High Nitrate Accumulation in the Vadose Zone after Land-Use Change from Croplands to Orchards

Stepwise H*-Mediated and Non-H* Reduction Processes for Highly Selective Transformation of Nitrate to Nitrogen Gas Using a ZVAl-Based Material

Improving the reduction efficiency and N2 selectivity is important for nitrate decontamination. A novel ternary ball-milled Al-Cu-AC material is reported to achieve a highly selective reduction of nitrate to N2. The reduction process, driven by the continuous dissolution of zero-valent aluminum (ZVAl), demonstrated a stepwise reduction scheme. The interesting shift in the electron-donating pathways was ascribed to the spontaneous change in the microenvironmental pH from neutral to alkaline. The Al-Cu-AC (1:1:5 mass ratio) material completely removed 30 mg/L of NO3--N over a wide pH range (5-9), achieving over 83% TN removal and N2-selectivity, without detectable copper leaching. The atomic hydrogen (H*)-mediated reduction occurring on the Cu component was proven to be crucial for the fast transformation from NO3- to NO2-, while the non-H* reduction process was dominated by the electrochemical reduction of NO2- to N2 on the AC cathode of Al || AC microgalvanic cells formed in the material. The primary reduction route from NO3- to N2 was identified as the *NOH pathway, and the superiority of the Al-Cu-AC material toward nitrate reduction was verified with actual wastewater. This study revealed how microenvironmental pH influenced the electron-donating pathways of ZVAl and provides a new approach to maximize the performance of zero-valent metals.

Deep nitrate accumulation in typical black soil critical zones of Northeast China

Deep nitrate accumulation below 1 m has been observed in various soil regions, yet remains undocumented in the black soil (mainly Phaeozems and Chernozems) region. Climatic and edaphic factors likely influence deep nitrate accumulation on a large scale, although existing studies primarily focus on individual sites. In order to evaluate the distribution and controlling factors of deep nitrate in the black soil region, inorganic nitrogen forms and regolith properties of nine boreholes spanning humid, semi-humid, and semi-arid areas in Fujin, Hailun, and Lindian in northeast China were analyzed down to a depth of 10 m. The results revealed significant nitrate accumulation in Lindian, peaking at 11.03 mg N kg-1 at a depth of 3 m underground. Nitrate storage from the land surface to a depth of 10 m in Lindian ranged from 459.65 kg N ha-1 to 1072.88 kg N ha-1, with over 70 % of nitrate stored below 1 m. Nitrate accounted for 97.74 % of the total N stock in Lindian. Ammonium accumulation has been observed at a deeper depth in Hailun, with no nitrate accumulation detected in Hainlun and Fujin. Regolith properties such as clay, silt, sand, and pH playing a crucial role in reshaping the vertical pattern of nitrate. The presence of nitrate pools at greater depths in intensively managed black soil regions should be taken into account for the sustainable utilization of soil resources and the mitigation of groundwater pollution risks.

Simulation analysis of the preventative effects of planting sweet corn on nitrate leaching in a cherry greenhouse soil

Introduction

To ensure higher productivity, fertilizers have been excessively applied to the fruit greenhouse soil yearly, thus resulting in the increasing risks of residual nitrate leaching in the North China Plain.

Methods

In this study, a water and solute transport HYDRUS-1D model was used to evaluate the effects of using sweet corn as a catch crop on deep water drainage and nitrate leaching in a sweet cherry greenhouse soil. A three-year (2019-2021) field experiment was conducted during the rainfall season from July to September in the post-harvest of sweet cherry, when the plastic cover was removed each year. In the experiment, the five treatments were designed. The three nitrate residue levels denoted by CKR, N1R, and N2R, represented nitrate residue amounts in the soil profile of three nitrogen fertilizer levels(0, 280 and 420kg ha-1) before the harvest of sweet cherry(March to June). Two other treatments with and without sweet corn as a catch crop based on the treatments of N1R and N2R were denoted by N1RC and N2RC, respectively. The data of both the spatial and temporal distribution of water and nitrate content during the rainy seasons of 2019, 2020 and 2021 in the field experiment were collected to calibrate and validate the model.

