Nitrogen addition enhances soil carbon and nutrient dynamics in Chinese croplands: a machine learning and nationwide synthesis

Nitrogen (N) addition is a critical driver of soil organic carbon (SOC) sequestration and nutrient cycling in croplands. However, its spatial variability and long-term effects under diverse environmental conditions remain poorly understood. We synthesised data from 479 cropland sites across China and apply machine learning models to evaluate the impacts of N addition on SOC and key soil nutrient indicators, including total nitrogen (TN), nitrate (NO₃⁻-N), ammonium (NH₄⁺-N), the carbon-to-nitrogen ratio (C/N), and available phosphorus (AP). We further evaluated the moderating roles of climate zones, fertiliser types, and fertilisation duration. Our findings demonstrate that N addition significantly increased SOC, TN, NO₃⁻-N, NH₄⁺-N, and AP contents, whereas the C/N ratio remains unaffected. SOC sequestration was greater in arid regions, whereas nutrient accumulation was more pronounced in humid zones. Organic and integrated (organic-inorganic) fertilisers outperformed chemical ones in enhancing SOC and nutrient cycling. Long-term N input (> 10 years) markedly intensified SOC storage and nutrient accumulation. We further developed the high-resolution (5 km) national-scale dataset that predicts the spatial responses of SOC and nutrient dynamics to nitrogen addition across China. This AI-derived dataset enables automated mapping of soil carbon and nutrient functions, capturing substantial spatial heterogeneity under varying environmental conditions. These results provide critical insights for optimising nitrogen management strategies, enhancing soil carbon sink functions, and informing precision agriculture policies in China.

Cadmium inhibits carbon and nitrogen cycling through soil microbial biomass and reduces soil nitrogen availability

Microbial mediated carbon and nitrogen cycling response to cadmium are often observed in soil; however, a unified framework of this response has not yet been established. By analyzing 1232 observations from 166 publications, we found that cadmium decreased microbial biomass carbon (-16 %) and nitrogen (-21 %), dissolved organic nitrogen (-27 %), nitrification rate (-17 %), microbial respiration rate (-12 %), and β-1,4-glucosidase (-21 %) and urease (-16 %) activities, but increased microbial metabolic quotient (+11 %) and fungal-to-bacterial ratio (+39 %). The cadmium impact was concentration-dependent, becoming more pronounced at higher concentrations. Increasing cadmium concentration reduced soil N mineralization rate and total N content, but increased microbial biomass carbon-to-nitrogen ratio. These results indicate that cadmium reduced carbon and nitrogen assimilation into microbial biomass and limited soil inorganic nitrogen production. Soil bulk density drove soil microbial biomass and nitrogen availability response to cadmium. Lower soil bulk density and higher initial carbon and clay contents and soil pH reduced the negative impact of cadmium on microbial biomass and nitrogen availability, suggesting that anthropogenic activities that enhance soil quality may mitigate the inhibitory effect of cadmium on soil carbon and nitrogen cycling. Our analysis provides critical implications for improving our understanding of the ecological consequences of cadmium on soil carbon and nitrogen cycling.

Towards a Mechanistic Understanding of Legume Functioning in Natural Restoration of Degraded Ecosystem: Legume-Specific Impacts on Nitrogen Transformation Processes

Legumes have important functions in degraded ecosystems as they can mediate atmospheric nitrogen (N) inputs and increase soil N availability. However, it remains unclear whether legumes affect N availability only through biological N fixation or stimulating microbial N transformations. In this study, nine native legumes and four non-legumes were collected following a 9-year natural vegetation restoration experiment in a karst rocky desertification area. Leaf N/phosphorus (P) ratios and various soil N pool compositions were analyzed and gross N transformation rates were determined by 15N tracing techniques. Legumes exhibited higher leaf δ15N values and increased contents of total N, microbial biomass N and inorganic N compared to non‒legumes. Legume leaf N content and N/P ratio (26.7 g kg‒1 and 20.7) significantly exceeded those of non‒legumes (14.2 g kg‒1 and 14.5). Our results indicate that legumes increased soil N availability and decreased plant N limitation after 9 years of natural vegetation succession, with effects varying between species and related to soil N transformation processes. Species with low plant N limitation exhibited high rates of organic N mineralization (MNorg) and ammonium oxidation to nitrate (ONH4), both of which increase inorganic N supply (especially nitrate). This effect was more pronounced in rhizosphere than bulk soil. MNorg and ONH4 rates were positively correlated (p < 0.01) with soil organic carbon, total N, water holding capacity, calcium content and microbial biomass as well as with leaf N:P ratios, indicating legumes improve soil quality and inorganic N supply, thereby alleviating plant N limitation. Our results highlight the importance of legumes in soil N cycling and availability, which is often a limiting factor for natural restoration of degraded ecosystems.

Nitrogen Acquisition by Invasive Plants: Species Preferential N Uptake Matching with Soil N Dynamics Contribute to Its Fitness and Domination

Plant invasions play a significant role in global environmental change. Traditionally, it was believed that invasive plants absorb and utilize nitrogen (N) more efficiently than native plants by adjusting their preferred N forms in accordance with the dominant N forms present in the soil. More recently, invasive plants are now understood to optimize their N acquisition by directly mediating soil N transformations. This review highlights how exotic species optimize their nitrogen acquisition by influencing soil nitrogen dynamics based on their preferred nitrogen forms, and the various mechanisms, including biological nitrification inhibitor (BNI) release, pH alterations, and changes in nutrient stoichiometry (carbon to nitrogen ratio), that regulate the soil nitrogen dynamics of exotic plants. Generally, invasive plants accelerate soil gross nitrogen transformations to maintain a high supply of NH4+ and NO3- in nitrogen-rich ecosystems irrespective of their preference. However, they tend to minimize nitrogen losses to enhance nitrogen availability in nitrogen-poor ecosystems, where, in such situations, plants with different nitrogen preferences usually affect different nitrogen transformation processes. Therefore, a comprehensive understanding requires more situ data on the interactions between invasive plant species' preferential N form uptake and the characteristics of soil N transformations. Understanding the combination of these processes is essential to elucidate how exotic plants optimize nitrogen use efficiency (NUE) and minimize nitrogen losses through denitrification, leaching, or runoff, which are considered critical for the success of invasive plant species. This review also highlights some of the most recent discoveries in the responses of invasive plants to the different forms and amounts of N and how plants affect soil N transformations to optimize their N acquisition, emphasizing the significance of how plant-soil interactions potentially influence soil N dynamics.

Comammox Nitrospira act as key bacteria in weakly acidic soil via potential cobalamin sharing

The discovery of comammox Nitrospira in low pH environments has reshaped the ammonia oxidation process in acidic settings, providing a plausible explanation for the higher nitrification rates observed in weakly acidic soils. However, the response of comammox Nitrospira to varying pH levels and its ecological role in these environments remains unclear. Here, a survey across soils with varying pH values (ranging from 4.4 to 9.7) was conducted to assess how comammox Nitrospira perform under different pH conditions. Results showed that comammox Nitrospira dominate ammonia oxidation in weakly acidic soils, functioning as a K-strategy species characterized by slow growth and stress tolerance. As a key species in this environment, comammox Nitrospira may promote bacterial cooperation under low pH conditions. Genomic evidence suggested that cobalamin sharing is a potential mechanism, as comammox Nitrospira uniquely encode a metabolic pathway that compensates for cobalamin imbalance in weakly acidic soils, where 86.8% of metagenome-assembled genomes (MAGs) encode cobalamin-dependent genes. Additionally, we used DNA stable-isotope probing (DNA-SIP) to demonstrate its response to pH fluctuations to reflect how it responds to the decrease in pH. Results confirmed that comammox Nitrospira became dominant ammonia oxidizers in the soil after the decrease in pH. We suggested that comammox Nitrospira will become increasingly important in global soils, under the trend of soil acidification. Overall, our work provides insights that how comammox Nitrospira perform in weakly acidic soil and its response to pH changes.

Enrichment and identification of a moderately acidophilic nitrite-oxidizing bacterium

This study enriched a novel nitrite-oxidizing bacterium (NOB, 'Candidatus Nitrobacter acidophilus') in a laboratory reactor operating at pH 4.5 for treating low-strength ammonia wastewater. Batch experiments showed that 'Ca. N. acidophilus' oxidized nitrite to nitrate at a rate of 20.7 ± 2.3 μM/h with optimal growth at pH 5, distinguishing it from most previously known NOB strains. Phylogenetic analysis showed that this Nitrobacter strain clustered with other Nitrobacter strains obtained from acidic environments but was divergent from each other with an average nucleotide identity (ANI) below 85 %. Genomic characteristics revealed that 'Ca. N. acidophilus' possesses versatile transporter systems. They are different from previously reported Nitrobacter strains and indicate acid adaptation mechanisms. Interestingly, the mutualistic interaction with acidophilic ammonia-oxidizing archaea (AOA) Nitrosotalea markedly increased the archaeal amoA gene expression by 149 times and enhanced ammonia oxidation rates by 5 times, highlighting the NOB's role in alleviating nitrite inhibition on the acidophilic AOA. These findings expand our understanding of bacterial nitrite oxidation and provide valuable insights into an important partnership between acidophilic AOA and NOB in acidic environments.

