrification remained the main contributor (56.4-70.7 %) to soil ammonium removal, followed by comammox (11.7-25.6 %), anammox (5.0-20.2 %) and Feammox (3.3-11.6 %). Compared with conventional nonbiodegradable MPs (i.e., PE and PP), biodegradable MPs (i.e., PLA and PBAT) exhibited more pronounced impacts on soil nutrient conditions and functional microbes, which collectively induced alterations in soil ammonium oxidation. Interestingly, low-dose PLA and PBAT remarkably enhanced the roles of anammox and Feammox in soil ammonium removal, contributing to the mitigation of soil acidification in agroecosystems. This study highlights the diverse responses of ammonium oxidization pathways to MPs, further deepening our understanding of how MPs affect biogeochemical cycling and enriching strategies for agricultural managements amid increasing MPs pollution.

The contribution of natural and anthropogenic causes to soil acidification rates under different fertilization practices and site conditions in southern China

Excessive application of mineral fertilizers has accelerated soil acidification in China, affecting crop production when the pH drops below a critical value. However, the contributions of natural soil acidification, induced by leaching of bicarbonate, and anthropogenic causes of soil acidification, induced by nitrogen (N) transformations and removal of base cations over acid anions, are not well quantified. In this study, we quantified soil acidification rates, in equivalents (eq) of acidity, by assessing the inputs and outputs of all major cations and anions, including calcium, magnesium, potassium, sodium, ammonium, nitrate, bicarbonate, sulphate, phosphate and chloride, for 13 long-term experimental sites in southern China. The acidification rates strongly varied among fertilizer treatments and with the addition of animal manure. Bicarbonate leaching was the dominant acid production process in calcareous soils (23 keq ha-1 yr-1) and in non-calcareous paddy soils (9.6 keq ha-1 yr-1), accounting for 80 % and 68 % of the total acid production rate, respectively. The calcareous soils were strongly buffered, and acidification led no or a limited decline in pH. In contrast, N transformations were the most important driver for soil acidification at one site with upland crops on a non-calcareous soil, accounting for 72 % of total acid production rate of 8.4 keq ha-1 yr-1. In this soil, the soil pH considerably decreased being accompanied by a substantial decline in exchangeable base cation. Reducing the N surplus decreased the acidification rate with 10 to 54 eq per kg N surplus with the lowest value occurring in paddy soils and the highest in the upland soil. The use of manure, containing base cations, partly mitigated the acidifying impact of N fertilizer inputs and crop removal, but enhanced phosphorus (P) accumulation. Combining mineral fertilizer, manure and lime in integrative management strategies can mitigate soil acidification and minimize N and P losses.

Soil organic matter and total nitrogen as key driving factors promoting the assessment of acid-base buffering characteristics in a tea (Camellia sinensis) plantation habitat

The issue of soil acidification in tea plantations has become a critical concern due to its potential impact on tea quality and plant health. Understanding the factors contributing to soil acidification is essential for implementing effective soil management strategies in tea-growing regions. In this study, a field study was conducted to investigate the effects of tea plantations on soil acidification and the associated acid-base buffering capacity (pHBC). We assessed acidification, pHBC, nutrient concentrations, and cation contents in the top 0-20 cm layer of soil across forty tea gardens of varying stand ages (0-5, 5-10, 10-20, and 20-40 years old) in Anji County, Zhejiang Province, China. The results revealed evident soil acidification due to tea plantation activities, with the lowest soil pH observed in tea gardens aged 10-20 and 20-40 years. Higher levels of soil organic matter (SOM), total nitrogen (TN), Olsen phosphorus (Olsen-P), available iron (Fe), and exchangeable hydrogen (H+) were notably recorded in 10-20 and 20-40 years old tea garden soils, suggesting an increased risk of soil acidification with prolonged tea cultivation. Furthermore, prolonged tea cultivation correlated with increased pHBC, which amplified with tea stand ages. The investigation of the relationship between soil pHBC and various parameters highlighted significant influences from soil pH, SOM, cation exchange capacity, TN, available potassium, Olsen-P, exchangeable acids (including H+ and aluminum), available Fe, and available zinc. Consequently, these findings underscore a substantial risk of soil acidification in tea gardens within the monitored region, with SOM and TN content being key driving factors influencing pHBC.

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.

Acidification of European croplands by nitrogen fertilization: Consequences for carbonate losses, and soil health

Soil acidification is an ongoing problem in intensively cultivated croplands due to inefficient and excessive nitrogen (N) fertilization. We collected high-resolution data comprising 19,969 topsoil (0-20 cm) samples from the Land Use and Coverage Area frame Survey (LUCAS) of the European commission in 2009 to assess the impact of N fertilization on buffering substances such as carbonates and base cations. We have only considered the impacts of mineral fertilizers from the total added N, and a N use efficiency of 60 %. Nitrogen fertilization adds annually 6.1 × 107 kmol H+ to European croplands, leading to annual loss of 6.1 × 109 kg CaCO3. Assuming similar acidification during the next 50 years, soil carbonates will be completely removed from 3.4 × 106 ha of European croplands. In carbonate-free soils, annual loss of 2.1 × 107 kmol of basic cations will lead to strong acidification of at least 2.6 million ha of European croplands within the next 50 years. Inorganic carbon and basic cation losses at such rapid scale tremendously drop the nutrient status and production potential of croplands. Soil liming to ameliorate acidity increases pH only temporarily and with additional financial and environmental costs. Only the direct loss of soil carbonate stocks and compensation of carbonate-related CO2 correspond to about 1.5 % of the proposed budget of the European commission for 2023. Thus, controlling and decreasing soil acidification is crucial to avoid degradation of agricultural soils, which can be done by adopting best management practices and increasing nutrient use efficiency. Regular screening or monitoring of carbonate and base cations contents, especially for soils, where the carbonate stocks are at critical levels, are urgently necessary.