Results

The simulated results have showed that using sweet corn as a catch crop increased the evapotranspiration rate, the upward flux of water and nitrate at a 100 cm soil depth reached a maximum of 1.5 mm d-1 and 1.0 kg N ha-1d-1, respectively, and the downward movement of water and nitrate leached to deeper soil layers was reduced. Compared with CKR, the treatments with catch crops (N1RC and N2RC) reduced the amount of water drainage by 16.4% -47.7% in the 0-180cm soil profile. The average amounts of nitrate leaching in the 1.8 m soil profile during the three-year experiment were 88.1, 113.3, and 58.2 kg N ha-1 for the treatment without catch crop (N1R and N2R) and 32.3, 54.8, and 31.4 kg N ha-1 for the treatment with catch crop (N1RC and N2RC), respectively. The treatments (N1RC and N2RC) with catch crops decreased the amount of nitrate leaching by 29.6%-69.1% compared with the treatments without catch crops (N1R and N2R).

Discussion

Sweet corn as summer catch crop can reduce nitrate leaching in the sweet cherry greenhouses. Our study has provided an effective method to reduce the risk of nitrate leaching for sweet cherry greenhouses in the North China Plain.

Global Ecosystem Nitrogen Cycling Reciprocates Between Land-Use Conversion and Its Reversal

Anthropogenic land-use practices influence ecosystem functions and the environment. Yet, the effect of global land-use change on ecosystem nitrogen (N) cycling remains unquantified despite that ecosystem N cycling plays a critical role in maintaining food security. Here, we analysed 2430 paired observations globally to show that converting natural to managed ecosystems increases ratios of autotrophic nitrification to ammonium immobilisation and nitrate to ammonium, but decreases soil immobilisation of mineral N, causing increased N losses via leaching and gaseous N emissions, such as nitrous oxide (e.g., via denitrification), resulting in a leaky N cycle. Changing land use from intensively managed to one that resembles natural ecosystems reversed N losses by 108% on average, resulting in a more conservative N cycle. Structural equation modelling revealed that changes in soil organic carbon, pH and carbon to N ratio were more important than changes in soil moisture content and temperature in predicting ecosystem N retention capacities following land-use conversion and its reversion. The hotspots of leaky N cycles were mostly in equatorial and tropical regions, as well as in Western Europe, the United States and China. Our results suggest that whether an ecosystem exhibits a conservative N cycle after land-use reversion depends on management practices.

Climate Change Impacts on and Response Strategies for Kiwifruit Production: A Comprehensive Review

Climate change, a pressing global concern, poses significant challenges to agricultural systems worldwide. Among the myriad impacts of climate change, the cultivation of kiwifruit trees (Actinidia spp.) faces multifaceted challenges. In this review, we delve into the intricate effects of climate change on kiwifruit production, which span phenological shifts, distributional changes, physiological responses, and ecological interactions. Understanding these complexities is crucial for devising effective adaptation and mitigation strategies to safeguard kiwifruit production amidst climate variability. This review scrutinizes the influence of rising global temperatures, altered precipitation patterns, and a heightened frequency of extreme weather events on the regions where kiwifruits are cultivated. Additionally, it delves into the ramifications of changing climatic conditions on kiwifruit tree physiology, phenology, and susceptibility to pests and diseases. The economic and social repercussions of climate change on kiwifruit production, including yield losses, livelihood impacts, and market dynamics, are thoroughly examined. In response to these challenges, this review proposes tailored adaptation and mitigation strategies for kiwifruit cultivation. This includes breeding climate-resilient kiwifruit cultivars of the Actinidia species that could withstand drought and high temperatures. Additional measures would involve implementing sustainable farming practices like irrigation, mulching, rain shelters, and shade management, as well as conserving soil and water resources. Through an examination of the literature, this review showcases the existing innovative approaches for climate change adaptation in kiwifruit farming. It concludes with recommendations for future research directions aimed at promoting the sustainability and resilience of fruit production, particularly in the context of kiwifruit cultivation, amid a changing climate.