Fire Reduces Soil Nitrate Retention While Increasing Soil Nitrogen Production and Loss Globally

Elucidating the response of soil gross nitrogen (N) transformations to fires could improve our understanding of how fire affects N availability and loss. Yet, how internal soil gross N transformation rates respond to fires remains unexplored globally. Here, we investigate the general response of gross soil N transformations to fire and its consequences for N availability and loss. The results showed that fire increased gross N mineralization rate (GNM; +38%) and ammonium concentration (+47%) as a result of decreased soil C/N ratio but decreased microbial nitrate immobilization (INO3; -56%), resulting in increased nitrous oxide (N2O; +50%) and nitric oxide (+121%) emissions and N leaching (+308%). Time since fire affected soil N cycling and loss. Fire increased GNM, ammonium concentration, and N2O emission, and decreased INO3 only when time since fire was less than one year, while increased N leaching in the short (one year) terms. Thus, the consequences of fire were a short-lived increase in N availability and N2O emissions (lasting less than one year) but with persistent risks of N loss by leaching over time. Overall, fire increased the potential risks of N loss by stimulating N production and inhibiting nitrate retention.

Effects of vegetation restoration in karst areas on soil nitrogen mineralisation

Background

Nitrogen mineralization plays a critical role in the ecosystem cycle, significantly influencing both the ecosystem function and the nitrogen biogeochemical cycle. Therefore, it is essential to investigate the evolutionary characteristics of soil nitrogen mineralization during the karst vegetation restoration to better understand its importance in the terrestrial nitrogen cycle.

Methods

This study analyzed from various stages of vegetation growth, including a 40-year-old woodland, 20-year-old shrubland, 15-year-old shrubland, 5-year-old grassland, and nearby cropland. The aerobic incubation technique was used for 35 days to evaluate soil N mineralization characteristics and their correlation with soil environmental factors. The study focused on examining the variations in soil N mineralization rate (NMR), N nitrification rate (NR), net nitrification rate (AR), and NH4 +-N and NO3 --N levels.

Results

Nitrate nitrogen, the primary form of inorganic nitrogen, increased by 19.38% in the 0-40 cm soil layer of the 20-year-old shrubland compared to the cultivated land. Soil NH4 +-N levels varied during the incubation period, decreasing by the 14th day and rising again by the 21st day. Soil NO3--N and total inorganic nitrogen levels initially increased, then declined, and eventually stabilized, reaching their highest levels on the 14th day. During vegetation restoration, the soil NR and NMR decreased gradually with increasing incubation time. The 15-year shrub, 20-year shrub, and 40-year woodland showed the potential to increase soil NR and NMR. Furthermore, the 15-year shrub and 20-year shrub also increased soil AR. The Mantel test analysis indicated positive correlations among total nitrogen (TN), total phosphorus (TP), total potassium (TK), silicon (Si), AR, NR, and NMR. While available phosphorus (AP) and NMR demonstrated positive correlations with NR and NMR. Furthermore, TN, TP, TK, and Si were found to be positively correlated with AR, NR, and NMR, whereas AP and NO3 --N showed negative correlations with AR, NR, and NMR. It is worth noting that NH4 +-N had the greatest effect on AR, while the bulk density (BD) significantly affected the NR. Furthermore, ammonium nitrogen (AN) and soil organic carbon (SOC) were identified as the primary contributors to NMR. This study provides a theoretical basis for comprehending the influence of vegetation restoration on soil nitrogen mineralization and its role in ecosystem restoration.

Identifying the spatial pattern and driving factors of nitrate in groundwater using a novel framework of interpretable stacking ensemble learning

Groundwater nitrate contamination poses a potential threat to human health and environmental safety globally. This study proposes an interpretable stacking ensemble learning (SEL) framework for enhancing and interpreting groundwater nitrate spatial predictions by integrating the two-level heterogeneous SEL model and SHapley Additive exPlanations (SHAP). In the SEL model, five commonly used machine learning models were utilized as base models (gradient boosting decision tree, extreme gradient boosting, random forest, extremely randomized trees, and k-nearest neighbor), whose outputs were taken as input data for the meta-model. When applied to the agricultural intensive area, the Eden Valley in the UK, the SEL model outperformed the individual models in predictive performance and generalization ability. It reveals a mean groundwater nitrate level of 2.22 mg/L-N, with 2.46% of sandstone aquifers exceeding the drinking standard of 11.3 mg/L-N. Alarmingly, 8.74% of areas with high groundwater nitrate remain outside the designated nitrate vulnerable zones. Moreover, SHAP identified that transmissivity, baseflow index, hydraulic conductivity, the percentage of arable land, and the C:N ratio in the soil were the top five key driving factors of groundwater nitrate. With nitrate threatening groundwater globally, this study presents a high-accuracy, interpretable, and flexible modeling framework that enhances our understanding of the mechanisms behind groundwater nitrate contamination. It implies that the interpretable SEL framework has great promise for providing valuable evidence for environmental management, water resource protection, and sustainable development, particularly in the data-scarce area.

Effects of straw management and N levels on gross nitrogen transformations in fluvo-aquic soil of the North China Plain

Straw incorporation with nitrogen (N) fertilization is crucial for enhancing soil fertility and minimizing negative environmental impacts by altering the magnitude and direction of soil N transformation processes. However, the response of soil N transformations to long-term carbon (C) and N inputs, and their primary driving factors, remain poorly understood. Thus, a 15N tracing study was conducted to investigate the effects of straw incorporation (AS) and straw removal (NS) with N levels of 0, 150 and 250 kg N ha-1 per season (N0, N150 and N250) on gross N transformation rates in the North China Plain after 6-year trial. Results indicated that at N0, AS significantly increased soil microbial immobilization of nitrate (NO3--N, INO3) and autotrophic nitrification rates (ONH4) compared to NS. With N fertilization, AS increased gross N immobilization (Itotal), ammonium-N immobilization (NH4+-N, INH4), net NH4+-N immobilization (InetNH4) and net NH4+-N absorption rates (AnetNH4). Specifically, at N150, AS significantly increased recalcitrant organic N mineralization rate (MNrec), while significantly reducing ONH4, labile organic N mineralization (MNlab), and gross N mineralization rates (Mtotal). At N250, AnetNH4, MNlab, MNrec and ONH4 under AS were significantly higher than under NS. Nitrogen application significantly increased ONH4, Itotal and INO3 under two straw management practices, and enhanced INH4 and InetNH4 under AS. Compared to N250, N150 significantly increased INH4 and InetNH4 under AS, while decreasing Mtotal. Opposite results were observed under NS. Meanwhile, NO3--N and dissolved organic carbon (DOC) were master factors controlling immobilization, total nitrogen (TN), hydrolysable NH4+-N (HNN) and stable organic N significantly affected AnetNH4, while labile organic N were the key environmental factors affecting MNrec, all of which positively influenced the rates of assimilation, mineralization and clay mineral adsorption. Overall, this study provides new insights into reducing N fertilization under straw incorporation by quantifying soil N transformation processes.

A global dataset of gross nitrogen transformation rates across terrestrial ecosystems

Rates of nitrogen transformations support quantitative descriptions and predictive understanding of the complex nitrogen cycle, but measuring these rates is expensive and not readily available to researchers. Here, we compiled a dataset of gross nitrogen transformation rates (GNTR) of mineralization, nitrification, ammonium immobilization, nitrate immobilization, and dissimilatory nitrate reduction to ammonium in terrestrial ecosystems. Data were extracted from 331 studies published from 1984-2022, covering 581 sites. Globally, 1552 observations were appended with standardized soil, vegetation, and climate data (49 variables in total) potentially contributing to the observed variations of GNTR. We used machine learning-based data imputation to fill in partially missing GNTR, which improved statistical relationships between theoretically correlated processes. The dataset is currently the most comprehensive overview of terrestrial ecosystem GNTR and serves as a global synthesis of the extent and variability of GNTR across a wide range of environmental conditions. Future research can utilize the dataset to identify measurement gaps with respect to climate, soil, and ecosystem types, delineate GNTR for certain ecoregions, and help validate process-based models.

Microbial nitrogen transformations in tundra soil depend on interactive effects of seasonality and plant functional types

Nitrogen (N) cycling in organic tundra soil is characterised by pronounced seasonal dynamics and strong influence of the dominant plant functional types. Such patterns in soil N-cycling have mostly been investigated by the analysis of soil N-pools and net N mineralisation rates, which, however, yield little information on soil N-fluxes. In this study we investigated microbial gross N-transformations, as well as concentrations of plant available N-forms in soils under two dominant plant functional types in tundra heath, dwarf shrubs and mosses, in subarctic Northern Sweden. We collected organic soil under three dwarf shrub species of distinct growth form and three moss species in early and late growing season. Our results showed that moss sites were characterised by significantly higher microbial N-cycling rates and soil N-availability than shrub sites. Protein depolymerisation, the greatest soil N-flux, as well as gross nitrification rates generally did not vary significantly between early and late growing season, whereas gross N mineralisation rates and inorganic N availability markedly dropped in late summer at most sites. The magnitude of the seasonal changes in N-cycling, however, clearly differed among plant functional types, indicating interactive effects of seasonality and plant species on soil N-cycling. Our study highlights that the spatial variation and seasonal dynamics of microbial N transformations and soil N availability in tundra heath are intimately linked with the distinct influence of plant functional types on soil microbial activity and the plant species-specific patterns of nutrient uptake and carbon assimilation. This suggests potential strong impacts of future global change-induced shifts in plant community composition on soil N-cycling in tundra ecosystems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10533-024-01176-6.

