Biochar as a tool to reduce environmental impacts of nitrogen loss in water-saving irrigation paddy field

Nitrogen (N) loss caused by excessive and incorrect application of N fertilizer in traditional rice-growing agriculture contributed to rural non-point source pollution and greenhouse gas emission, reduced crop N utilization efficiency. The objective of this study is to explore the response of N loss (leaching, NH3 volatilization and N2O emission) to different biochar applications (0,20,40 t/hm2) and irrigation patterns (controlled irrigation, flooding irrigation) in paddy fields of the Taihu Lake region (TLR) with the combination of field experiments and laboratory analysis. Based on the long-term observation of rice growth period, the ammonium N (NH4+–N), nitrate N (NO3−–N) and total N (TN) leaching of paddy fields applied with water-saving irrigation (WSI) and biochar was measured in the plots of lysimeter. The impact of biochar application on NH3 volatilization, N2O emission in different water-carbon modes were clarified and the effect of water-carbon management modes on rice yield and yield components was analyzed. The results showed that the application of biochar reduced the NO3−–N and TN leaching by 29.98%–38.63% and 12.77%–13.36%, increased rice seed setting rate and actual yield by 12.15%–15.28% and 1.67%–5.54%. Biochar addition inhibited NH3 volatilization and N2O emission. Under low-volume biochar (20 t/hm2), the main form of N leaching in rice fields was NO3−–N, the NO3−–N leaching accounted for 33.67% – 50.24% of the TN leaching; under high-volume biochar (40 t/hm2), N leaching in rice field was dominated by NH4+–N, the NH4+–N leaching accounted for 16.50% – 47.47% of the TN leaching. Controlled irrigation can not only reduce the water leakage in paddy fields but also reduce the N concentration in water leakage compared with flooding irrigation so that the total leaching of NH4+–N, NO3−–N and TN in rice fields can be reduced by 23.44%, 47.89% and 50.69%. This study suggests that to make biochar beneficial for reducing environmental impacts of soil N effluxes and realizing cleaner production in rice cultivation, WSI technique and adapted use of biochar are of great importance.