Tracing the contribution and fate of synthetic nitrogen fertilizer in young apple orchard agrosystems

Excessive synthetic nitrogen (N) inputs in intensive orchard agrosystems of developing countries are a growing concern regarding their adverse impacts on fruit production and the environment. Quantifying the distribution and contribution of fertilizer N is essential for increasing N use efficiency and minimizing N loss in orchards. A 15N tracer experiment was performed in a young dwarf apple orchard over two growing seasons to determine the fertilizer N transformation and fate. Fertilizer N primarily contributed to 25 % to 75 % of soil nitrate in the top 60 cm, but the contribution to soil microbial biomass N and fixed ammonium was <8 %, with the contribution to plant N ranging from 9 % to 19 %. In most growth periods, soil nitrate and fixed ammonium contents derived from native soil with N fertilization were higher than those not receiving N fertilizer. The N use efficiency of plants was only 2.6 % and 4.9 % in the first and second seasons, respectively, in contrast to 56.6 % and 54.0 % of N recovered in soil. Meanwhile, N assimilated into microbial biomass accounted for 0.8 %, and the proportion fixed by clay minerals was 3.5 %-5.2 %. One season after N fertilization, the nitrate below the 1 m soil layers accounted for 4.6 % of the applied N fertilizer, and the proportion increased to 22.5 % after two seasons. The N loss rate via N2O emission was 0.4 % over two years. The application of N fertilizer facilitated indigenous soil N mineralization, and abiotic ammonium fixation more efficiently retained synthetic N than microbial immobilization. These findings provide new insight into orchard N cycling, and attention should be given to the improvement of soil N retention and turnover capacity regulated by soil microbial and abiotic processes, as well as the potential environmental impacts of additional soil N mineralization resulting from prolonged chemical N fertilization.

Extreme precipitation accelerates nitrate leaching in the intensive agricultural region with thick unsaturated zones

Nitrate accumulation in the soil profile in the intensive agricultural region has been widely concerned in the world. However, the changes in nitrate accumulation characteristics caused by climate change, such as extremely high precipitation, are not well quantified, particularly for the regions with thick unsaturated zones. Here, we resampled the soil profiles taken in normal year (2020) after extreme precipitation year (2021) (>800 cm) in three regions in the southern Loess Plateau (LP) with three different water managements including rainfed orchards (n = 10), well-irrigated orchards (n = 4) and canal-irrigated orchards (n = 8). The accumulation amounts, peak depths, and accumulation depths of nitrate soil profiles of the different regions of two years were compared. The results showed that average nitrate accumulation in normal year at the rainfed region (800-cm depth), well-irrigated region (800-cm depth) and canal-irrigated region (1400-cm depth) were 5995 kg N ha-1, 9765 kg N ha-1, and 19,608 kg N ha-1, respectively. Compared with 2020, extreme precipitation in 2021 led to 56-91% reductions (2060-3702 kg N ha-1) in nitrate accumulation in 0-200 cm soil layer, and average nitrate leaching into the aquifer was >1390 kg N ha-1 in the canal-irrigated region. Average migration depths of nitrate peak in rainfed, well-irrigated and canal-irrigated regions were 92 cm, 115 cm, and 188 cm, respectively; as for nitrate accumulation depths, they were 10 cm, 80 cm and 108 cm, respectively. Vertically, the dried soil layer and paleosol layer (high clay content) in the canal-irrigated region significantly hindered nitrate deep migration caused by the extreme precipitation. The result highlights that extreme precipitation significantly accelerated nitrate leaching in the deep soil profiles, and future vulnerability and risk assessment studies must account for the impacts of extreme precipitation on nitrate leaching.

Microbial nitrogen mineralization is slightly affected by conversion from farmland to apple orchards in thick loess deposits