In the Qaidam Basin, Soil Nutrients Directly or Indirectly Affect Desert Ecosystem Stability under Drought Stress through Plant Nutrients

The low nutrient content of soil in desert ecosystems results in unique physiological and ecological characteristics of plants under long-term water and nutrient stress, which is the basis for the productivity and stability maintenance of the desert ecosystem. However, the relationship between the soil and the plant nutrient elements in the desert ecosystem and its mechanism for maintaining ecosystem stability is still unclear. In this study, 35 sampling sites were established in an area with typical desert vegetation in the Qaidam Basin, based on a drought gradient. A total of 90 soil samples and 100 plant samples were collected, and the soil's physico-chemical properties, as well as the nutrient elements in the plant leaves, were measured. Regression analysis, redundancy analysis (RDA), the Theil-Sen Median and Mann-Kendall methods, the structural equation model (SEM), and other methods were employed to analyze the distribution characteristics of the soil and plant nutrient elements along the drought gradient and the relationship between the soil and leaf nutrient elements and its impact on ecosystem stability. The results provided the following conclusions: Compared with the nutrient elements in plant leaves, the soil's nutrient elements had a more obvious regularity of distribution along the drought gradient. A strong correlation was observed between the soil and leaf nutrient elements, with soil organic carbon and alkali-hydrolyzed nitrogen identified as important factors influencing the leaf nutrient content. The SEM showed that the soil's organic carbon had a positive effect on ecosystem stability by influencing the leaf carbon, while the soil's available phosphorus and the mean annual temperature had a direct positive effect on stability, and the soil's total nitrogen had a negative effect on stability. In general, the soil nutrient content was high in areas with a low mean annual temperature and high precipitation, and the ecosystem stability in the area distribution of typical desert vegetation in the Qaidam Basin was low. These findings reveal that soil nutrients affect the stability of desert ecosystems directly or indirectly through plant nutrients in the Qaidam Basin, which is crucial for maintaining the stability of desert ecosystems with the background of climate change.

Enhancing boreal forest resilience: A four-year impact of biochar on soil quality and fungal communities

Boreal forests commonly suffer from nutrient deficiency due to restricted biological activity and decomposition. Biochar has been used as a promising strategy to improve soil quality, yet its impacts on forest soil microbes, particularly in cold environment, remains poorly understood. In this study, we investigated the effects of biochar, produced at different pyrolysis temperatures (500 °C and 650 °C) and applied at different amounts (0.5 kg·m-2 and 1.0 kg·m-2), on soil property, soil enzyme activity, and fungal community dynamics in a boreal forest over a span of two to four years. Our results showed that, four-year post-application of biochar produced at 650 °C and applied at 1.0 kg·m-2, significantly increased the relative abundance of Mortierellomycota and enhanced fungal species richness, α-diversity and evenness compared to the control (CK) (P < 0.05). Notably, the abundance of Phialocephala fortinii increased with the application of biochar produced at 500 °C and applied at 0.5 kg·m-2, exhibiting a positively correlation with the carbon cycling-related enzyme β-cellobiosidase. Functionally, distinct fungal gene structures were formed between different biochar pyrolysis temperatures, and between application amounts in four-year post-biochar application (P < 0.05). Additionally, correlation analyses revealed the significance of the duration post-biochar application on the soil properties, soil extracellular enzymes, soil fungal dominant phyla, fungal community and gene structures (P < 0.01). The interaction between biochar pyrolysis temperature and application amount significantly influenced fungal α-diversity (P < 0.01). Overall, these findings provide theoretical insights and practical application for biochar as soil amendment in boreal forest ecosystems.

Climate controls on nitrate dynamics and gross nitrogen cycling response to nitrogen deposition in global forest soils

Understanding the patterns and controls regulating nitrogen (N) transformation and its response to N enrichment is critical to re-evaluating soil N limitation or availability and its environmental consequences. Nevertheless, how climatic conditions affect nitrate dynamics and the response of gross N cycling rates to N enrichment in forest soils is still only rudimentarily known. Through collecting and analyzing 4426-single and 769-paired observations from 231 15N labeling studies, we found that nitrification capacity [the ratio of gross autotrophic nitrification (GAN) to gross N mineralization (GNM)] was significantly lower in tropical/subtropical (19%) than in temperate (68%) forest soils, mainly due to the higher GNM and lower GAN in tropical/subtropical regions resulting from low C/N ratio and high precipitation, respectively. However, nitrate retention capacity [the ratio of dissimilatory nitrate reduction to ammonium (DNRA) plus gross nitrate immobilization (INO3) to gross nitrification] was significantly higher in tropical/subtropical (86%) than in temperate (54%) forest soils, mainly due to the higher precipitation and GNM of tropical/subtropical regions, which stimulated DNRA and INO3. As a result, the ratio of GAN to ammonium immobilization (INH4) was significantly higher in temperate than in tropical/subtropical soils. Climatic rather than edaphic factors control heterotrophic nitrification rate (GHN) in forest soils. GHN significantly increased with increasing temperature in temperate regions and with decreasing precipitation in tropical/subtropical regions. In temperate forest soils, gross N transformation rates were insensitive to N enrichment. In tropical/subtropical forests, however, N enrichment significantly stimulated GNM, GAN and GAN to INH4 ratio, but inhibited INH4 and INO3 due to reduced microbial biomass and pH. We propose that temperate forest soils have higher nitrification capacity and lower nitrate retention capacity, implying a higher potential risk of N losses. However, tropical/subtropical forest systems shift from a conservative to a leaky N-cycling system in response to N enrichment.

Warming-Induced Stimulation of Soil N2O Emissions Counteracted by Elevated CO2 from Nine-Year Agroecosystem Temperature and Free Air Carbon Dioxide Enrichment

Globally, agricultural soils account for approximately one-third of anthropogenic emissions of the potent greenhouse gas and stratospheric ozone-depleting substance nitrous oxide (N2O). Emissions of N2O from agricultural soils are affected by a number of global change factors, such as elevated air temperatures and elevated atmospheric carbon dioxide (CO2). Yet, a mechanistic understanding of how these climatic factors affect N2O emissions in agricultural soils remains largely unresolved. Here, we investigate the soil N2O emission pathway using a 15N tracing approach in a nine-year field experiment using a combined temperature and free air carbon dioxide enrichment (T-FACE). We show that the effect of CO2 enrichment completely counteracts warming-induced stimulation of both nitrification- and denitrification-derived N2O emissions. The elevated CO2 induced decrease in pH and labile organic nitrogen (N) masked the stimulation of organic carbon and N by warming. Unexpectedly, both elevated CO2 and warming had little effect on the abundances of the nitrifying and denitrifying genes. Overall, our study confirms the importance of multifactorial experiments to understand N2O emission pathways from agricultural soils under climate change. This better understanding is a prerequisite for more accurate models and the development of effective options to combat climate change.

Monotonic trends of soil microbiomes, metagenomic and metabolomic functioning across ecosystems along water gradients in the Altai region, northwestern China

To investigate microbial communities and their contributions to carbon and nutrient cycling along water gradients can enhance our comprehension of climate change impacts on ecosystem services. Thus, we conducted an assessment of microbial communities, metagenomic functions, and metabolomic profiles within four ecosystems, i.e., desert grassland (DG), shrub-steppe (SS), forest (FO), and marsh (MA) in the Altai region of Xinjiang, China. Our results showed that soil total carbon (TC), total nitrogen, NH4+, and NO3- increased, but pH decreased with soil water gradients. Microbial abundances and richness also increased with soil moisture except the abundances of fungi and protists being lowest in MA. A shift in microbial community composition is evident along the soil moisture gradient, with Proteobacteria, Basidiomycota, and Evosea proliferating but a decline in Actinobacteria and Cercozoa. The β-diversity of microbiomes, metagenomic, and metabolomic functioning were correlated with soil moisture gradients and have significant associations with specific soil factors of TC, NH4+, and pH. Metagenomic functions associated with carbohydrate and DNA metabolisms, as well as phages, prophages, TE, plasmids functions diminished with moisture, whereas the genes involved in nitrogen and potassium metabolism, along with certain biological interactions and environmental information processing functions, demonstrated an augmentation. Additionally, MA harbored the most abundant metabolomics dominated by lipids and lipid-like molecules and organic oxygen compounds, except certain metabolites showing decline trends along water gradients, such as N'-Hydroxymethylnorcotinine and 5-Hydroxyenterolactone. Thus, our study suggests that future ecosystem succession facilitated by changes in rainfall patterns will significantly alter soil microbial taxa, functional potential, and metabolite fractions.