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.

Physico-chemical assessment of on-farm bioconversion of organic waste in dairy farms in context to sustainability and circular bioeconomy

On a milk-producing dairy farm, milk production is correlated with manure production and the number of cattle, and manure is widely used as a soil fertilizer. However, excessive dairy manure production is linked with greenhouse gas emissions and water quality issues. On-farm planning of manure storage and application to enhance soil nutrients are essential in a circular economy to reduce environmental impact, where manure is not landfilled and incinerated. Instead, it creates a nutrient resource for crops and soil. Dairy manure, which is rich in nutrients, is a valuable fertilizer that contains many nutrients such as nitrogen (N), organic matter (OM), phosphorous (P), Potassium (K) and micronutrients. In this work, a pilot field research was conducted between 2016 and 2018 in various parts of California, USA (San Joaquin Valley, Sacramento Valley, Shasta Cascade, and the North Coast of California) to assess physio-chemical characteristics of solid fractions of dairy manure among various dairy farms. A total of 156 samples were collected from the gut (n = 107) and toe (n = 49) of the manure piles across California for determining total solid (TS), volatile solid (VS), temperature, moisture content and carbon-nitrogen ratio (C: N). Here, using the observations of field study and analysis, we show that C: N, OM and MC of solid fractions of dairy manure vary significantly among dairy farms. The average C: N ratio of manure (26-32) among various regions was close to an ideal C: N value of 24:1 for soil microbes to stimulate nutrient release to crops. Manure pH ranged between 7.0 and 8.0, which was close to an optimal pH range for common crops (6.0-8.0). Moreover, considering less cost and surplus availability, manure will likely continue providing a cost-effective organic fertilizer resource compared to commercial chemical fertilizers.

Revolutionizing agriculture: Harnessing nano-innovations for sustainable farming and environmental preservation

The agricultural sector is currently confronted with a significant crisis stemming from the rapid changes in climate patterns, declining soil fertility, insufficient availability of essential macro and micronutrients, excessive reliance on chemical fertilizers and pesticides, and the presence of heavy metals in soil. These numerous challenges pose a considerable threat to the agriculture industry. Furthermore, the exponential growth of the global population has led to a substantial increase in food consumption, further straining agricultural systems worldwide. Nanotechnology holds great promise in revolutionizing the food and agriculture industry, decreasing the harmful effects of agricultural practices on the environment, and improving productivity. Nanomaterials such as inorganic, lipid, and polymeric nanoparticles have been developed for increasing productivity due to their unique properties. Various strategies can enhance product quality, such as the use of nano-clays, nano zeolites, and hydrogel-based materials to regulate water absorption and release, effectively mitigating water scarcity. The production of nanoparticles can be achieved through various methods, each of which has its own unique benefits and limitations. Among these methods, chemical synthesis is widely favored due to the impact that various factors such as concentration, particle size, and shape have on product quality and efficiency. This review provides a detailed examination of the roles of nanotechnology and nanoparticles in sustainable agriculture, including their synthetic methods, and presents an analysis of their associated advantages and disadvantages. To date, there are serious concerns and awareness about healthy agriculture and the production of healthy products, therefore the development of nanotech-enabled devices that act as preventive and early warning systems to identify health issues, offering remedial measures is necessary.

Fine mapping of QTL conferring resistance to calcareous soil in mungbean reveals VrYSL3 as candidate gene for the resistance

Iron is a crucial nutrient for biological functions in plants. High-pH and calcareous soil is a major stress causing iron deficiency chlorosis (IDC) symptoms and yield losses in crops. Use of calcareous soil-tolerance genetic resources is the most effective preventative method to combat the effects of high-pH and calcareous soils. A previous study using a mungbean recombinant inbred line (RIL) population of the cross Kamphaeg Saen 2 (KPS2; IDC susceptible) × NM-10-12 identified a major quantitative trait locus (QTL), qIDC3.1, which controls resistance and explains more than 40% of IDC variation. In this study, we fine-mapped qIDC3.1 and identified an underlying candidate gene. A genome wide association analysis (GWAS) using 162 mungbean accessions identified single nucleotide polymorphisms (SNPs) on chromosome 6; several SNPs were associated with soil plant analysis development (SPAD) values and IDC visual scores of mungbeans planted on calcareous soil, respectively. These SNPs corresponded to qIDC3.1. Using the same RIL population as in the previous study and an advanced backcross population developed from KPS2 and IDC-resistant inbred line RIL82, qIDC3.1 was further confirmed and fine-mapped to an interval of 217 kilobases harboring five predicted genes, including LOC106764181 (VrYSL3), which encodes a yellow stripe1-like-3 (YSL3) protein, YSL3 is involved in iron deficiency resistance. Gene expression analysis revealed that VrYSL3 was highly expressed in mungbean roots. In calcareous soil, expression of VrYSL3 was significantly up-regulated, and it was more obviously upregulated in the roots of RIL82, than in those of KPS2. Sequence comparison of VrYSL3 between the RIL82 and KPS2 revealed four SNPs that result in amino acid changes in the VrYSL3 protein and a 20-bp insertion/deletion in the promoter where a cis-regulatory element resides. Transgenic Arabidopsis thaliana plants overexpressing VrYSL3 showed enhanced iron and zinc contents in the leaves. Taken together, these results indicate that VrYSL3 is a strong candidate gene responsible for calcareous soil resistance in mungbean.

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.