Organic nitrogen mineralization, indispensable to soil carbon and nitrogen cycles, is the largest contributor to nitrate reservoirs in deep vadose zones. The microbial nitrogen mineralization (MNM) within deep soils, particularly in regions with intensive agricultural activities and thick soil horizons, has been largely disregarded. As such, this study aims to address this knowledge gap by investigating the chiA-harboring microbial structure and network within nine 10-m profiles beneath cultivated farmland and two apple orchards. The results showed that apple orchards, compared to farmland, had considerable water deficit and nitrogen accumulation within deeper soil layers due to well-developed root systems and the overuse of chemical fertilizers. However, the chiA-harboring microbial diversity, composition, and abundance all exhibited significant variations with soil depths rather than being influenced by different land use types. Moreover, the diversity indices and gene abundances decreased with soil depths, and the related soil microbes included 19 phyla, 29 classes, 72 orders, 114 families, and 197 genera, with Actinobacteria and Proteobacteria being the two major bacterial phyla. The microbial co-occurrence network was simper beneath apple orchards. The chiA-harboring microorganisms within deep unsaturated zones were greatly influenced by the depth-dependent soil nutrients, such as total nitrogen, organic carbon, and available potassium. The limited plant root biomass and the inhibitory effects of dried soil layers both restricted the availability of carbon sources, which further interfered with the MNM processes within deep soils insignificantly. Therefore, despite the considerable plant-induced ecohydrological consequences, the depth-dependent MNM processes were slightly affected after the transformation from farmland to apple orchards within thick loess deposits. This study offers crucial insights into microbial dynamics of the deep biosphere, thereby contributing to our understanding of depth-dependent biogeochemical cycles within global deep unsaturated zones.

Unveiling the biogeochemical mechanism of nitrate in the vadose zone-groundwater system: Insights from integrated microbiology, isotope techniques, and hydrogeochemistry

Clarifying the biogeochemical mechanism of nitrate (NO3-) in the vadose zone-groundwater system, particularly in agricultural contexts, is crucial for mitigating groundwater NO3- pollution. However, comprehensive studies on the impacts of changes in chemical indicators and microbial communities on NO3- are still lacking. This paper aims to address this gap by employing hydrogeochemistry, stable isotopes, and microbial techniques to assess the NO3- biogeochemical processes in the vadose zone-groundwater system. The findings suggested that NO3- in upper soil layers was primarily influenced by plant root absorption, assimilation, and nitrification processes. The oxygen contents gradually decreased with the nitrification process, resulting in the occurrence of the denitrification. However, denitrification predominantly occurred in the 60-80 cm soil layer in the study area. The limited thickness of the denitrification layer results in less NO3- consumption, leading to increased NO3- leaching into groundwater. Hydrochemical and isotopic characteristics further indicated that groundwater NO3- concentrations were mainly controlled by nitrification, followed by denitrification and mixing processes. The 16S rRNA sequencing analysis revealed great influences of soil sampling depths and groundwater NO3- concentrations on the microbial community structure. Additionally, the PICRUSt2-based prediction results demonstrated a stronger potential for dissimilatory reduction of NO3- to ammonium (DNRA) in both soil and groundwater compared to the other processes, potentially due to the widespread presence of the nrfH functional genes. However, the chemical indicators and isotopes used in this study did not support the occurrence of DNRA process in the vadose zone-groundwater system. This finding highlights the importance of an integrated approach combining microbiological, isotopic, and hydrogeochemical data to comprehensive understanding biogeochemical processes. The study developed a conceptual model elucidating the NO3- biogeochemical processes in the vadose zone-groundwater system within an agricultural area, contributing to enhanced comprehension and advancement of sustainable management practices for groundwater nitrogen.

Distribution, sources and main controlling factors of nitrate in a typical intensive agricultural region, northwestern China: Vertical profile perspectives

Nitrate (NO3-) pollution of groundwater is a global concern in agricultural areas. To gain a comprehensive understanding of the sources and destiny of nitrate in soil and groundwater within intensive agricultural areas, this study employed a combination of chemical indicators, dual isotopes of nitrate (δ15N-NO3- and δ18O-NO3-), random forest model, and Bayesian stable isotope mixing model (MixSIAR). These approaches were utilized to examine the spatial distribution of NO3- in soil profiles and groundwater, identify key variables influencing groundwater nitrate concentration, and quantify the sources contribution at various depths of the vadose zone and groundwater with different nitrate concentrations. The results showed that the nitrate accumulation in the cropland and kiwifruit orchard at depths of 0-400 cm increased, leading to subsequent leaching of nitrate into deeper vadose zones and ultimately groundwater. The mean concentration of nitrate in groundwater was 91.89 mg/L, and 52.94% of the samples exceeded the recommended grade III value (88.57 mg/L) according to national standards. The results of the random forest model suggested that the main variables affecting the nitrate concentration in groundwater were well depth (16.6%), dissolved oxygen (11.6%), and soil nitrate (10.4%). The MixSIAR results revealed that nitrate sources vary at different soil depths, which was caused by the biogeochemical process of nitrate. In addition, the highest contribution of nitrate in groundwater, both with high and low concentrations, was found to be soil nitrogen (SN), accounting for 56.0% and 63.0%, respectively, followed by chemical fertilizer (CF) and manure and sewage (M&S). Through the identification of NO3- pollution sources, this study can take targeted measures to ensure the safety of groundwater in intensive agricultural areas.

