Exploring dissolved N2O characteristics and unearthing indirect N2O emission factors in the shallow groundwater of paddy and upland fields.

Indirect emissions of nitrous oxide (N2O) stemming from nitrogen (N) leaching in agricultural fields constitute a significant contributor to atmospheric N2O. Groundwater nitrate (NO3--N) pollution is severe in the Ningxia Yellow River Irrigation Area (NYRIA), coupled with high NO3--N leaching, exacerbates the risk of indirect N2O emissions from groundwater. Over two years of field observations, this study investigated the characteristics and interannual variations of dissolved N2O (dN2O) concentrations and indirect N2O emission factors (EF5g) in shallow groundwater. The research focused on three typical farmlands in the NYRIA, each subjected to six levels of N fertilizer application. The mean dN2O concentrations in the groundwater of paddy, corn and vegetable fields were 5.17, 8.40 and 16.35 µg N·L-1, respectively. Notably, the dN2O concentrations in the shallow groundwater of upland fields exceeded those in paddy fields, with maximum levels in vegetable fields nearly an order of magnitude higher. Elevated N application significantly increased dN2O concentrations across various farmlands, showing statistically significant variation. However, differences in EF5g-A and EF5g-B within the same farmland were negligible. Denitrification was the primary process contributing to N2O production in groundwater, with nitrification also played a crucial role in upland fields. Factors such as NO3--N, NH4+-N, dissolved oxygen (DO), and pH critically influenced N2O production. EF5g-B, which considers the NO3--N consumption during denitrification processes in groundwater, was deemed more appropriate than EF5g-A for assessing the indirect N2O emission in the NYRIA. The EF5g of agricultural fields exhibited minimal sensitivity to N input but was significantly affected by other factors, such as the planting pattern. The study revealed the rationality of adopting EF5g-B in assessing indirect N2O emissions, providing valuable insights for N management strategies in regions with high NO3--N leaching. Minimizing N fertilizer application while ensuring crop yield, especially in upland fields, is beneficial for reducing N2O emissions.

Coping with groundwater pollution in high-nitrate leaching areas: The efficacy of denitrification.

The Ningxia Yellow River irrigation area, characterized by an arid climate and high leaching of NO3--N, exhibits complex and unique groundwater nitrate (NO3--N) pollution, with denitrification serving as the principal mechanism for NO3--N removal. The characteristics of N leaching from paddy fields and NO3--N removal by groundwater denitrification were investigated through a two-year field observation. The leaching losses of total nitrogen (TN) and NO3--N accounted for 10.81-27.34% and 7.59-12.74%, respectively, of the N input. The linear relationship between NO3--N leaching and N input indicated that the fertilizer-induced emission factor (EF) of NO3--N leaching in direct dry seeding and seedling-raising and transplanting paddy fields was 8.2% (2021, R2 = 0.992) and 6.7% (2022, R2 = 0.994), respectively. The study highlighted that the quadratic relationship between the NO3--N leaching loss and N input (R2 = 0.999) significantly outperformed the linear relationship. Groundwater denitrification capacity was characterized by monitoring the concentrations of dinitrogen (N2) and nitrous oxide (N2O). The results revealed substantial seasonal fluctuations in excess N2 and N2O concentrations in groundwater, particularly following fertilization and irrigation events. The removal efficiency of NO3--N via groundwater denitrification ranged from 42.70% to 74.38%, varying with depth. Groundwater denitrification capacity appeared to be linked to dissolved organic carbon (DOC) concentration, redox conditions, fertilization, irrigation, and soil texture. The anthropogenic-alluvial soil with limited water retention accelerated the leaching of NO3--N into groundwater during irrigation. This process enhances the groundwater recharge capacity and alters the redox conditions of groundwater, consequently impacting groundwater denitrification activity. The DOC concentration emerged as the primary constraint on the groundwater denitrification capacity in this region. Hence, increasing carbon source concentration and enhancing soil water retention capacity are vital for improving the groundwater denitrification capacity and NO3--N removal efficiency. This study provides practical insights for managing groundwater NO3--N pollution in agricultural areas, optimizing fertilization strategies and improving groundwater quality.

Ditch level-dependent N removal capacity of denitrification and anammox in the drainage system of the Ningxia Yellow River irrigation district.

Drainage networks, consisting of different levels of ditches, play a positive role in removing reactive nitrogen (N) via self-purification before drainage water returns to natural water bodies. However, relatively little is known about the N removal capacity of irrigation agricultural systems with different drainage ditch levels. In this study, we employed soil core incubation and soil slurry 15N paired tracer techniques to investigate the N removal rate (i.e., N2 flux), denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) rates in the Ningxia Yellow River irrigation district at various ditch levels, including field ditches (FD), paddy field ditches (PFD), lateral ditches (LD1 and LD2), branch ditches (BD1, BD2, BD3), and trunk ditches (TD). The results indicated that the N removal rate ranged from 44.7 to 165.22 nmol N g-1 h-1 in the ditches, in the following decreasing order: trunk ditches > branch ditches > paddy field ditches > lateral ditches > field ditches. This result suggested that the N removal rate in drainage ditches is determined by the ditch level. In addition, denitrification and anammox were the primary pathways for N removal in the ditches, contributing 68.40-76.64 % and 21.55-30.29 %, respectively, to the total N removal. In contrast, DNRA contributed only 0.82-2.15 % to the total nitrate reduction. The N removal rates were negatively correlated with soil EC and pH and were also constrained by the abundances of denitrification functional genes. Overall, our findings suggest that the ditch level should be considered when evaluating the N removal capacity of agricultural ditch systems.

Drainage ditches are significant sources of indirect N2O emissions regulated by available carbon to nitrogen substrates in salt-affected farmlands.