Initial evidence on the effect of copper on global cropland nitrogen cycling: A meta-analysis

Copper (Cu) is a key cofactor in ammonia monooxygenase functioning responsible for the first step of nitrification, but its excess availability impairs soil microbial functions and plant growth. Yet, the impact of Cu on nitrogen (N) cycling and process-related variables in cropland soils remains unexplored globally. Through a meta-analysis of 1209-paired and 319-single observations from 94 publications, we found that Cu (Cu addition or Cu-polluted soil) reduced soil potential nitrification by 33.8% and nitrite content by 73.5% due to reduced soil enzyme activities of nitrification and urease, microbial biomass content, and ammonia oxidizing archaea abundance. The response ratio of potential nitrification decreased with increasing Cu concentration, soil total N, and clay content. We further noted that soil potential nitrification inhibited by 46.5% only when Cu concentration was higher than 150 mg kg-1, while low Cu concentration (less than 150 mg kg-1) stimulated soil nitrate by 99.0%. Increasing initial soil Cu content stimulated gross N mineralization rate due to increased soil organic carbon and total N, but inhibited gross nitrification rate, which ultimately stimulated gross N immobilization rate as a result of increased the residence time of ammonium. This resulted in a lower ratio of gross nitrification rate to gross N immobilization rate, implying a lower potential risk of N loss as evidenced by decreased nitrous oxide emissions with increasing initial soil Cu content. Our analysis offers initial global evidence that Cu has an important role in controlling soil N availability and loss through its effect on N production and consumption.

Global patterns of soil available N production by mineralization-immobilization turnover in the tropical forest ecosystems

Available N (Navail) is important to nurish plant-microbial system and sequestrate carbon (C) in terrestrial ecosystems. For forest ecosystem, Navail is usually calculated as the sum of N2 fixation (NN2-fixed), N deposition (Ndeposition) and soil available N production (Navail-soil), in which Navail-soil determined the Navail production and its temporal changes. While, there is still a lack of Navail-soil estimation at the global and regional level due to the temporal and spatial variability of influencing factors, such as climate and soil physicochemical properties. By assembling a dataset of gross rates of soil N mineralization (GRmin), immobilization of ammonium (NH4+) (GRac) and nitrate (NO3-) (GRnc), as well as their corresponding geographic information, climate and main soil physicochemical properties, the Navail-soil produced from organic N (Norg) mineralization and inorganic N (Ninorg) immobilization turnover (MIT) was calculated via building a random forest (RF) model in global tropical forests. The results revealed a good fit between the observed and predicted GRmin (R2 = 0.76), GRac (R2 = 0.77) and GRnc (R2 = 0.67). We further estimated that the total mineralized N, immobilized NH4+ and NO3- was 23.97 (10.48-37.46), 17.98 (5.81-30.15) and 4.86 (1.46-8.26) Pg N year-1, respectively, leading to the total Navail-soil of 1.13 (-0.95-3.21) Pg N year-1. Referring to the reported average density of NN2-fixed and Ndeposition, the total NN2-fixed and Ndeposition was 0.03-0.05 and 0.01 Pg N year-1, respectively, by producting density and square meter of global tropic forest. Then the total Navail of global tropic forest ecosystem was 1.18 (-0.91-3.27) Pg N year-1 (Navail-soil + NN2-fixed + Ndeposition). According to the tight stoichiometric relationship between C and N in the production of gross primary productivity (GPP) and soil respiration (Rs), C:N ratio of 31.8-41.9 and 22.7-48.2 was calculated, respectively, which all fall into the C:N ratio range of plants and litter (13.9-75.9) in tropical forest ecosystem. These results confirmed the prediction of Navail-soil production from MIT was in line with theoretic estimates by applying RF machine learning. To our knowledge, this is the first estimation of Navail-soil and the results provide the theoretical basis to evaluate soil C sequestration potential in tropical (e.g. southern America, southeast Asia and Africa) forest ecosystem.

Plant composition change mediates climate drought, nitrogen addition, and grazing effects on soil net nitrogen mineralization in a semi-arid grassland in North China

Human activities induce alterations of the nitrogen (N) cycle, climate drought, and disturbance (e.g., livestock grazing) regimes at the global scale. Their individual, interactive, and combined effects on soil N cycling in grasslands are unclear. We investigated the N addition, drought, and grazing effects on the N mineralization, as well as their correlations with N-related variables, including the C4 species, shoot biomass (SB), root biomass (RB), plant total nitrogen (PTN), plant total carbon (PTC), soil total nitrogen (STN), soil total carbon (STC), and soil microbial N and C, during a three-year field experiment conducted in a semi-arid grassland in North China. The results showed that N addition increased the nitrate N (NO3--N) and ammonium N (NH4+-N) concentrations, whereas drought decreased the NO3--N concentration because of strengthened N limitation. Pronounced temporal variation in the N mineralization occurred under seasonal drought (maxima in August and September) and under its combination with N addition and grazing (minima in August). RB and the C4 species were positively correlated, whereas STC and the NO3--N concentration were negatively correlated with the N mineralization under the combined influence of the three factors. The structural equation model showed that at the site affected by all three factors, drought indirectly increased the N mineralization by reducing the NO3--N concentration, whereas N addition and grazing did not alter the N mineralization. N addition directly increased while indirectly reduced N mineralization by increasing the NO3--N concentration. Additionally, N addition and grazing increased the C4 species and decreased the STC, consequently enhanced N mineralization. These results highlight the predominant role of drought, when combined with N addition and grazing, in controlling the N mineralization. The N supply balance in semi-arid grasslands could be stabilized in response to increased N addition, climate drought, and grazing.

The differential assimilation of nitrogen fertilizer compounds by soil microorganisms

The differential soil microbial assimilation of common nitrogen (N) fertilizer compounds into the soil organic N pool is revealed using novel compound-specific amino acid (AA) 15N-stable isotope probing. The incorporation of fertilizer 15N into individual AAs reflected the known biochemistry of N assimilation-e.g. 15N-labelled ammonium (15NH4+) was assimilated most quickly and to the greatest extent into glutamate. A maximum of 12.9% of applied 15NH4+, or 11.7% of 'retained' 15NH4+ (remaining in the soil) was assimilated into the total hydrolysable AA pool in the Rowden Moor soil. Incorporation was lowest in the Rowden Moor 15N-labelled nitrate (15NO3-) treatment, at 1.7% of applied 15N or 1.6% of retained 15N. Incorporation in the 15NH4+ and 15NO3- treatments in the Winterbourne Abbas soil, and the 15N-urea treatment in both soils was between 4.4% and 6.5% of applied 15N or 5.2% and 6.4% of retained 15N. This represents a key step in greater comprehension of the microbially mediated transformations of fertilizer N to organic N and contributes to a more complete picture of soil N-cycling. The approach also mechanistically links theoretical/pure culture derived biochemical expectations and bulk level fertilizer immobilization studies, bridging these different scales of understanding.

Integrative knowledge-based nitrogen management practices can provide positive effects on ecosystem nitrogen retention

Knowledge-based nitrogen (N) management provides better synchronization of crop N demand with N supply to enhance crop production while reducing N losses. Yet, how these N management practices contribute to reducing N losses globally is unclear. Here we compiled 5,448 paired observations from 336 publications representing 286 sites to assess the impacts of four common knowledge-based N management practices, including balanced fertilization, organic fertilization, co-application of synthetic and organic fertilizers, and nitrification inhibitors, on global ecosystem N cycling. We found that organic and balanced fertilization rather than N-only fertilization stimulated soil nitrate retention by enhancing microbial biomass, but also stimulated soil N leaching and emissions relative to no fertilizer addition. Nitrification inhibitors, however, stimulated soil ammonium retention and plant N uptake while reducing N leaching and emissions. Therefore, integrative application of knowledge-based N management practices is imperative to stimulate ecosystem N retention and minimize the risk of N loss globally.

Dynamics of soil nitrogen availability following conversion of natural forests to various coffee cropping systems in northern Thailand

Land conversion critically affects soil physiochemical and biological properties, yet very little remains clear about the impact of forest conversion on the N pool and related microbial N transformations. Therefore, this study aimed to examine the dynamics of soil N availability following forest conversion into the different coffee cropping systems, and explore the mechanisms behind these dynamics from the microbial N transformation. Disturbed soil samples from two depths (0-20 and 20-40 cm) were collected from four land uses consisting of three different coffee cropping systems (coffee monocultures (C), coffee agroforestry (FC), coffee associated with persimmon (Diospyros kaki L.) (CH)) converted from natural forest and adjacent natural forest (F) in northern Thailand. The soil labile N pools (including ammonium (NH4+), nitrate (NO3-), inorganic N (IN), dissolved organic N (DON) contents and microbial biomass N (MBN)) were measured, as well as the soil total N (STN) content. Soil N transformation rates, including net N mineralization, nitrification, and immobilization, were determined using a laboratory incubation experiment. The results showed that the forest conversion to coffee agroforestry significantly increased soil N content by 39.83 % in topsoil, but no significant difference was observed in C and CH soils as compared to F soil (p ≤ 0.05). The three labile N forms (NH4+, NO3- and DON content) were significantly higher under the C, FC and CH soils in both depths, while the coffee monoculture decreased the MBN content. The increases in soil IN, IN/DON and NO3-/NH4+ ratios used as an N availability indicator were positively associated with an increase in the N mineralization and nitrification processes following the forest conversion. Interestingly, the N immobilization processes in the F and FC soils were significantly higher than those in the C and CH soils, which indirectly regulated a decreased nitrification rate in F and FC soils in our study. With the exception of the FC soil, the nitrification/N immobilization ratios in the C (4.95) and CH (4.08) soils were higher than those in the F (0.70) soil, indicating an increased N loss risk after forest conversion. Therefore, coffee agroforestry systems have the potential to be effective management strategies for improving soil nitrogen sequestration following forest conversion.