Combination of Aspergillus niger MJ1 with Pseudomonas stutzeri DSM4166 or mutant Pseudomonas fluorescens CHA0-nif improved crop quality, soil properties, and microbial communities in barrier soil

Soil salinization and acidification seriously damage soil health and restricts the sustainable development of planting. Excessive application of chemical fertilizer and other reasons will lead to soil acidification and salinization. This study focus on acid and salinized soil, investigated the effect of phosphate-solubilizing bacteria, Aspergillus niger MJ1 combined with nitrogen-fixing bacteria Pseudomonas stutzeri DSM4166 or mutant Pseudomonas fluorescens CHA0-nif on crop quality, soil physicochemical properties, and microbial communities. A total of 5 treatments were set: regular fertilization (T1), regular fertilization with MJ1 and DSM4166 (T2), regular fertilization with MJ1 and CHA0-nif (T3), 30%-reducing fertilization with MJ1 and DSM4166 (T4), and 30%-reducing fertilization with MJ1 and CHA0-nif (T5). It was found that the soil properties (OM, HN, TN, AP, AK, and SS) and crop quality of cucumber (yield production, protein, and vitamin C) and lettuce (yield production, vitamin C, nitrate, soluble protein, and crude fiber) showed a significant response to the inoculated strains. The combination of MJ1 with DSM4166 or CHA0-nif influenced the diversity and richness of bacterial community in the lettuce-grown soil. The organismal system-, cellular process-, and metabolism-correlated bacteria and saprophytic fungi were enriched, which were speculated to mediate the response to inoculated strains. pH, OM, HN, and TN were identified to be the major factors correlated with the soil microbial community. The inoculation of MJ1 with DSM4166 and CHA0-nif could meet the requirement of lettuce and cucumber growth after reducing fertilization in acid and salinized soil, which provides a novel candidate for the eco-friendly technique to meet the carbon-neutral topic.

Sick or rich: Assessing the selected soil properties and fertility status across the tea-growing region of Dooars, West Bengal, India

Harnessing the potential yields of evergreen perennial crops like tea (Camellia sinensis L.) essentially requires the application of optimum doses of nutrients based on the soil test reports. In the present study, the soil pH, organic carbon (OC), available potassium as K₂O (AK), and available sulphur (AS) of 7300 soil samples from 115 tea estates spread over the Dooars ranging from 88°52'E to 89°86'E longitude and 26°45'N to 27°00'N latitude of West Bengal, India have been documented. About 54% of soil samples were found within the optimum range of soil pH (4.50-5.50) for tea cultivation. The overall range of OC was found from 0.28% to 6.00% of which, 94% of the analyzed samples were within the range of satisfactory to excellent level of OC i.e. (>0.80% to 6.00%). Around 36.3% of soil samples were found to have high AK (>100 mg kg^-1) but 37.1% of soils were found to have high AS content (>40 mg kg^-1). The nutrient index status of soil pH was low in Dam Dim, Chulsa, Nagrakata, Binnaguri, and Jainti sub-districts. Soils from five sub-districts had a high nutrient index (2.47 to 2.83) for soil organic carbon. However, it existed in the medium index (1.69 and 2.22) for Dalgaon and Kalchini sub-districts. Only Nagrakata sub-district soil samples were in the high nutrient index (2.65) for AK. All analyzed samples showed a medium nutrient index (1.97 to 2.27) for AS. The result indicated that soil pH was significantly negatively correlated with soil OC (-0.336) and AK (-0.174). However, the soil OC was significantly positive correlated with AK (0.258) and AS (0.100). It could be concluded that a balanced fertilizer application would be needed as a part of the soil improvement program through soil chemical tests for sustainable tea cultivation.

Higher pH is associated with enhanced co-occurrence network complexity, stability and nutrient cycling functions in the rice rhizosphere microbiome

The rice rhizosphere microbiota is crucial for crop yields and nutrient use efficiency. However, little is known about how co-occurrence patterns, keystone taxa and functional gene assemblages relate to soil pH in the rice rhizosphere soils. Using shotgun metagenome analysis, the rice rhizosphere microbiome was investigated across 28 rice fields in east-central China. At higher pH sites, the taxonomic co-occurrence network of rhizosphere soils was more complex and compact, as defined by higher average degree, graph density and complexity. Network stability was greatest at medium pH (6.5 < pH < 7.5), followed by high pH (7.5 < pH). Keystone taxa were more abundant at higher pH and correlated significantly with key ecosystem functions. Overall functional genes involved in C, N, P and S cycling were at a higher relative abundance in higher pH rhizosphere soils, excepting C degradation genes (e.g. key genes involved in starch, cellulose, chitin and lignin degradation). Our results suggest that the rice rhizosphere soil microbial network is more complex and stable at higher pH, possibly indicating increased efficiency of nutrient cycling. These observations may indicate routes towards more efficient soil management and understanding of the potential effects of soil acidification on the rice rhizosphere system.

Growth status and physiological changes of sugar beet seedlings in response to acidic pH environments

Sugar beet (Beta vulgaris L.) is an important sugar crop that is popularly cultivated in a variety of agriculture conditions. Here, we studied sugar beet growth in different pH soils (pH 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0) and analyzed their growth status and physiology. Sugar beet growth was best at pH 9.0 and worst at pH 5.0. As the soil pH decreased from 9.0 to 5.0, the osmoregulatory substances, antioxidant enzyme activity, and elemental contents in leaves and roots showed increasing trends, while photosynthesis and macronutrient contents showed decreasing trends. To explore the physiological mechanisms sugar beet use to respond to different pH environments, we analyzed the correlations between leaf net photosynthesis rate and physiological changes and nutrient contents of sugar beet. One of the factors inhibiting sugar beet growth in low pH soils was a reduction in photosynthetic capacity. The accumulation of osmoregulatory substances and increased peroxidative damage may have led to the decrease in leaf net photosynthesis rate. Furthermore, the decrease in nutrient content and accumulation of metal elements were correlated with the decrease in leaf photosynthetic rate. QRT-PCR analysis showed higher expression levels of antioxidant enzyme genes in the leaves and roots of sugar beet grown in low pH environments compared to those in high pH environments. Correspondingly, antioxidant enzyme activity was significantly higher in beets in low pH environments than in beets in high pH environments. These results provide important insight into the physiological responses by which sugar beet can adapt to different pH soils.

