Abiotic nitrate loss and nitrogenous trace gas emission from Chinese acidic forest soils

Response of soil N2O production pathways to biochar amendment and its isotope discrimination methods

Reducing nitrous oxide (N2O) emission from farmland is crucial for alleviating global warming since agriculture is an important contributor of atmospheric N2O. Returning biochar to agricultural fields is an important measure to mitigate soil N2O emissions. Accurately quantifying the effect of biochar on the process of N2O production and its driving factors is critical for achieving N2O emission mitigation. Recently, stable isotope techniques such as isotope labeling, natural abundance, and site preference (SP) value, have been widely used to distinguish N2O production pathways. However, the different isotope methods have certain limitations in distinguishing N2O production in biochar-amended soils where it is difficult to identify the relative contribution of individual pathways for N2O production. This paper systematically reviews the pathways of soil N2O production (nitrification, nitrifier denitrification, bacterial denitrification, fungal denitrification, coupled nitrification-denitrification, dissimilatory nitrate reduction to ammonium and abiotic processes) and their response mechanism to the addition of biochar, as well as the development history and advantages of isotopes in differentiating N2O production pathways in biochar-amended soils. Moreover, the limitations of current research methods and future research directions are proposed. These results will help resolve how biochar affects different processes that lead to soil N2O generation and provide a scientific basis for sustainable agricultural carbon sequestration and the fulfilment of carbon neutrality goals.

Contribution and Driving Mechanism of N2O Emission Bursts in a Chinese Vegetable Greenhouse after Manure Application and Irrigation

Solar greenhouse vegetable fields have been found to be hotspots of nitrous oxide (N2O) emissions in China, mainly due to excessive manure application and irrigation. Pulses of N2O emissions have been commonly reported by field monitoring works conducted in greenhouse fields, though their significance regarding total N2O emissions and the driving mechanism behind them remain poorly understood. N2O fluxes were monitored in situ using a static opaque chamber method in a typical greenhouse vegetable field. Then, laboratory incubations were conducted under different soil moisture and manure application gradients to monitor nitrous oxide emissions and related soil properties, using a robotized incubation system. Field monitoring showed that the occurrence of clear N2O emission bursts closely followed fertilization and irrigation events, accounting for 76.7% of the annual N2O efflux. The soil N2O flux increased exponentially with the water-filled pore space (WFPS), causing extremely high N2O emissions when the WFPS was higher than 60%. During the lab incubation, emission bursts led to N2O peaks within 40 h, synchronously changing with the transit soil NO2−. An integrated analysis of the variations in the gas emission and soil properties indicated that the denitrification of transit NO2− accumulation was the major explanation for N2O emission bursts in the greenhouse filed. Nitrous oxide emission bursts constituted the major portion of the N2O emissions in the Chinese greenhouse soils. Nitrite (NO2−) denitrification triggered by fertilization and irrigation was responsible for these N2O emission pulses. Our results clarified the significance and biogeochemical mechanisms of N2O burst emissions; this knowledge could help us to devise and enact sounder N2O mitigation measures, which would be conducive to sustainable development in vegetable greenhouse fields.

Nitrogen deposition increases N2O emission from an N-saturated subtropical forest in southwest China

Nitrous oxide (N₂O) is a major greenhouse gas, with elevated emission being reported from subtropical forests that receive high nitrogen (N) deposition. After 10 years of monthly addition of ammonium nitrate (NH₄NO₃) or sodium nitrate (NaNO₃) to a Mason pine forest at Tieshanping, near Chongqing city in Southwest China, the simulated N deposition was stopped in October 2014. The results of soil N₂O emissions monitoring in different seasons during the nitrogen application period showed that nitrogen addition significantly increased soil N₂O emission. In general, the N₂O emission fluxes were positively correlated to nitrate (NO₃^-) concentrations in soil solution, supporting the important role of denitrification in N₂O production, which was also modified by environmental factors such as soil temperature and moisture. After stopping the application of nitrogen, the soil N₂O emissions from the treatment plots were no longer significantly higher than those from the reference plots, implying that a decrease in nitrogen deposition in the future would cause a decrease in N₂O emission. Although the major forms of N deposition, NH₄⁺ and NO₃^-, had not shown significantly different effects on soil N₂O emission, the reduction in NH₄⁺ deposition may decrease the NO₃^- concentrations in soil solution faster than the reduction in NO₃^- deposition, and thus be more effective in reducing N₂O emission from N-saturated forest soil in the future.