Spatiotemporal Variations of Soil Reactive Nitrogen Oxide Fluxes across the Anthropogenic Landscape

Volatile reactive nitrogen oxides (NOy) are significant atmospheric pollutants, including NOx (nitric oxide [NO] + nitrogen dioxide [NO2]) and NOz (nitrous acid [HONO] + nitric acid [HNO3] + nitrogen trioxide [NO3] + ...). NOy species are products of nitrogen (N) cycle processes, particularly nitrification and denitrification. Biogenic sources, including soil, account for over 50% of natural NOy emissions to the atmosphere, yet emissions from soils are generally not included in atmospheric models as a result of a lack of mechanistic data. This work is a unique investigation of NOy fluxes on a landscape scale, taking a comprehensive set of land-use types, human influence, and seasonality into account to determine large-scale heterogeneity to provide a basis for future modeling and hypothesis generation. By coupling 16S rRNA amplicon sequencing and quantitative polymerase chain reaction, we have linked significant differences in functional potential and activity of nitrifying and denitrifying soil microbes to NOy emissions from soils. Further, we have identified soils subject to increased N deposition that are less microbially active despite increased available N, potentially as a result of poor soil health from anthropogenic pollution. Structural equation modeling suggests human influence on soils to be a more significant effector of soil NOy emissions than land-use type.

Mechanisms Driving the Distribution and Activity of Mineralization and Nitrification in the Reservoir Riparian Zone

The riparian zone ecosystems have greater energy flow and elemental cycling than adjacent terrestrial and aquatic ecosystems. Mineralization and nitrification are important initiating processes in the nitrogen cycle, but their distribution and activity under different environmental conditions in the riparian zone and the driving mechanisms are still not clear. We investigated the effects of environmental and microbial factors on mineralization and nitrification activities by analyzing the community of alkaline (apr) and neutral (npr) metallopeptidase, ammonia-oxidizing archaea (AOA), and bacteria (AOB) in soils and sediments under different land-use types in the riparian zone of Miyun Reservoir, as well as measuring potential nitrogen mineralization and ammonia oxidation rates (AOR). The results showed that the mineralization and nitrification activities of soils were greater than those of sediments. AOA and AOB dominate the ammonia oxidation activity of soil and sediment, respectively. NH4+ content was a key factor influencing the ecological niche differentiation between AOA and AOB. The high carbon and nitrogen content of the woodland significantly increased mineralization and nitrification activity. Microbial communities were significantly clustered in the woodland. The land-use type, not the flooding condition, determined the distribution of microbial community structure. The diversity of npr was significantly correlated with potential N mineralization rates, while the transcript abundance of AOA was significantly correlated with ammonia oxidation rates. Our study suggests that environmental changes regulate the distribution and activity of mineralization and nitrification processes in the reservoir riparian zone by affecting the transcript abundance, diversity and community structure of the microbial functional genes.

Effects of straw and plastic film mulching on microbial functional genes involved in soil nitrogen cycling

Introduction

Microorganisms regulate soil nitrogen (N) cycling in cropping systems. However, how soil microbial functional genes involved in soil N cycling respond to mulching practices is not well known.

Methods

We collected soil samples from a spring maize field mulched with crop straw (SM) and plastic film (FM) for 10-year and with no mulching (CK) in the Loess Plateau. Microbial functional genes involved in soil N cycling were quantified using metagenomic sequencing. We collected soil samples from a spring maize field mulched with crop straw (SM) and plastic film (FM) for 10-year and with no mulching (CK) in the Loess Plateau. Microbial functional genes involved in soil N cycling were quantified using metagenomic sequencing.

Results

Compared to that in CK, the total abundance of genes involved in soil N cycling increased in SM but had no significant changes in FM. Specifically, SM increased the abundances of functional genes that involved in dissimilatory nitrate reduction to ammonium (nirB, napA, and nrfA), while FM decreased the abundances of functional genes that involved in ammonification (ureC and ureA) in comparison with CK. Other genes involved in assimilatory nitrate reduction, denitrification, and ammonia assimilation, however, were not significantly changed with mulching practices. The nirB and napA were derived from Proteobacteria (mainly Sorangium), and the ureC was derived from Actinobacteria (mainly Streptomyces). Mental test showed that the abundance of functional genes that involved in dissimilatory nitrate reduction was positively correlated with the contents of soil microbial biomass N, potential N mineralization, particulate organic N, and C fractions, while ammonification related gene abundance was positively correlated with soil pH, microbial biomass C and N, and mineral N contents.

Discussion

Overall, this study showed that SM could improve soil N availability and promote the soil N cycling by increasing the abundance of functional genes that involved in DNRA, while FM reduced the abundance of functional genes that involved in ammonification and inhibited soil N cycling.

Threshold responses of soil gross nitrogen transformation rates to aridity gradient

The responses of soil nitrogen (N) transformations to climate change are crucial for biome productivity prediction under global change. However, little is known about the responses of soil gross N transformation rates to drought gradient. Along an aridity gradient across the 2700 km transect of drylands on the Qinghai-Tibetan Plateau, this study measured three main soil gross N transformation rates in both topsoil (0-10 cm) and subsoil (20-30 cm) using the laboratorial 15 N labeling. The relevant soil abiotic and biotic variables were also determined. The results showed that gross N mineralization and nitrification rates steeply decreased with increasing aridity when aridity was less than 0.5 but just slightly decreased with increasing aridity when aridity was larger than 0.5 at both soil layers. In topsoil, the decreases of the two gross rates were accompanied by the similar decreased patterns of soil total N content and microbial biomass carbon with increasing aridity (p < .05). In subsoil, although the decreased pattern of soil total N with increasing aridity was still similar to the decreases of the two gross rates (p < .05), microbial biomass carbon did not change (p > .05). Instead, bacteria and ammonia oxidizing archaea abundances decreased with increasing aridity when aridity was larger than 0.5 (p < .05). With an aridity threshold of 0.6, gross N immobilization rate increased with increasing aridity in wetter region (aridity < 0.6) accompanied with an increased bacteria/fungi ratio, but decreased with increasing aridity in drier region (aridity > 0.6) where mineral N and microbial biomass N also decreased at both soil layers (p < .05). This study provided new insight to understand the differential responses of soil N transformation to drought gradient. The threshold responses of the gross N transformation rates to aridity gradient should be noted in biogeochemical models to better predict N cycling and manage land in the context of global change.

Forms of nitrogen inputs regulate the intensity of soil acidification

Soil acidification induced by reactive nitrogen (N) inputs can alter the structure and function of terrestrial ecosystems. Because different N-transformation processes contribute to the production and consumption of H+ , the magnitude of acidification likely depends on the relative amounts of organic N (ON) and inorganic N (IN) inputs. However, few studies have explicitly measured the effects of N composition on soil acidification. In this study, we first conducted a meta-analysis to test the effects of ON or IN inputs on soil acidification across 53 studies in grasslands. We then compared soil acidification across five different ON:IN ratios and two input rates based on long-term field N addition experiments. The meta-analysis showed that ON had weaker effects on soil acidification than IN when the N addition rate was above 20 g N m-2  year-1 . The field experiment confirmed the findings from meta-analysis: N addition with proportions of ON ≥ 20% caused less soil acidification, especially at a high input rate (30 g N m-2  year-1 ). Structural equation model analysis showed that this result was largely due to a relatively low rate of H+ production from ON as NH3 volatilization and uptake of ON and NH4 + by the dominant grass species Leymus chinensis (which are both lower net contributors to H+ production) result in less NH4 + available for nitrification (which is a higher net contributor to H+ production). These results indicate that the evaluation of soil acidification induced by N inputs should consider N forms and manipulations of relative composition of N inputs may provide an effective approach to alleviate the N-induced soil acidification.

Degradation reduces greenhouse gas emissions while weakening ecosystem carbon sequestration of Moso bamboo forests

Moso bamboo (Phyllostachys heterocycla cv. Pubescens) is well known for its high capacity to sequester atmospheric carbon, which has a unique role to play in combating global warming. Many Moso bamboo forests are gradually degrading due to rising labor costs and falling prices for bamboo timber. However, the mechanisms of carbon sequestration of Moso bamboo forest ecosystems in response to degradation are unclear. In this study, a space-for-time substitution approach was used to select Moso bamboo forest plots with the same origin and similar stand types, but different years of degradation, and four degradation sequences, continuous management (CK), 2 years of degradation (D-I), 6 years of degradation (D-II) and 10 years of degradation (D-III). A total of 16 survey sample plots were established based on the local management history files. After a 12-month monitoring, the response characteristics of soil greenhouse gases (GHG) emissions, vegetation, and soil organic carbon sequestration in different degradation sequences were evaluated to reveal the differences in the ecosystem carbon sequestration. The results indicated that under D-I, D-II, and D-III, the global warming potential (GWP) of soil GHG emissions decreased by 10.84 %, 17.75 %, and 31.02 %, while soil organic carbon (SOC) sequestration increased by 2.82 %, 18.11 %, and 4.68 %, and vegetation carbon sequestration decreased by 17.30 %, 33.49 %, and 44.76 %, respectively. In conclusion, compared to CK, the ecosystem carbon sequestration was reduced by 13.79 %, 22.42 %, and 30.31 %, respectively. This suggests that degradation reduces soil GHG emissions but weakens the ecosystem carbon sequestration capability. Therefore, in the background of global warming and the strategic goal of carbon neutrality, restorative management of degraded Moso bamboo forests is critically needed to improve the carbon sequestration potential of the ecosystem.