Expression levels of genes involved in metal homeostasis, physiological adaptation, and growth characteristics of rice (Oryza sativa L.) genotypes under Fe and/or Al toxicity

Acid sulphate soil contains high amounts of iron (Fe) and aluminum (Al), and their contamination has been reported as major problems, especially in rainfed and irrigated lowland paddy fields. Rice is sensitive to Fe and Al grown in acid soil (pH < 5.5), leading to growth inhibition and grain yield loss. The objective of this study was to evaluate Fe and/or Al uptake, translocation, physiological adaptation, metal toxicity, and growth inhibition in rice genotypes grown in acid soil. Fe and Al in the root tissues of all rice genotypes were enriched depending on the exogenous application of either Fe or Al in the soil solution, leading to root growth inhibition, especially in the KDML105 genotype. Expression level of OsYSL1 in KDML105 was increased in relation to metal uptake into root tissues, whereas OsVIT2 was downregulated, leading to Fe (50.3 mg g^-1 DW or 13.1 folds over the control) and Al (4.8 mg g^-1 DW or 2.2 folds over the control) translocation to leaf tissues. Consequently, leaf greenness (SPAD), net photosynthetic rate (P_n), stomatal conductance (g_s), and transpiration rate (E) in the leaf tissues of genotype KDML105 under Fe + Al toxicity significantly declined by 28.4%, 35.3%, 55.6%, and 51.6% over the control, respectively. In Azucena (AZU; Fe/Al tolerant), there was a rapid uptake of Fe and Al by OsYSL1 expression in the root tissues, but a limited secretion into vacuole organelles by OsVIT2, leading to a maintenance of low level of toxicity driven by an enhanced accumulation of glutathione together with downregulation of OsGR expression level. In addition, Fe and Al restrictions in the root tissues of genotype RD35 were evident; therefore, crop stress index (CSI) of Fe + Al-treated plants was the maximum, leading to an inhibition of g_s (53.6% over the control) and E (49.0% over the control). Consequently, free proline, total phenolic compounds, and ascorbic acid in the leaf tissues of rice under Fe + Al toxicity significantly increased by 3.2, 1.2, and 1.5 folds over the control, respectively, indicating their functions in non-enzymatic antioxidant defense. Moreover, physiological parameters including leaf temperature (T_leaf) increment, high level of CSI (>0.6), SPAD reduction, photon yield of PSII (Φ_PSII) diminution, P_n, g_s, and E inhibition in rice genotype IR64 (Fe/Al-sensitive) under Fe + Al treatment were clearly demonstrated as good indicators of metal-induced toxicity. Our results on Fe- and/or Al-tolerant screening to find out the candidate genotypes will contribute to present screening and breeding efforts, which in turn help increase rice production in the Fe/Al-contaminated acid soil under lowland conditions.

Physiological Essence of Magnesium in Plants and Its Widespread Deficiency in the Farming System of China

Magnesium (Mg) is an essential nutrient for a wide array of fundamental physiological and biochemical processes in plants. It largely involves chlorophyll synthesis, production, transportation, and utilization of photoassimilates, enzyme activation, and protein synthesis. As a multifaceted result of the introduction of high-yielding fertilizer-responsive cultivars, intensive cropping without replenishment of Mg, soil acidification, and exchangeable Mg (Ex-Mg) leaching, Mg has become a limiting nutrient for optimum crop production. However, little literature is available to better understand distinct responses of plants to Mg deficiency, the geographical distribution of soil Ex-Mg, and the degree of Mg deficiency. Here, we summarize the current state of knowledge of key plant responses to Mg availability and, as far as possible, highlight spatial Mg distribution and the magnitude of Mg deficiency in different cultivated regions of the world with a special focus on China. In particular, ~55% of arable lands in China are revealed Mg-deficient (< 120 mg kg-1 soil Ex-Mg), and Mg deficiency literally becomes increasingly severe from northern (227-488 mg kg-1) to southern (32-89 mg kg-1) China. Mg deficiency primarily traced back to higher depletion of soil Ex-Mg by fruits, vegetables, sugarcane, tubers, tea, and tobacco cultivated in tropical and subtropical climate zones. Further, each unit decline in soil pH from neutral reduced ~2-fold soil Ex-Mg. This article underscores the physiological importance of Mg, potential risks associated with Mg deficiency, and accordingly, to optimize fertilization strategies for higher crop productivity and better quality.

Soil chemical quality assessment and spatial distribution of pomelo orchards in acidic red soil hilly regions of China

BACKGROUND

Soil quality assessment is a critical strategy for determining optimum fertilization in intensive pomelo production. In this study, we evaluated the soil quality status and mapped the spatial distribution of 347 soil samples collected from pomelo orchards in Pinghe County, southern China. We analyzed nine chemical parameters and an altitude indicator.

RESULTS

The mean soil quality index (SQI) was 0.355 in the total data set (TDS) and 0.292 in the minimum data set (MDS). Available Ca (Avail-Ca), pH value, organic matter and altitude were selected as indicators of soil quality in the MDS. The SQI in mature orchards (>10 years) was higher than that in young orchards (<10 years), while no differences between soil types and altitude gradients were identified. We detected a significant positive correlation between the SQI based on TDS (SQI_TDS ) and the SQI based on MDS (SQI_MDS ), and the spatial distribution of soil properties and SQI_TDS showed a uniform pattern, except for Avail-N, Avail-B and SQI_MDS . Overall, unfavorable soil quality indicators, including rich in Avail-P, deficient in Avail-Ca, -Mg and -B, soil acidification and high altitude, were considered to be limiting factors for pomelo production.

