Mechanism of Modified Biochar in Mitigating Carbon and Nitrogen Loss in Drought Soil with Green Manure Application

With the frequent occurrence of global droughts, modified biochar has demonstrated the potential to be an efficient soil amendment, which could affect carbon and nitrogen sequestration in arid soil. Therefore, this study investigated the co-application of pristine biochar (BC), Fe-modified biochar (FB) and H2O2-modified biochar (HB) with green manure during a 70-day laboratory incubation under drought conditions and normal moisture conditions. The emissions were quantified using gas chromatography, while microbial necromass carbon and nitrogen were measured by quantifying the amino sugar content by gas chromatography–mass spectrometry, and other soil carbon and nitrogen fractions were determined through chemical analysis. The results revealed that under drought conditions, compared to BC co-application with green manure, the total carbon loss of FB and HB co-application with green manure was reduced from 24.38% to 13.14% and 14.27%, respectively, and the total nitrogen loss was also reduced from 14.61% to 7.23% and 7.27%, respectively. This reduction occurred because FB and HB protected soil organic matter through iron oxide binding and micropore adsorption, thereby increasing the content of soil total humus acid (>5%) and microbial necromass nitrogen (>16%). In addition, under normal moisture conditions, BC, FB and HB co-application with green manure enhanced microbial activity and promoted the formation of stable total humus acid, thereby enhancing carbon and nitrogen sequestration. In conclusion, this study provides crucial theoretical support for the optimization of the green manure return via modified biochar co-application in arid environments.