Successive corn stover and biochar applications mitigate N2O emissions by altering soil physicochemical properties and N-cycling-related enzyme activities: A five-year field study in Northeast China

As the most important source of commodity grain in China, Northeast China plays a crucial role in guaranteeing national food security. The application of biochar in agricultural ecosystems has shown immense potential in ensuring crop yields while mitigating N2O emissions. However, the long-term impacts and mechanisms of suc-cessive application of corn stover and its derived biochar on soil N2O emission in the corn monocropping region, such as Northeast China, remain unclear. A five-year field experiment was conducted in Northeast China to fill this research gap regarding the soil nitrogen (N) cycle. Three experimental treatments were established: a control with no corn stover or biochar application (CK), corn stover returned yearly at a rate of 7.5 t ha-1 (ST), and biochar added yearly at a rate of 2.6 t ha-1 (BC) corresponding to 35% of the stover addition rate in ST. Identical mineral fertilizers were applied in all three treatments. The total accumulated N2O emissions across five analyzed growing seasons within the CK, ST, and BC treatments were 260.2, 184.0, and 151.0 mg N2O m- 2, respectively, with significant 29.3% and 42.0% decreases observed in ST and BC treatments, respectively, compared with the control. Based on the annual average from 2013 to 2017, ST and BC treatments significantly increased soil pH and total N (TN) content and significantly decreased the levels of NO3--N, NH4+-N, and soil bulk density (BD). Additionally, BC treatment remarkably lowered the average NH4+-N content than that under ST treatment by 24.1%. Both ST and BC showed a significant inhibitory effect on nitrate reductase (NR), nitrite reductase (NiR), and hydroxylamine reductase (HyR) activities related to denitrification. Soil N2O emission was significantly positively correlated with BD, NO3--N, NH4+-N, NR, NiR, and HyR activities. Path analysis suggested that N2O emissions were primarily driven by NR and HyR activities and NO3--N content. Overall, the applications of corn stover and its derived biochar are a feasible means of mitigating soil N2O emissions in the context of continuous corn cropping systems in mid-latitude regions like Northeast China, with biochar being more beneficial than stover application.