Effects of recharge process on groundwater nitrate concentration in an oasis of Tengger Desert hinterland, China

Groundwater nitrate concentrations cannot be effectively diluted in an oasis of desert hinterland without direct recharge from external rivers. Therefore, there is an urgent need to understand the relationship between groundwater nitrate concentration and the groundwater recharge process. Using hydrochemicals, stable isotopes, LUCC, and combining these with the MixSIAR model, the distributions of groundwater nitrate concentration in the Dengmaying Basin (DMYB) in 2006 and 2020 were obtained. The contributions of groundwater recharge and nitrate sources were also quantified. With the development of agriculture in the DMYB, groundwater irrigation leakage has gradually become a crucial recharge source of groundwater, with a recharge proportion reaching 30.3%. From 2006 to 2020, under the influence of well irrigation and agricultural fertilization, the groundwater nitrate concentration in the DMYB increased significantly, with an increased range of 1.3 ~ 14.3 mg L-1. Moreover, the δ15N-NO3- and δ18O-NO3- values of nitrate in cultivated soil water were similar to those in groundwater, which also proves the process of carrying nitrate from the vadose zone into groundwater by irrigation water. The contribution of anthropogenic sources (54.9%) to groundwater nitrate exceeded that of natural sources (45.1%) to groundwater nitrate in the DMYB. These results indicate that the potential for nitrate pollution in groundwater must be considered, even in desert oases.

Characteristics of dissolved N2O and indirect N2O emission factor in the groundwater of high nitrate leaching areas in northwest China

Numerous studies have demonstrated high concentrations of dissolved N₂O and indirect N₂O emission factors in groundwater affected by agriculture. However, the characteristics of seasonal and vertical dimensional difference in groundwater in high nitrate leaching areas are relatively lacking. We monitored the concentrations of dissolved and wellhead N₂O of 23 groundwater wells over a one year period to understand the seasonal characteristics of dissolved and wellhead N₂O concentrations and indirect N₂O emission factors (EF₅) of the shallow and deep groundwater in a high nitrogen leaching area and analyze the reasons for their differences. The mean dissolved N₂O concentration in groundwater was 9.71 (9.03) μg/L, which was 1.5-fold higher during the wet season relative to the dry season. Furthermore, the leaching of soil N₂O caused by rainfall and irrigation could be a pivotal factor affecting seasonal variation in the dissolved N₂O. Shallow wells were found to have higher dissolved and wellhead N₂O concentrations compared with deep wells in all seasons. The low wellhead N₂O concentrations during the dry season were attributed to the seasonal decrease of the groundwater table and dissolved N₂O concentrations. We concluded that indirect N₂O emission factors did not vary in the vertical dimension but were higher during the wet season than that during the dry season. In addition, the mean indirect N₂O emission factor in the groundwater was 0.025 %, which was one order of magnitude below the current IPCC value (0.25 %). Thus, we proposed that such a low indirect N₂O emissions factor could imply a low indirect N₂O emission potential in groundwater with high dissolved oxygen and nitrogen loads. Our study further indicated that seasonal differences in dissolved N₂O concentrations and indirect N₂O emission factors should be considered when estimating the potential emissions of dissolved N₂O in groundwater.