CONCLUSION

The soil chemical quality in pomelo orchards is generally low, indicating that integrated management by controlling acidification, reducing planting altitude, regulating fertilization and monitoring soil properties is required for sustainable pomelo production. © 2021 Society of Chemical Industry.

Spatiotemporal Patterns of Cultivated Land Quality Integrated with Multi-Source Remote Sensing: A Case Study of Guangzhou, China

Scientifically revealing the spatiotemporal patterns of cultivated land quality (CLQ) is crucial for increasing food production and achieving United Nations Sustainable Development Goal (SDG) 2: Zero Hunger. Although studies on the evaluation of CLQ have been conducted, an effective evaluation system that is suitable for the macro-regional scale has not yet been developed. In this study, we first defined the CLQ from four aspects: soil fertility, natural conditions, construction level, and cultivated land productivity. Then, eight indicators were selected by integrating multi-source remote sensing data to create a new CLQ evaluation system. We assessed the spatiotemporal patterns of CLQ in Guangzhou, China, from 2010 to 2018. In addition, we identified the main factors affecting the improvement of CLQ. The results showed that the CLQ continuously improved in Guangzhou from 2010 to 2018. The area of high-quality cultivated land increased by 13.7%, which was mainly distributed in the traditional agricultural areas in the northern and eastern regions of Guangzhou. The areas of medium- and low-quality cultivated land decreased by 8.1% and 5.6%, respectively, which were scattered throughout the whole study area. The soil fertility and high productivity capacity were the main obstacle factors that affected the improvement of CLQ. Simultaneously, the obstacle degree of stable productivity capacity gradually increased during the study period. Therefore, the targeted improvement measures could be put forward by applying biofertilizers, strengthening crop management and constructing well-facilitated farmland. The new CLQ evaluation system we proposed is particularly practical at the macro-regional scale, and the results provided targeted guidance for decision makers to improve CLQ and promote food security.

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.

Root-Associated Microbiota Response to Ecological Factors: Role of Soil Acidity in Enhancing Citrus Tolerance to Huanglongbing

The citrus orchards in southern China are widely threatened by low soil pH and Huanglongbing (HLB) prevalence. Notably, the lime application has been used to optimize soil pH, which is propitious to maintain root health and enhance HLB tolerance of citrus; however, little is known about the interactive effects of soil acidity on the soil properties and root-associated (rhizoplane and endosphere) microbial community of HLB-infected citrus orchard. In this study, the differences in microbial community structures and functions between the acidified and amended soils in the Gannan citrus orchard were investigated, which may represent the response of the host-associated microbiome in diseased roots and rhizoplane to dynamic soil acidity. Our findings demonstrated that the severity of soil acidification and aluminum toxicity was mitigated after soil improvement, accompanied by the increase in root activity and the decrease of HLB pathogen concentration in citrus roots. Additionally, the Illumina sequencing-based community analysis showed that the application of soil amendment enriched functional categories involved in host-microbe interactions and nitrogen and sulfur metabolisms in the HLB-infected citrus rhizoplane; and it also strongly altered root endophytic microbial community diversity and structure, which represented by the enrichment of beneficial microorganisms in diseased roots. These changes in rhizoplane-enriched functional properties and microbial composition may subsequently benefit the plant's health and tolerance to HLB disease. Overall, this study advances our understanding of the important role of root-associated microbiota changes and ecological factors, such as soil acidity, in delaying and alleviating HLB disease.

Zai Technology and Integrated Nutrient Management for Improved Soil Fertility and Increased Sorghum Yields in Kitui County, Kenya

Deteriorating soil fertility, low unreliable rainfall and soil moisture stress has resulted to low crop yields among farmers of sub-Saharan Africa (SSA), necessitating a search for more sustainable production practices. Zai technology has the ability to promote soil moisture retention and enhances soil fertility. A four-seasons field experiment was conducted to assess the impact of Zai technology combined with cattle manure and inorganic fertilizer on selected soil properties and sorghum yields in Kabati, Kitui County. The experiment was set up in a Randomized Complete Block Design (RCBD) with eight treatments replicated thrice with sorghum Gadam as the test crop. Soil sampling was done at the beginning of the first season and at the end of the fourth season at a dept of 0–15 cm across each plot for laboratory analyses. From the results, the increase in electrical conductivity was significant at p &lt; 0.05 in all the treatments after four cropping seasons. Total organic carbon significantly increased in Zai with cattle manure (p = 0.045), conventional with no input (p = 0.038) and conventional with cattle manure (p = 0.045). Available phosphorous significantly (p &lt; 0.05) increased in treatments under Zai technology while total nitrogen significantly (p &lt; 0.05) reduced after the four cropping seasons. There was a significant (p &lt; 0.05) interactive effect of the tested factors on soil pH, electrical conductivity, total nitrogen, and available phosphorous at the end of the experiment. Moreover, there was significant (p &lt; 0.05) interactive effects on grain yields (SR18 and SR19 seasons) and stover yields (SR18, LR19, and SR19 seasons), with higher yields being recorded in treatments under Zai technology. This study demonstrates the importance of Zai technology in increasing crop yield by trapping water and enhancing its retention and infiltration into the soil for uptake by plants. This study concluded that positive impacts on important soil properties and crop yield could be realized when Zai technology is utilized alongside either sole inorganics or a combination of organic and inorganic amendments and this could be used as a strategy to improve crop production in eastern Kenya and other similar areas.