Effects of nitrogen deposition on carbon and nutrient cycling along a natural soil acidity gradient as revealed by metagenomics

Nitrogen (N) deposition and soil acidification are environmental challenges affecting ecosystem functioning, health, and biodiversity, but their effects on functional genes are poorly understood. Here, we utilized metabarcoding and metagenomics to investigate the responses of soil functional genes to N deposition along a natural soil pH gradient. Soil N content was uncorrelated with pH, enabling us to investigate their effects separately. Soil acidity strongly and negatively affected the relative abundances of most cluster of orthologous gene categories of the metabolism supercategory. Similarly, soil acidity negatively affected the diversity of functional genes related to carbon and N but not phosphorus cycling. Multivariate analyses showed that soil pH was the most important factor affecting microbial and functional gene composition, while the effects of N deposition were less important. Relative abundance of KEGG functional modules related to different parts of the studied cycles showed variable responses to soil acidity and N deposition. Furthermore, our results suggested that the diversity-function relationship reported for other organisms also applies to soil microbiomes. Since N deposition and soil pH affected microbial taxonomic and functional composition to a different extent, we conclude that N deposition effects might be primarily mediated through soil acidification in forest ecosystems.

Effects of vegetation restoration on soil properties and vegetation attributes in the arid and semi-arid regions of China

Driven by the goal of reversing desertification and recovering degraded lands, a wide range of vegetation restoration practices (such as planting and fencing) have been implemented in China's drylands. It is essential to examine the effects of vegetation restoration and environmental factors on soil nutrients to optimize restoration approaches. However, quantitative evaluation on this topic is insufficient due to a lack of long-term field monitoring data. This study evaluated the effects of sandy steppe restoration and sand dune fixation in the semi-arid desert, and natural and artificial vegetation restoration in the arid desert. It considered soil and plant characteristics using long-term (2005-2015) data from the Naiman Research Station located in the semi-arid region and Shapotou Research Station in the arid region of China's drylands. Results showed the sandy steppe had higher soil nutrient contents, vegetation biomass and rate of accumulating soil organic matter (OM) than the fixed dunes and moving dunes. Soil nutrient contents and vegetation biomass of the natural vegetation of Artemisia ordosica were higher than those of the artificial restoration of Artemisia ordosica since 1956. Artificial restoration had a higher rate of accumulating soil OM, total nitrogen (TN) and grass litter biomass than natural restoration. Soil water indirectly affected soil OM by affecting vegetation. Grass diversity was the main influencing factor on soil OM variance in the semi-arid Naiman desert while shrub diversity was the main factor in the arid Shapotou desert. These findings indicate that sand fixation in the semi-arid desert and vegetation restoration in the arid desert bring benefits for soil nutrient accumulation and vegetation improvement, and that natural restoration is preferable to artificial restoration. Results can be used to formulate sustainable vegetation restoration strategies, such as encouraging natural restoration, considering local resource constraints, and giving priority to restoring shrubs in arid areas with limited water.

Soil acidification induced variation of nitrifiers and denitrifiers modulates N2O emissions in paddy fields

Soil acidification is a major land degradation process globally, and impacts soil nitrogen (N) transformation. However, it is still not well known how soil acidification affects net N mineralization and nitrification, especially N-cycling microbes and nitrous oxide (N₂O) emissions. Hence, three soils characterized by different soil pH values (5.5, 6.3, and 7.7) were collected from the paddy fields, and experiments were conducted to evaluate the effect of soil acidification on net N mineralization and nitrification, and N₂O emissions. Compared to those in the soils with pH 7.7 and 6.3, net N mineralization, net nitrification, and N₂O emissions were decreased by 75-76 %, 89-91 %, and 19-48 %, respectively, in the soil with pH 5.5, while net N nitrification and N₂O emissions decreased by 18 % in the soil with pH 6.3 when compared to those in the soil with pH 7.7. The significantly decreased net nitrification in the soils with pH 6.3 and 5.5 was mainly attributed to the limited N availability and abundance of nitrification-related microbes including ammonia-oxidizing bacteria and complete ammonia-oxidizers. The decrease in N₂O emissions of soils with pH 6.3 and 5.5 had mainly resulted from decreasing nitrification and denitrification via suppressing microbes including nirS and fungal nirK and limiting N availability. Hence, this study provides new insights and improves our understanding of how soil acidification regulates N mineralization, nitrification, and N₂O emissions in paddy soils, which gives guidance on developing N management strategies for sustainable production and N₂O mitigation in acid soils.

Effects of Solidago canadensis L. on mineralization-immobilization turnover enhance its nitrogen competitiveness and invasiveness

The effects of exotic plants on soil nitrogen (N) transformations may influence species invasion success. However, the complex interplay between invasive plant N uptake and N transformation in soils remains unclear. In the present study, a series of ¹⁵N-labeled pot experiments were carried out with Solidago canadensis L. (S. canadensis), an invasive plant, and the Ntrace tool was used to clarify the preferred inorganic N form and its effects on soil N transformation. According to the results, nitrate-N (NO₃^--N) uptake rates by S. canadensis were 2.38 and 2.28 mg N kg^-1 d^-1 in acidic and alkaline soil, respectively, which were significantly higher than the ammonium-N (NH₄⁺-N) uptake rates (1.76 and 1.56 mg N kg^-1 d^-1, respectively), indicating that S. canadensis was a NO₃^--N-preferring plant, irrespective of pH condition. Gross N mineralization rate was 0.41 mg N kg^-1 d^-1 in alkaline soil in the presence of S. canadensis L., which was significantly lower than that in the control (no plant, CK, 2.44 mg N kg^-1 d^-1). Gross autotrophic nitrification rate also decreased from 5.95 mg N kg^-1 d^-1 in the CK to 0.04 mg N kg^-1 d^-1 in the presence of S. canadensis in alkaline soil. However, microbial N immobilization rate increased significantly from 1.09 to 2.16 mg N kg^-1 d^-1, and from 0.02 to 2.73 mg N kg^-1 d^-1 after S. canadensis planting, in acidic and alkaline soil, respectively. Heterotrophic nitrification rate was stimulated in the presence of S. canadensis to provide NO₃^--N to support the N requirements of plants and microbes. The results suggested that S. canadensis can influence the mineralization-immobilization turnover (MIT) to optimize its N requirements while limiting N supply for other plants in the system. The results of the present study enhance our understanding of the competitiveness and mechanisms of invasion of alien plants.

Understanding the relative contributions of fungi and bacteria led nitrous oxide emissions in an acidic soil amended with industrial waste

Amendment of fertilized arable soil with alkaline industrial waste has the potential to ameliorate soil acidification whilst also improving crop yield. Another co-benefit is nitrous oxide (N₂O) emission abatement but the contribution of fungi and bacteria involved in this process remains unclear. Two incubation experiments were conducted to: 1) examine how amendment of acidic soils with a mixture of phosphorus tailings mixture and insoluble potassium-containing rocks (PT) affect N₂O emissions and 2) understand the microbial mechanisms and relative contributions of fungi and bacteria responsible for N₂O emissions. In the first incubation experiment, the four treatments consisted of: i) the study control, ii) urea, iii) PT amendment and iv) PT amendment plus urea. Results showed that the PT amendment significantly increased soil pH from 4.8 to above 6.0, and reduced N₂O emissions by 65.7%. PT-amended soils had higher N₂ emissions and faster O₂ consumption. The PT amendment significantly increased extracellular enzyme activities of leucine aminopeptidase and N-Acetyl-β-glucosaminidase, while it significantly decreased activities of β-1, 4-glucosidase and β-cellobiosidase. Two antibiotics (cycloheximide and streptomycin) combined with substrate-induced respiration method were used in the second incubation experiment. Compared to soil with urea, urea with PT amendment raised soil bacteria-related N₂O from 9.2% to 18.8% while decreasing fungi-related N₂O from 50.5% to 43.2%. These findings suggest that the N₂O emissions from acidic soils can be considerably mitigated by the application of alkaline industrial wastes. The contribution of fungi should be considered when designing and applying N₂O mitigation strategies in acidic soils. DATA AVAILABILITY: Data will be made available on request.

Effects of Temperature and Humidity on Soil Gross Nitrogen Transformation in a Typical Shrub Ecosystem in Yanshan Mountain and Hilly Region

Shrubland is a pivotal terrestrial ecosystem in China. Soil nitrogen transformations play a crucial role in maintaining the productivity of this ecosystem, yet the driving forces underlying it have not been sufficiently addressed, particularly under ongoing climate changes. Herein, by incorporating 15N isotope pool dilution method in laboratory incubation, the rates of gross N ammonification, nitrification, and inorganic N consumption in soils in response to varying temperature and humidity conditions were determined at different depths (SL10: 0–10 cm, and SL20: 10–20 cm) in a typical shrub ecosystem in the Yanshan mountain and hilly region, North China. The gross rates of ammonification and nitrification of soils in SL10 were higher than those in SL20, which was likely affected by the higher soil organic matter and total N contents at a shallower depth. Both temperature and humidity significantly affected the N transformations. The gross ammonification and nitrification were significantly stimulated as the incubation temperature increased from 5 to 35 °C. The gross ammonification increased exponentially, while the gross nitrification increased differently in different temperature ranges. The increment of soil water contents (from 30% WHC to 60% and 100% WHC) promoted the gross nitrification rate more significantly than the gross ammonification rate. The gross nitrification ceased until soil water content reached 60%WHC, indicating that soil water availability between 60% and 100% WHC was not a limiting factor in the nitrification process for the shrubland soils in this study. The ammonium (NH4+) immobilization was significantly lower than nitrification irrespective of varying environmental conditions, even though the NH4+ consumption rate might be overestimated, uncovering two putative processes: (1) heterotrophic nitrification process; (2) and more competitive nitrifying bacteria than NH4+-immobilizing microorganisms. Our study is indispensable for assessing the stability and sustainability of soil N cycling in the shrub ecosystem under climate changes.