Deep accumulation of soluble organic nitrogen after land-use conversion from woodlands to orchards in a subtropical hilly region

Accumulation of soluble organic nitrogen (SON) in soil poses a significant threat to groundwater quality and plays an important role in regulating the global nitrogen cycle; however, most related studies have focused only on the upper 100-cm soil layers. Surface land-use management and soil properties may affect the vertical distribution of SON; however, their influence is poorly understood in deep soil layers. Therefore, this study assessed the response of SON concentration, pattern, and storage in deep regoliths to land-use conversion from woodlands to orchards in a subtropical hilly region. Our results showed that the SON stocks of the entire soil profile (up to 19.5 m) ranged from 254.5 kg N ha^-1 to 664.1 kg N ha^-1. Land-use conversion not only reshaped the distribution pattern of SON, but also resulted in substantial accumulation of SON at the 0-200 cm soil profile in the orchards compared to that in the woodlands (124.1 vs 190.5 kg N ha^-1). Land-use conversion also altered the SON/total dissolved nitrogen ratio throughout the regolith profile, resulting in a relatively low (<50 %) ratio in orchard soils below 200 cm. Overall, 76.8 % of SON (338.4 ± 162.0 kg N ha^-1) was stored in the layers from 100 cm below the surface to the bedrock. Regolith depth (r = -0.52 and p < 0.05) was found to be significantly correlated with SON concentration, explaining 17.8 % of the variation in SON, followed by total nitrogen (14.4 %), total organic carbon/total nitrogen ratio (10.1 %), and bulk density (9.3 %). This study provides insights into the estimation of terrestrial nitrogen and guidance for mitigation of groundwater contamination risk due to deep accumulation of SON.

Anthropogenic N input increases global warming potential by awakening the "sleeping" ancient C in deep critical zones

Even a small net increase in soil organic carbon (SOC) mineralization will cause a substantial increase in the atmospheric CO₂ concentration. It is widely recognized that the SOC mineralization within deep critical zones (2 to 12 m depth) is slower and much less influenced by anthropogenic disturbance when compared to that of surface soil. Here, we showed that 20 years of nitrogen (N) fertilization enriched a deep critical zone with nitrate, almost doubling the SOC mineralization rate. This result was supported by corresponding increases in the expressions of functional genes typical of recalcitrant SOC degradation and enzyme activities. The CO₂ released and the SOC had a similar ¹⁴C age (6000 to 10,000 years before the present). Our results indicate that N fertilization of crops may enhance CO₂ emissions from deep critical zones to the atmosphere through a previously disregarded mechanism. This provides another reason for markedly improving N management in fertilized agricultural soils.

Nitrate as an alternative electron acceptor destabilizes the mineral associated organic carbon in moisturized deep soil depths

Numerous studies have investigated the effects of nitrogen (N) addition on soil organic carbon (SOC) decomposition. However, most studies have focused on the shallow top soils <0.2 m (surface soil), with a few studies also examining the deeper soil depths of 0.5-1.0 m (subsoil). Studies investigating the effects of N addition on SOC decomposition in soil >1.0 m deep (deep soil) are rare. Here, we investigated the effects and the underlying mechanisms of nitrate addition on SOC stability in soil depths deeper than 1.0 m. The results showed that nitrate addition promoted deep soil respiration if the stoichiometric mole ratio of nitrate to O₂ exceeded the threshold of 6:1, at which nitrate can be used as an alternative acceptor to O₂ for microbial respiration. In addition, the mole ratio of the produced CO₂ to N₂O was 2.57:1, which is close to the theoretical ratio of 2:1 expected when nitrate is used as an electron acceptor for microbial respiration. These results demonstrated that nitrate, as an alternative acceptor to O₂, promoted microbial carbon decomposition in deep soil. Furthermore, our results showed that nitrate addition increased the abundance of SOC decomposers and the expressions of their functional genes, and concurrently decreased MAOC, and the ratio of MAOC/SOC decreased from 20% before incubation to 4% at the end of incubation. Thus, nitrate can destabilize the MAOC in deep soils by stimulating microbial utilization of MAOC. Our results imply a new mechanism on how above-ground anthropogenic N inputs affect MAOC stability in deep soil. Mitigation of nitrate leaching is expected to benefit the conservation of MAOC in deep soil depths.