Effect of Different Rates of Nitrogen Fertilization on Crop Yield, Soil Properties and Leaf Physiological Attributes in Banana Under Subtropical Regions of China

Excessive nitrogen (N) application is widespread in Southern China. The effects of N fertilization on soil properties and crop physiology are poorly understood in tropical red loam soil. We conducted a field experiment to evaluate the effect of nitrogen fertilization rates on physiological attributes (chlorophyll, plant metabolic enzymes, soluble matters) on banana leaves, soil properties (soil enzymes, soil organic matter (SOM), soil available nutrients) as well as banana crop yield in a subtropical region of southern China. The N rates tested were 0 (N₀), 145 (N₁₄₅), 248 (N₂₄₈), 352 (N₃₅₂), 414 (N_FT), and 455 (N₄₅₅) g N per plant. The correlations among soil factors, leaf physiological factors and crop yield were evaluated. The results indiated that the high rates of N fertilization (N_FT and N₄₅₅) significantly decreased soil available potassium (K) content, available phosphorus (P) content, glutamine synthetase (GS) activity, and soluble protein and sugar contents compared with lower N rates. The N₃₅₂ treatment had the highest crop yields compared with higher N rates treatments, followed by the N₄₅₅ treatment. However, there were no significant differences in crop yields among N fertilization treatments. Factor analysis showed that the N₃₅₂ treatment had the highest integrated score for soil and leaf physiological factors among all treatments. Moreover, the N₃₅₂ treatment was the most effective in improving carbon and nitrogen metabolism in banana. Crop yield was significantly and positively linearly correlated with the integrated score (r = 0.823, p < 0.05). Path analysis revealed that invertase, SOM and sucrose synthase (SS) had a strong positive effect on banana yield. Canonical correspondence analysis (CCA) suggested that available K, invertase, acid phosphatase and available P were the most important factors impacting leaf physiological attributes. Cluster analysis demonstrated distinct differences in N application treatment related to variations in soil and leaf factors. This study suggested that excessive N fertilization had a negative effect on soil fertility, crop physiology and yield. The lower N rates were more effective in improving crop yield than higher rates of N fertilization. The N rate of 352 g N per plant (N₃₅₂) was recommended to reduce excess N input while maintaining the higher yield for local farmers' banana planting.

Environmental impacts of nitrogen emissions in China and the role of policies in emission reduction

Atmospheric reactive nitrogen (N_r) has been a cause of serious environmental pollution in China. Historically, China used too little N_r in its agriculture to feed its population. However, with the rapid increase in N fertilizer use for food production and fossil fuel consumption for energy supply over the last four decades, increasing gaseous N_r species (e.g. NH₃ and NO_x) have been emitted to the atmosphere and then deposited as wet and dry deposition, with adverse impacts on air, water and soil quality as well as plant biodiversity and human health. This paper reviews the issues associated with this in a holistic way. The emissions, deposition, impacts, actions and regulations for the mitigation of atmospheric N_r are discussed systematically. Both NH₃ and NO_x make major contributions to environmental pollution but especially to the formation of secondary fine particulate matter (PM_2.5), which impacts human health and light scattering (haze). In addition, atmospheric deposition of NH₃ and NO_x causes adverse impacts on terrestrial and aquatic ecosystems due to acidification and eutrophication. Regulations and practices introduced by China that meet the urgent need to reduce N_r emissions are explained and resulting effects on emissions are discussed. Recommendations for improving future N management for achieving 'win-win' outcomes for Chinese agricultural production and food supply, and human and environmental health, are described. This article is part of a discussion meeting issue 'Air quality, past present and future'.

Incentive mechanism to promote corn stalk return sustainably in Henan, China

Corn stalk return (CSR) can manage agricultural residues on the spot to avoid field open burning and protect the environment. However, the implementation of this measure encounters reluctance from farmers which hinders its sustainability. This study combined the economic (cost) and technical (return amount, crushing quality, and decomposition of corn stalk) aspects to examine the factors affecting farmers' willingness to participate in the CSR by using a logistic regression model. The level of willingness to accept (WTA) compensation and its determinants were analyzed by using a tobit model. Based on the survey of 925 farmers, this study found the likelihood of farmers' participation in CSR will be decreased when CSR has high machinery cost, an excessive amount of stalk, poor quality of crushing, and slow decomposing rate. The farmers' WTA for CSR was estimated at about 711 Chinese Yuan (RMB) per ha annually, much higher than the current compensation level of 75-225 RMB per ha in Henan. Farmers were willing to be compensated more because of the high cost and slow decomposing rate. The issues in economic and technical sides should attract more attention, and the compensation should be increased and the technical problems should be solved to stimulate farmers' willingness of CSR. By providing a fuller understanding of farmers' CSR behavior, this study can serve as a reference for the Chinese government to develop and implement better policies.

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.

Using organic fertilizers to increase crop yield, economic growth, and soil quality in a temperate farmland

We used a constant total N application base rate to conduct a two-year field experiment comparing the effects of three organic fertilizers (rapeseed meal (RSM), soybean meal (SBM), and cattle manure (CM)) on the crop yield, economic growth, and soil quality of a winter wheat-summer maize rotation system. Winter wheat and summer maize in rapeseed meal treatment (RSMT), soybean meal treatment (SBMT), and cattle manure treatment (CMT) showed yield increases of 161%, 299%, and 256%, respectively, when compared to no organic fertilizer treatment (CK) (P < 0.05). The annual net incomes of SBMT and CMT were 1.46 and 1.42 times higher, respectively, than RSMT. Compared to the results of the CK group, RSM, SBM, and CM stimulated the soil physically, chemically, and biologically. We found the highest soil macroaggregate proportions, soil organic matter (SOM) levels, total N (TN) levels, and phospholipid fatty acid (PLFA) levels in SBMT. The highest soil pH, microbial biomass carbon (MBC) levels, and microbial biomass nitrogen (MBN) levels were observed in CMT. We used a soil quality index (SQI) to evaluate soil quality. After the two-year fertilization treatments, we calculated the SQI using a minimum data set (MDS). We used SOM levels and actinomycete quantity for the MDS properties. The SQI values were significantly different across the four treatments, with the highest values occurring in SBMT, then CMT and RSMT. In conclusion, SBM and CM were more effective than RSM at maintaining crop yield, economic growth, and soil quality.