Effects of Soil Nutrients on Plant Nutrient Traits in Natural Pinus tabuliformis Forests

In light of global warming, the interaction between plant nutrient traits and soil nutrients is still unclear. Plant nutrient traits (e.g., N and P) and their stoichiometric relationships (N/P ratio) are essential for plant growth and reproduction. However, the specific role of soil nutrients in driving variation in plant nutrient traits remains poorly understood. Fifty natural Pinus tabuliformis forests were used as the research object to clarify the interaction between plant nutrient traits and soil nutrients. We show that: (1) The N_mass, P_mass and N/P ratios of leaves were significantly higher than those of roots. The N/P ratio of both leaves and roots was less than 14. (2) Leaf nutrient traits showed diverse relationship patterns with root nutrient traits throughout the growing period. Significant changes were found in root nutrient PC2 (the second principal component of root nutrient traits) and leaf nutrient PC1 (the first principal component of leaf traits), and non-significant changes were found in other relationships between leaf and root traits (p > 0.05). Root nutrient traits explained 36.4% of the variance in leaf nutrient traits. (3) With the increase in soil nutrient PC2 (related to N), leaf PC2 (related to N) showed a significant trend of first decreasing and then increasing (p < 0.05). Only the soil N_mass was significantly correlated with the leaf N_mass (p < 0.05), which demonstrated that the growth and survival of Pinus tabuliformis forests were mainly affected by N-limitation.

Performance of graphene and traditional soil improvers in limiting nutrients and heavy metals leaching from a sandy Calcisol

Given the large amount of Graphene produced in the last years, there is the need to introduce this new material into a green and circular economy loop. In this study, for the first time, the fate of nutrients and heavy metals in a sandy Calcisol amended with Graphene was monitored and compared to other traditional improvers such as Compost, Zeolites, and Biochar. This was performed via saturated and unsaturated columns' experiments with two different fertilization regimes: one with NPK fertilizer and one with an innovative fertigation water (FW) produced from a pilot wastewater treatment plant. The breakthrough curves of each nutrient and heavy metal were analysed to understand the main processes occurring in saturated and unsaturated conditions, comparing the columns amended with the improvers versus the unamended Controls. Mass balances for each nutrient and heavy metal were developed to infer whether the different soil improvers were effective in minimizing leaching. Graphene, for most cases, behaved as the Control in nutrients' leaching for all the saturated and unsaturated experiments, both with NPK and FW. Biochar increased EC, K⁺, and pH of the leaching water, which could be an issue for the growth of some plants. Compost increased NO₃^- leaching in all the experiments. Zeolites showed the best N compounds retention, but great PO₄^3- leaching in saturated conditions. Heavy metals leachates were analysed only for unsaturated columns (as more representative of field conditions) and found at concentrations well below the limits suggested by the U.S. Environmental Protection Agency. Overall, Graphene performed well in minimizing nutrients and heavy metals leaching, respect to classical improvers. This study is a starting point for field studies that will be critical to have a clear understanding of how Graphene behaves in the environment.

Soil net N mineralization and hydraulic properties of carbonate-derived laterite under different vegetation types in Karst forests of China

Soil net nitrogen (N) mineralization (N_min) is a key process in the forest N cycle regulating the N availability of plant growth. However, it is unclear how N transformation responds to soil hydraulic properties changes. The soil inorganic N pools and N transformation in the early growing season in karst forestlands were investigated by using an intact soil core in situ incubation method. Three different typical vegetation types were selected. The results showed that the mean values of NH₄⁺-N, NO₃^--N, and inorganic N were 1.05-1.36, 1.55-3.85, and 1.05-2.34 times greater for ferns than for shrubs. NO₃^--N and NH₄⁺-N mainly occur at soil depths of 0-5 cm and 5-15 cm, respectively. The soil N_min was 2.21-232.03 times higher at 0-5 cm than at the 10-15 cm. Net N immobilization was found for the juvenile ferns and shrubs at 5-15 cm. The N_min of juvenile and mature ferns was 1.90-11.78 times and 1.17-16.20 times higher than shrubs, respectively, and shrubs had the highest K_s (69.91 mm h^-1) but the lowest water-holding capacity. Both ferns and shrubs were able to hold more water and available water was richest in mature fern soil, which provided an extra water source for fern growth. Principal component analysis (PCA) was used to test whether the measured variables affected N_min, and the results showed that soil organic matter (SOM), pH, and saturated volumetric water content (θ_s) were the main soil factors affecting N_min. In addition, the NH₄⁺-N, NO₃^--N, and inorganic N stocks were reduced by 3.98 %-59.04 %, 48.07 %-63.30 % and 8.18 %-57.37 % after rainwater input, respectively. Our findings suggest that soil inorganic N and N_min in the karst forest were regulated by soil hydraulic properties. Changes in the soil hydraulic properties might therefore influence the functioning of soil N transformation.

Expanding agroforestry can increase nitrate retention and mitigate the global impact of a leaky nitrogen cycle in croplands

The internal soil nitrogen (N) cycle supplies N to plants and microorganisms but may induce N pollution in the environment. Understanding the variability of gross N cycling rates resulting from the global spatial heterogeneity of climatic and edaphic variables is essential for estimating the potential risk of N loss. Here we compiled 4,032 observations from 398 published 15N pool dilution and tracing studies to analyse the interactions between soil internal potential N cycling and environmental effects. We observed that the global potential N cycle changes from a conservative cycle in forests to a less conservative one in grasslands and a leaky one in croplands. Structural equation modelling revealed that soil properties (soil pH, total N and carbon-to-N ratio) were more important than the climate factors in shaping the internal potential N cycle, but different patterns in the potential N cycle of terrestrial ecosystems across climatic zones were also determined. The high spatial variations in the global soil potential N cycle suggest that shifting cropland systems towards agroforestry systems can be a solution to improve N conservation.

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.

Challenges in measuring nitrogen isotope signatures in inorganic nitrogen forms: An interlaboratory comparison of three common measurement approaches

RATIONALE

Stable isotope approaches are increasingly applied to better understand the cycling of inorganic nitrogen (Ni ) forms, key limiting nutrients in terrestrial and aquatic ecosystems. A systematic comparison of the accuracy and precision of the most commonly used methods to analyze δ15 N in NO3 - and NH4 + and interlaboratory comparison tests to evaluate the comparability of isotope results between laboratories are, however, still lacking.

METHODS

Here, we conducted an interlaboratory comparison involving 10 European laboratories to compare different methods and laboratory performance to measure δ15 N in NO3 - and NH4 + . The approaches tested were (a) microdiffusion (MD), (b) chemical conversion (CM), which transforms Ni to either N2 O (CM-N2 O) or N2 (CM-N2 ), and (c) the denitrifier (DN) methods.

RESULTS

The study showed that standards in their single forms were reasonably replicated by the different methods and laboratories, with laboratories applying CM-N2 O performing superior for both NO3 - and NH4 + , followed by DN. Laboratories using MD significantly underestimated the "true" values due to incomplete recovery and also those using CM-N2 showed issues with isotope fractionation. Most methods and laboratories underestimated the at%15 N of Ni of labeled standards in their single forms, but relative errors were within maximal 6% deviation from the real value and therefore acceptable. The results showed further that MD is strongly biased by nonspecificity. The results of the environmental samples were generally highly variable, with standard deviations (SD) of up to ± 8.4‰ for NO3 - and ± 32.9‰ for NH4 + ; SDs within laboratories were found to be considerably lower (on average 3.1‰). The variability could not be connected to any single factor but next to errors due to blank contamination, isotope normalization, and fractionation, and also matrix effects and analytical errors have to be considered.

CONCLUSIONS

The inconsistency among all methods and laboratories raises concern about reported δ15 N values particularly from environmental samples.

Functional microbial ecology in arctic soils: the need for a year-round perspective

The microbial ecology of arctic and sub-arctic soils is an important aspect of the global carbon cycle, due to the sensitivity of the large soil carbon stocks to ongoing climate warming. These regions are characterized by strong climatic seasonality, but the emphasis of most studies on the short vegetation growing season could potentially limit our ability to predict year-round ecosystem functions. We compiled a database of studies from arctic, subarctic, and boreal environments that include sampling of microbial community and functions outside the growing season. We found that for studies comparing across seasons, in most environments, microbial biomass and community composition vary intra-annually, with the spring thaw period often identified by researchers as the most dynamic time of year. This seasonality of microbial communities will have consequences for predictions of ecosystem function under climate change if it results in: seasonality in process kinetics of microbe-mediated functions; intra-annual variation in the importance of different (a)biotic drivers; and/or potential temporal asynchrony between climate change-related perturbations and their corresponding effects. Future research should focus on (i) sampling throughout the entire year; (ii) linking these multi-season measures of microbial community composition with corresponding functional or physiological measurements to elucidate the temporal dynamics of the links between them; and (iii) identifying dominant biotic and abiotic drivers of intra-annual variation in different ecological contexts.

