Biochar amendment reduces paddy soil nitrogen leaching but increases net global warming potential in Ningxia irrigation, China

The efficacy of biochar as an environmentally friendly agent for non-point source and climate change mitigation remains uncertain. Our goal was to test the impact of biochar amendment on paddy rice nitrogen (N) uptake, soil N leaching, and soil CH4 and N2O fluxes in northwest China. Biochar was applied at four rates (0, 4.5, 9 and13.5 t ha−1 yr−1). Biochar amendment significantly increased rice N uptake, soil total N concentration and the abundance of soil ammonia-oxidizing archaea (AOA), but it significantly reduced the soil NO3−-N concentration and soil bulk density. Biochar significantly reduced NO3−-N and NH4+-N leaching. The C2 and C3 treatments significantly increased the soil CH4 flux and reduced the soil N2O flux, leading to significantly increased net global warming potential (GWP). Soil NO3−-N rather than NH4+-N was the key integrator of the soil CH4 and N2O fluxes. Our results indicate that a shift in abundance of the AOA community and increased rice N uptake are closely linked to the reduced soil NO3−-N concentration under biochar amendment. Furthermore, soil NO3−-N availability plays an important role in regulating soil inorganic N leaching and net GWP in rice paddies in northwest China.