Agriculture is a main source of nitrous oxide (N2O) emissions. In agricultural systems, direct N2O emissions from nitrogen (N) addition to soils have been widely investigated, whereas indirect emissions from aquatic ecosystems such as ditches are poorly known, with insufficient data available to refine the IPCC emission factor. In this contribution, in situ N2O emissions from two ditch water‒air interfaces based on a diffusion model were investigated (almost once per month) from June 2021 to December 2022 in an intensive arable catchment with high N inputs and salt-affected conditions in the Qingtongxia Irrigation District, northwestern China. Our results implied that agricultural ditches (mean 148 µg N m-2 h-1) were significant sources for N2O emissions, and were approximately 2.1 times greater than those of the Yellow River directly connected to ditches. Agronomic management strategies increased N2O fluxes in summer, while precipitation events decreased N2O fluxes. Agronomic management strategies, including fertilization (294--540 kg N hm-2) and irrigation on farmland, resulted in enhanced diffuse N loads in drain water, whereas precipitation diluted the dissolved N2O concentration in ditches and accelerated the ditch flow rate, leading to changes in the residence time of N-containing substances in water. The spatial analysis showed that N2O fluxes (202-233 µg N m-2 h-1) in the headstream and upstream regions of ditches due to livestock and aquaculture pollution sources were relatively high compared to those in the midstream and downstream regions (100-114 µg N m-2 h-1). Furthermore, high available carbon (C) relative to N reduced N2O fluxes at low DOC:DIN ratio levels by inhibiting nitrification. Spatiotemporal variations in the N2O emission factor (EF5) across ditches with higher N resulted in lower EF5 and a large coefficient of variation (CV) range. EF5 was 0.0011 for the ditches in this region, while the EF5 (0.0025) currently adopted by the IPCC is relatively high. The EF5 variation was strongly controlled by the DOC:DIN ratio, TN, and NO3--N, while salinity was also a nonnegligible factor regulating the EF5 variation. The regression model incorporating NO3--N and the DOC:DIN ratio could greatly enhance the predictions of EF5 for agricultural ditches. Our study filled a key knowledge gap regarding EF5 from agricultural ditches in salt-affected farmland and offered a field investigation for refining the EF5 currently used by the IPCC.

Regulation of denitrification/ammonia volatilization by periphyton in paddy fields and its promise in rice yield promotion.

BACKGROUND: Nitrogen (N) is the most limiting nutrient in rice production. N loss via denitrification and ammonia (NH3 ) volatilization decreases N utilization efficiency. The effect of periphyton (a widespread soil surface microbial aggregate in paddy soil) on N-cycling processes and rice growth in paddy soils remain unclear. The purpose of this study was to reveal the interactions of periphyton with the overlying water and sediment in paddy soils on denitrification/NH3 emissions and rice yield by combining pot experiments and path analysis modeling. RESULTS: The sediment exerted significant direct and positive effects on denitrification. The periphyton both directly and indirectly enhanced denitrification, mainly by regulating the ammonium (NH4 + )-N content in the sediment. The total contribution of periphyton to denitrification was stronger than that of the overlying water but smaller than that of the sediment. The pH in the overlying water and the NH4 + -N content in the sediment had a strong positive effect on NH3 volatilization. Although the periphyton biomass and chlorophyll a directly prohibited NH3 emissions, this was counterbalanced by the indirect stimulation effects of the periphyton due to its positive alteration of the pH. Moreover, periphyton facilitated rice yield by 10.2% by releasing N. CONCLUSION: Although the periphyton may have driven N loss by regulating the NH4 + -N content in the sediment and the pH in the overlying water, our study also found that the periphyton was considered a temporary N sink and provided a sustained release of N for rice, thus increasing the rice yield. © 2022 Society of Chemical Industry.

Application of biochar and polyacrylamide to revitalize coastal saline soil quality to improve rice growth.

Poor soil quality is affected by salinity, which limits land productivity and sustainable agricultural development in coastal China. Hence, it is essential to choose suitable and efficient approaches to revitalize coastal saline soil quality and improve agricultural productivity. Biochar and polyacrylamide (PAM) have been widely applied as soil amendments to enhance soil structure, but the interactive effects of biochar and PAM on rice growth are unclear. The experiment described in this study was conducted over five consecutive growing seasons (from 2016 to 2020) with biochar (at 0, 32, and 79 t/hm2) and PAM (at 0, 0.6, and 1.6 t/hm2) applications to study the effects of amendments on soil properties, rice photosynthesis, and rice yield in coastal saline land. The soil property results showed that wheat straw biochar and PAM lowered soil total salt and bulk density, but increased the soil organic matter (SOM), mean weight diameter of water-stable aggregates (MWD), and macroaggregate (> 0.25 mm) content. The application of either biochar or PAM increased the rice net photosynthetic rate, transpiration rate, and stomatal conductance. The combined application of 32 t/hm2 biochar + 0.6 t/hm2 PAM increased the net photosynthetic rate by 26.0% and the transpiration rate by 24.8% relative to the control. The application of 32 t/hm2 biochar and 1.6 t/hm2 PAM significantly increased the rice grain yield. The path analysis model showed that spikelets per panicle and canopy gross photosynthesis had strong and significant positive effects on grain yield, whereas soil total salt had a negative effect on grain yield. The combined application of 32 t/hm2 biochar + 0.6 t/hm2 PAM was identified as the most effective for rice growth. Biochar and PAM amendments at an optimal level may enhance soil properties by reducing salinity. These findings indicate that biochar and PAM have the potential to remediate coastal saline soil quality and the environment, which would simultaneously increase the sustainable use of coastal land resources and food production to preserve the ecological environment.