Biomass, Carbon and Nitrogen Partitioning and Water Use Efficiency Differences of Five Types of Alpine Grasslands in the Northern Tibetan Plateau

(1) Background: Grassland covers most areas of the northern Tibetan Plateau along with important global terrestrial carbon (C) and nitrogen (N) pools, so there is a need to better understand the different alpine grassland growth associated with ecosystem C, N storage and water use efficiency (WUE). (2) Methods: The plant biomass and C, N concentrations, stocks and vegetation WUE of five kinds of alpine grassland types were investigated in northern Tibetan Plateau. (3) Results: The results showed that there were significant differences among five types of alpine grasslands in aboveground biomass (AGB), belowground biomass (BGB), total biomass (TB) and root:shoot (R/S) ratio while the highest value of different indices was shown in alpine meadow type (AM). The AGB and BGB partitioning results significantly satisfied the allometric biomass partitioning theory. The C, N concentrations and C/N of the vegetation in AGB and BGB showed significant grassland type differences. The highest C, N stocks of BGB were in AM which was almost six or seven times more than the C, N stocks of AGB in alpine desert type (AD). There were significant differences in δ¹³C and intrinsic water use efficiency (WUEi) under five alpine grassland types while the highest mean values of foliar δ¹³C and WUEi were in AD. Significant negative correlations were found between WUEi and C, N concentrations, C/N of AGB and soil water content (SWC) while the correlation with BGB C/N was not significant. For AGB, BGB, TB and R/S, there were positive correlations with C, N concentrations of AGB, BGB and SWC while it had significant negative correlations with C/N of BGB. (4) Conclusions: With regard to its types, it is suggested that the AM or AS may be an actively growing grassland type in the northern Tibetan Plateau.

Levels and variations of soil bioavailable nitrogen among forests under high atmospheric nitrogen deposition

To examine the perturbation of atmospheric nitrogen (N) deposition on soil N status and the biogeochemical cycle is meaningful for understanding forest function evolution with environmental changes. However, levels of soil bioavailable N and their environmental controls in forests receiving high atmospheric N deposition remain less investigated, which hinders evaluating the effects of enhanced anthropogenic N loading on forest N availability and N losses. This study analyzed concentrations of soil extractable N, microbial biomass N, net rates of N mineralization and nitrification, and their relationships with environmental factors among 26 temperate forests under the N deposition rates between 28.7 and 69.0 kg N ha^-1 yr^-1 in the Beijing-Tianjin-Hebei (BTH) region of northern China. Compared with other forests globally, forests in the BTH region showed higher levels of soil bioavailable N (NH₄⁺, 27.1 ± 0.8 mg N kg^-1; NO₃^-, 7.0 ± 0.8 mg N kg^-1) but lower net rates of N mineralization and nitrification (0.5 ± 0.1 mg N kg^-1 d^-1 and 0.4 ± 0.1 mg N kg^-1 d^-1, respectively). Increasing N deposition levels increased soil nitrification and NO₃^- concentrations but did not increase microbial biomass N and N mineralization among the study forests. Soil moisture and C availability were found as dominant factors influencing microbial N mineralization and bioavailable N. In addition, by budgeting the differences in soil total N densities between the 2000s and 2010s, atmospheric N inputs to the forests were more retained in soils than lost proportionally (84% vs. 16%). We concluded that the high N deposition enriched soil N without stimulating microbial N mineralization among the study forests. These results clarified soil N status and the major controlling factors under high anthropogenic N loading, which is helpful for evaluating the fates and ecological effects of atmospheric N pollution.

Soil microbial nitrogen-cycling gene abundances in response to crop diversification: A meta-analysis

Single planting structure has a significant impact on the maintenance of nitrogen in managed ecosystems. Although the effect of crop diversity on soil nitrogen-cycling microbes is mainly related to the influence of environmental factors, there is a lack of quantitative research. This study aims to determine the effect of diversified cropping mode on the abundance of functional genes in the soil nitrogen cycle based on the quantitative integration of a meta-analysis database containing 189 observation data pairs. The results show that the soil nifH (nitrogenase coding gene), nirS and nirK (nitrite reductase coding gene), and narG (nitrate reductase coding gene) abundances are positively affected by the diversity of plant species, whereas the amoA (ammonia monooxygenase coding gene) and nosZ (nitrous oxide reductase coding gene) show no response. Diversification duration and ecosystem type are important factors that regulate soil nitrogen fixation and nitrification gene abundances. Denitrification genes are mainly affected by categorical variables such as the planting pattern, soil layer, application species, duration, and soil texture. Among them, the long-term continuous diversification is mainly manifested in the reduction of soil nifH and increase of nirK abundances. Soil organic carbon and nitrogen linearly affect the responses of nifH, amoA, nirS, and nirK. Therefore, to maintain the soil ecological function, diversity of planting patterns needs to be applied flexibly by regulating the abundance of nitrogen-cycling genes. Our study draws conclusions in order to provide theoretical references for the sustainability of nitrogen and improvement of management measures in the process of terrestrial managed ecosystem diversification.

Inhibition of Elevated Atmospheric Carbon Dioxide to Soil Gross Nitrogen Mineralization Aggravated by Warming in an Agroecosystem

The response of soil gross nitrogen (N) cycling to elevated carbon dioxide (CO₂) concentration and temperature has been extensively studied in natural and semi-natural ecosystems. However, how these factors and their interaction affect soil gross N dynamics in agroecosystems, strongly disturbed by human activity, remains largely unknown. Here, a ¹⁵N tracer study under aerobic incubation was conducted to quantify soil gross N transformation rates in a paddy field exposed to elevated CO₂ and/or temperature for 9 years in a warming and free air CO₂ enrichment experiment. Results show that long-term exposure to elevated CO₂ significantly inhibited or tended to inhibit gross N mineralization at elevated and ambient temperatures, respectively. The inhibition of soil gross N mineralization by elevating CO₂ was aggravated by warming in this paddy field. The inhibition of gross N mineralization under elevated CO₂ could be due to decreased soil pH. Long-term exposure to elevated CO₂ also significantly reduced gross autotrophic nitrification at ambient temperature, probably due to decreased soil pH and gross N mineralization. In contrast, none of the gross N transformation rates were affected by long-term exposure to warming alone. Our study provides strong evidence that long-term dual exposure to elevated CO₂ and temperature has a greater negative effect on gross N mineralization rate than the single exposure, potentially resulting in progressive N limitation in this agroecosystem and ultimately increasing demand for N fertilizer.

Microbiogeochemical Traits to Identify Nitrogen Hotspots in Permafrost Regions

Permafrost-affected tundra soils are large carbon (C) and nitrogen (N) reservoirs. However, N is largely bound in soil organic matter (SOM), and ecosystems generally have low N availability. Therefore, microbial induced N-cycling processes and N losses were considered negligible. Recent studies show that microbial N processing rates, inorganic N availability, and lateral N losses from thawing permafrost increase when vegetation cover is disturbed, resulting in reduced N uptake or increased N input from thawing permafrost. In this review, we describe currently known N hotspots, particularly bare patches in permafrost peatland or permafrost soils affected by thermokarst, and their microbiogeochemical characteristics, and present evidence for previously unrecorded N hotspots in the tundra. We summarize the current understanding of microbial N cycling processes that promote the release of the potent greenhouse gas (GHG) nitrous oxide (N2O) and the translocation of inorganic N from terrestrial into aquatic ecosystems. We suggest that certain soil characteristics and microbial traits can be used as indicators of N availability and N losses. Identifying N hotspots in permafrost soils is key to assessing the potential for N release from permafrost-affected soils under global warming, as well as the impact of increased N availability on emissions of carbon-containing GHGs.

Nitrogen availability determines ecosystem productivity in response to climate warming

One of the major uncertainties for carbon-climate feedback predictions is an inadequate understanding of the mechanisms governing variations in ecosystem productivity response to warming. Temperature and water availability are regarded as the primary controls over the direction and magnitude of warming effects, but some unexplained results signal that our understanding is incomplete. Using two complementary meta-analyses, we present evidence that soil nitrogen (N) availability drives the warming effects on ecosystem productivity more strongly than thermal and hydrological factors over a broad geographical scale. First, by synthesizing temperature manipulation experiments, meta-regression model analysis showed that the warming effect on productivity is mainly driven by its effect on soil N availability. Sites with higher warming-induced increase in N availability were characterized by stronger productivity enhancement and vice versa, suggesting that N is a limiting factor across sites. Second, a synthesis of full-factorial warming×N addition experiments demonstrated that N addition significantly weakened the positive warming effect, because the additional N induced by warming may not further benefit plant growth when N limitation is relieved, providing experimental evidence that N regulates the warming effect. Further, we demonstrated that warming effects on soil N availability were modulated by changes in dissolved organic N and soil microbes. Overall, our findings enrich a new mechanistic understanding of the varying magnitudes of observed productivity response to warming, and the N scaling of warming effects may help constrain climate projections.