Biochar retards Al toxicity to maize (Zea mays L.) during soil acidification: The effects and mechanisms

Biochar can effectively alleviate the Al phytotoxicity in acidic soils due to its alkaline nature. However, the longevity of this alleviation effect of biochar under re-acidification conditions is still unclear. In the present study, the maize root growth responding to the simulated re-acidification of two acidic soils amended by peanut straw biochar or Ca(OH)₂ was investigated to evaluate the long-term effect of biochar on alleviating Al toxicity in acidic soils. Compared with Ca(OH)₂ amendment, the application of biochar significantly retarded Al toxicity to plant during soil re-acidification. When 4.0 mM HNO₃ was added, the maize seedling root elongation in an Oxisol with biochar was 99% higher than that in the Oxisol with Ca(OH)₂. Also, the Evans blue uptake and Al content in the root tip in the biochar treatment were 60% and 51% lower than those in the Ca(OH)₂ treatment. The retarding effect was mainly attributed to the slow decrease in soil pH during acidification and the release of dissolved organic carbon (DOC) in the soils amended by biochar. The slower decrease in soil pH resulting from the increased pH buffering capacity after biochar application inhibited the increase of soluble and exchangeable Al during re-acidification. The increased DOC after biochar application decreased the toxic soluble Al speciation at the same pH value and total Al concentration in soil solution. Therefore, given the re-acidification of soils, biochar presented a longer-term effect on alleviating Al toxicity of acidic soil than liming.

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.

Assessing Soil Acidification of Croplands in the Poyang Lake Basin of China from 2012 to 2018

Soil acidification, caused by intensified fertilizer application and acid deposition, has threatened the sustainability of agricultural ecosystems and soil quality in parts of China since the 1980s. However, little is known about the spatio-temporal change of soil pH in cropland at a large basin scale. Poyang Lake Basin of China was selected as the study area to identify the spatio-temporal change of cropland pH and detect potential soil acidification factors. A total of 507 and 503 topsoil samples were collected in 2012 and 2018, respectively, and methods including one-way analysis of variance (ANOVA), Pearson’s correlation analyses, and Inverse Distance Weighted (IDW) were applied. Results showed that soil pH ranged from 3.96 to 7.95 in 2012 and from 3.34 to 8.19 in 2018, with most samples being acidic (pH < 7) in both sets of data. The two soil datasets showed a significant decline (p < 0.05) of 0.1 pH units over the past six years and several soil samples that exhibited obvious uptrends in the groups of pH < 4.5 and 4.5–5.0 from 2012 to 2018. Overall, the distribution patterns of pH at the two sampling dates were similar, whereas local details of the pH spatial distribution patterns differed. While we found a significant correlation (p < 0.05) between soil pH and aspect, elevation and slope showed no significant correlation with pH. ANOVA showed that pH values in the water density (river or lake network density) range of 6.27–19.94 were significantly higher (p < 0.05) than the other water densities. Large amounts of precipitation with low pH values were found to significantly influence soil pH, whereas N-fertilizer inputs exerted limited effects on soil pH over the entire study area. These findings provided new insights on soil acidification assessment and potential factor detection at the basin scale.

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.

Cropland acidification increases risk of yield losses and food insecurity in China

Distinct cropland acidification has been reported in China due to nitrogen (N) fertilizer overuse. However, the impacts on food production and thereby on food security are largely unknown. Yield losses in the period 1980-2050 were therefore assessed by simulating soil pH changes combined with derived pH-yield relationships for wheat, maize and rice. If the N fertilizer input continues to increase at 1% annually, the predicted average soil pH decline is about one unit and relative yield losses are expected to increase from approximately 4%-24% during 2010-2050. If the N fertilizer increase stops in 2020 (N2020), the expected losses are approximately 16% in 2050, which is comparable to a scenario of 100% crop residue return (100%RR). However, if 30% of the N fertilizer is replaced by manure N (30%MR), the losses reduce to near 5% in 2050. Soil acidification was predicted to reverse and expected losses are only 2.5% in 2050 in a combined scenario of N2020, 100%RR and 30%MR. Our results illustrate the potential food insecurity induced by cropland acidification and address the necessity of mitigation.

Beneficial dual role of biochars in inhibiting soil acidification resulting from nitrification

The dual role of biochar for inhibiting soil acidification induced by nitrification was determined through two-step incubation experiments in this study. Ca(OH)₂ or biochar was added respectively to adjust soil pH to the same values (pH 5.15 and 5.85), and then the amended soils were incubated in the presence of urea for 70 days. The results showed that compared with Ca(OH)₂ treatment, both rice straw biochar and peanut straw biochar inhibited the decrease in soil pH and the increase in exchangeable acidity during the incubation. The application of biochars suppressed soil nitrification during the incubation, and thus reduced 7.5 mmol kg^-1 and 1.4 mmol kg^-1 protons released from nitrification compared to Ca(OH)₂ treatments. Compared with Ca(OH)₂ treatment, the ammonia-oxidizing bacteria population size was decreased by 8% and 12% in rice straw biochar and peanut straw biochar treatments respectively, which was the main responsibility for the inhibited nitrification after biochar application. In addition, the application of rice straw biochar and peanut straw biochar increased soil pH buffering capacity (pHBC) respectively by 22% and 32%. The increased pHBC played the main role (75%) in inhibiting the acidification of the soil amended with peanut straw biochar, while the rice straw biochar inhibited soil acidification mainly through suppressing nitrification during the incubation. Overall, compared with lime application, biochars can inhibit soil acidification caused by urea application through suppressing the nitrification process and improving the resistance of soils to acidification. The crop residue biochars presented a longer-lasting effect on ameliorating acidic soils than mineral lime.

