Nitrous oxide production and mRNA expression analysis of nitrifying and denitrifying bacterial genes under floodwater disappearance and fertilizer application

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.

Remarkable N2O emissions by draining fallow paddy soil and close link to the ammonium-oxidizing archaea communities

Fallow paddies experience natural flooding and draining water status due to rainfall and evaporation, which could induce considerable nitrous oxide (N₂O) emissions and need to be studied specially. In this study, intact soil columns were collected from a fallow paddy field and the flooding-draining process was simulated in a microcosm experiment. The results showed that both N₂O concentrations in the soil and N₂O emission rates were negligible during flooding period, which were greatly elevated by draining the fallow paddy soil. The remarkable N₂O concentrations in the soil and N₂O emission/h during draining both had significant relationships with the Arch-amoA gene (P < 0.01) but not the Bac-amoA, narG, nirK, nirS, and nosZ genes, indicating that the ammonium-oxidizing archaea (AOA) might be the important players in soil N₂O net production and emissions after draining. Moreover, we observed that N₂O concentrations in the upper soil layers (0-10 cm) were not significantly different from that in the 10-20 cm layer under draining condition (P > 0.05). However, the number of AOA and the nitrification substrate (NH₄⁺-N) in the 0-10 cm layer were significantly higher than in the 10-20 cm layer (P < 0.01), indicating N₂O production in the 0-10 cm layer might be higher than the measured concentration and would contribute considerably to N₂O emissions as shorter distance of gas diffusion to the soil surface.