Conversion from rice fields to vegetable fields alters product stoichiometry of denitrification and increases N2O emission

Information about effects of conversion from rice fields to vegetable fields on denitrification process is still limited. In this study, denitrification rate and product ratio (i.e., N₂O/(N₂O + N₂) ratio) were investigated by soil-core incubation based N₂/Ar technique in one rice paddy field (RP) and two vegetable fields (VF4 and VF7, 4 and 7 years vegetable cultivating after conversion from rice fields, respectively). Genes related to denitrification and bacterial community composition were quantified to investigate the microbial mechanisms behind the effects of land-use conversion. The results showed that conversion of rice fields to vegetable fields did not significantly change denitrification rate although the abundance of denitrification related genes was significantly reduced by 79.22%-99.84% in the vegetable soils. Whereas, compared with the RP soil, N₂O emission rate was significantly (P < 0.05) increased by 53.5 and 1.6 times in the VF4 and VF7 soils, respectively. Correspondingly, the N₂O/(N₂O + N₂) ratio increased from 0.18% (RP soil) to 5.65% and 0.65% in the VF4 and VF7 soils, respectively. These changes were mainly attributed to the lower pH, higher nitrate content, and the altered bacterial community composition in the vegetable soils. Overall, our results showed that conversion of rice fields to vegetable fields increased the N₂O emission rate and altered the product ratio of denitrification. This may increase the contribution of land-use conversion to global warming and stratospheric ozone depletion.

Predicting Potential Climate Change Impacts on Groundwater Nitrate Pollution and Risk in an Intensely Cultivated Area of South Asia

One of the potential impacts of climate change is enhanced groundwater contamination by geogenic and anthropogenic contaminants. Such impacts should be most evident in areas with high land-use change footprint. Here, we provide a novel documentation of the impact on groundwater nitrate (GWNO3 ) pollution with and without climate change in one of the most intensely groundwater-irrigated areas of South Asia (northwest India) as a consequence of changes in land use and agricultural practices at present and predicted future times. We assessed the probabilistic risk of GWNO3 pollution considering climate changes under two representative concentration pathways (RCPs), i.e., RCP 4.5 and 8.5 for 2030 and 2040, using a machine learning (Random Forest) framework. We also evaluated variations in GWNO3 distribution against a no climate change (NCC) scenario considering 2020 status quo climate conditions. The climate change projections showed that the annual temperatures would rise under both RCPs. The precipitation is predicted to rise by 5% under RCP 8.5 by 2040, while it would decline under RCP 4.5. The predicted scenarios indicate that the areas at high risk of GWNO3 pollution will increase to 49 and 50% in 2030 and 66 and 65% in 2040 under RCP 4.5 and 8.5, respectively. These predictions are higher compared to the NCC condition (43% in 2030 and 60% in 2040). However, the areas at high risk can decrease significantly by 2040 with restricted fertilizer usage, especially under the RCP 8.5 scenario. The risk maps identified the central, south, and southeastern parts of the study area to be at persistent high risk of GWNO3 pollution. The outcomes show that the climate factors may impose a significant influence on the GWNO3 pollution, and if fertilizer inputs and land uses are not managed properly, future climate change scenarios can critically impact the groundwater quality in highly agrarian areas.

Tracing the Sources and Fate of NO3- in the Vadose Zone-Groundwater System of a Thousand-Year-Cultivated Region

Excess nitrate (NO₃^-) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO₃^--rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO₃^- in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO₃^- isotope quantification. The results demonstrated that the NO₃^- concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO₃^- stock in the 0-10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the cropland-to-orchard transition aggravated NO₃^- accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO₃^- isotopes (δ¹⁵N and δ¹⁸O), NO₃^- accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27-54%), soil N (SN, 0-64%), and chemical N fertilizer (FN, 4-46%). MN was, by far, the greatest contributor to groundwater NO₃^- (58-82%). The results also indicated that groundwater NO₃^- was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO₃^- in the vadose zone varied from decades to centuries; however, NO₃^- might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.

