Effects of biochar on N2O emission in denitrification pathway from paddy soil: A drying incubation study.

N2O emission from paddy soil is a potential environmental risk, especially when the soil moisture content of paddy soil changes and excessive nitrogen retention occurs. Biochar is known to have a positive effect on reducing N2O emissions. However, the influence of different types of biochar on N2O emission with varying soil moisture contents is unclear. The objective of this study was to investigate the effects of biochar made from different feedstocks and at different pyrolysis temperatures on the release of N2O during drying process of paddy soil. An incubation experiment with four kinds of biochar (rice straw and rice husk biochar pyrolyzed at 400 °C and 700 °C, respectively) applied at 1% (w/w) was conducted on paddy soil with the same initial moisture content (105% water-filled pore space). The emission rate of N2O, concentrations of ammonium and nitrate, and the abundance of N2O related microbial functional genes (narG and nosZ) were monitored throughout the incubation period. Biochar amendments reduced cumulative N2O emissions by 56.8-90.1% compared to the control. Low-temperature rice straw biochar decreased nosZ gene abundance, downregulated the denitrification pathway, and reduced nitrogen loss and N2O emission. The low-temperature pyrolysis rice husk biochar and the control showed similar trends in narG and nosZ gene abundance and N2O emission. The high-temperature pyrolysis of rice straw and rice husk biochar showed opposite trends in narG gene abundance, but both increased nosZ gene abundance at the later incubation period. Different feedback on denitrification-derived N2O emission in biochar application was revealed in this study by establishing a link between biotic and abiotic factors, showing that caution should be exercised when considering the use of biochar to mitigate N2O emission under drying soil conditions.