Transcriptome sequencing reveals the effect of biochar improvement on the development of tobacco plants before and after topping

The application of biochar is one of the most useful methods for improving soil quality, which is of the utmost significance for the continuous production of crops. As there are no conclusive studies on the specific effects of biochar application on tobacco quality, this study aimed to improve the yield and quality of tobacco as a model crop for economic and genetic research in southern China, by such application. We used transcriptome sequencing to reveal the effects of applied biochar on tobacco development before and after topping. Our results showed that topping affected carbon and nitrogen metabolism, photosynthesis and secondary metabolism in the tobacco plants, while straw biochar-application to the soil resulted in amino acid and lipid synthesis; additionally, it affected secondary metabolism of the tobacco plants through carbon restoration and hormonal action, before and after topping. In addition to the new insights into the impact of biochar on crops, our findings provide a basis for biochar application measures in tobacco and other crops.

Effect of hydrothermal carbonization on heavy metals in swine manure: Speciation, bioavailability and environmental risk

The speciation, bioavailability and environmental risk of heavy metals (HMs) in the hydrochar derived from hydrothermal carbonization (HTC) of swine manure were investigated in this study. The results indicated that the majority of HMs originally in swine manure were retained in the hydrochar by HTC, and CaO addition substantially reduced the HMs concentration in the hydrochar. HTC especially CaO assisted HTC significantly promoted the HMs transformation from the bioavailable fraction to the relatively stable fraction, and thus decreased their environmental risk in the hydrochar. Moreover, the Toxicity Characteristic Leaching Procedure and diethylenetriamine pentaacetic acid test revealed that the leachability and plant-bioavailability of HMs in swine manure were greatly declined by HTC especially for HTC with 15% CaO addition. The present study suggested that HTC was an effective disposal approach for swine manure from the perspective of HMs immobilization, especially for CaO assisted HTC.

Properties of hydrochars derived from swine manure by CaO assisted hydrothermal carbonization

The hydrochars derived from swine manure were prepared by CaO assisted hydrothermal carbonization (HTC), and their properties were investigated for the first time. The results showed that the pH and yield of the hydrochar were largely increased by CaO addition. HTC of swine manure increased the phosphorus (P) content in the hydrochar, and appropriately 100% of P as apatite-P was enriched in the hydrochar by CaO assisted HTC. Additionally, the CaO addition during HTC improved the porosity of the hydrochar. The FTIR analysis revealed that substantial functional groups were present on the surface of the hydrochar, indicating the facilitated exchange between the hydrochar and hydrophilic soil when the hydrochar was used for soil amendment. This study demonstrated that CaO assisted HTC was a novel strategy to quickly convert swine manure to the promising soil amendment especially for acidic soils.

Effects of soil acidification on the toxicity of organophosphorus pesticide on Eisenia fetida and its mechanism

Organophosphorus pesticides (OPs) have been widely used to control agricultural insects. Soil acidification is a major problem in soil of intensive agricultural systems, especially in red soil with a low pH buffer capacity. However, the effects of soil acidification on the toxicity of pesticides are still unclear. In the present study, the toxicity of three OPs on E. fetida was determined at individual (14-day lethal test) and biochemical levels (antioxidative defence enzymes) by using acidified soils (pH = 5.5, 4.3 and 3.1). The results showed that the toxicity of tested OPs was slightly increased with the decrease of soil pH. To interpret the phenomena, an optimum Quantitative Structure Activity Relationship (QSAR) model was developed based on the toxicity mechanism and the partial least squares regression (PLS) method. The model indicated bioavailability and toxicodynamics are key factors of soil acidification affecting the toxicity of the OPs. Further results revealed the bioavailability of the OPs was strongly related to their hydrolysis and biodegradation character, whereas the effects of soil acidification on toxicodynamics were mainly caused by the interaction between the acetylcholinesterase (AchE) and the OPs. Results will increase understanding of the effects of soil acidification on the toxicity of pesticides and its mechanism.

Distribution, fractions, and potential release of thallium in acidic soils nearby a waste copper mining site from southern China

Although thallium (Tl) is a highly toxic element, little information is available on the environmental risks of Tl in agricultural soils with intensive practices, particularly nearby mining sites. Therefore, we investigated the potential release of Tl in acidic soils with intensive cultivation nearby a waste copper mining site from southern China based on its level and chemical fractions as well as simulated release under artificial acid rain. Results showed that the average Tl content was 1.31 mg/kg in the studied area, which significantly exceeds the permissible thallium value of 1 mg/kg for agricultural soil in China. Some vertical increases of soil Tl from different land uses indicate the potential transport of Tl downward to groundwater. High positive correlations between surficial soil Tl and rubidium (Rb) and copper (Cu) indicated that Tl has the lithophile and chalcophile behavior. Tl in soils is mainly entrapped in residual fraction. The exchangeable fraction of Tl in agricultural soils was less than undisturbed natural soils and copper mined soils. Additionally, the percentage of Tl release from undisturbed natural soils and soils of copper ore area was more than that from agricultural soils in simulated acid rain. Furthermore, the releases of Tl from the soils increased with the acidity of artificial acid rain. Thus, more attention must be paid to land management of this similar area to avoid the risk of Tl impact on human health.