Direct Electron Transfer Coordinated by Oxygen Vacancies Boosts Selective Nitrate Reduction to N2 on a Co-CuOx Electroactive Filter

Atomic hydrogen (H*) is used as an important mediator for electrochemical nitrate reduction; however, the Faradaic efficiency (FE) and selective reduction to N₂ are likely compromised due to the side reactions (e.g., ammonia generation and hydrogen evolution reactions). This work reports a Co-CuO_x electrochemical filter with CoO_x nanoclusters rooted on vertically aligned CuO_x nanowalls for selective nitrate reduction to N₂, utilizing the direct electron transfer between oxygen vacancies and nitrate to suppress the contribution by H*. At a cathodic potential of -1.1 V (vs Ag/AgCl), the Co-CuO_x filter showed 95.2% nitrate removal and 96.0% N₂ selectivity at an influent nitrate concentration of 20 N-mg L^-1. Meanwhile, the energy consumption and FE were 0.60 kW h m^-3 and 53.5%, respectively, at a permeate flux of 60 L m^-2 h^-1. The presence of abundant oxygen vacancies on Co-CuO_x was due to the change in the electron density of the Cu atom and a decrease of the coordination numbers of Cu-O via cobalt doping. Theoretical calculations and electrochemical tests showed that the oxygen vacancies coordinated nitrate adsorption and subsequent reduction reactions, thus suppressing the contribution of H* to nitrate reduction and leading to a thermodynamically favorable process to N₂ via direct electron transfer.

Enhancing the Understanding of Soil Nitrogen Fate Using a 3D-Electrospray Sensor Roll Casted with a Thin-Layer Hydrogel

Accurate and continuous monitoring of soil nitrogen is critical for determining its fate and providing early warning for swift soil nutrient management. However, the accuracy of existing electrochemical sensors is hurdled by the immobility of targeted ions, ion adsorption to soil particles, and sensor reading noise and drifting over time. In this study, polyacrylamide hydrogel with a thickness of 0.45 μm was coated on the surface of solid-state ion-selective membrane (S-ISM) sensors to absorb water contained in soil and, consequently, enhance the accuracy (R² > 0.98) and stability (drifting < 0.3 mV/h) of these sensors monitoring ammonium (NH₄⁺) and nitrate (NO₃^-) ions in soil. An ion transport model was built to simulate the long-term NH₄⁺ dynamic process (R² > 0.7) by considering the soil adsorption process and soil complexity. Furthermore, a soil-based denoising data processing algorithm (S-DDPA) was developed based on the unique features of soil sensors including the nonlinear mass transfer and ion diffusion on the heterogeneous sensor-hydrogel-soil interface. The 14 day tests using real-world soil demonstrated the effectiveness of S-DDPA to eliminate false signals and retrieve the actual soil nitrogen information for accurate (error: <2 mg/L) and continuous monitoring.

Intensive vegetable production results in high nitrate accumulation in deep soil profiles in China

A comprehensive understanding of the patterns and controlling factors of nitrate accumulation in intensive vegetable production is essential to solve this problem. For the first time, the national patterns and controlling factors of nitrate accumulation in soil of vegetable systems in China were analysed by compiling 1262 observations from 117 published articles. The results revealed that the nitrate accumulation at 0-100 cm, 100-200 cm, 200-300 cm, and >300 cm were 504, 390, 349, and 244 kg N ha^-1, with accumulation rates of 62, 54, 19, and 16 kg N ha^-1 yr^-1 for plastic greenhouse vegetables (PG); for open field vegetables (OF), they were 264, 217, 228, and 242 kg N ha^-1 with accumulation rates of 26, 24, 18, and 10 kg N ha^-1 yr^-1, respectively. Nitrate accumulation at 0-100 cm, 0-200 cm, and 0-400 cm accounted for 5%, 11%, and 17% of accumulated nitrogen (N) inputs for PG, and represented 4%, 9%, and 13% of accumulated N inputs for OF. Nitrogen input rates and soil pH had positive effects and soil organic carbon, water input rate, and carbon to nitrogen ratio (C/N) had negative effects on nitrate accumulation in root zone (0-100 cm soil). Nitrate accumulation in deep vadose zone (>100 cm soil) was positively correlated with N and water input rates, and was negatively correlated with soil organic carbon, C/N, and the clay content. Thus, for a given vegetable soil with relatively stable soil pH and soil clay content, reducing N and water inputs, and increasing soil organic carbon and C/N are effective measures to control nitrate accumulation.