Straw Return Strategies Compensate for Nitrogen Reduction by Enhancing Chitinase Gene Abundance and Soil Quality in a Mollisol

In Northeast China’s black soil region, we examined how different straw return strategies—no return (CK), direct straw incorporation (SD), and biochar application (BC)—interact with three nitrogen levels (N0, N60, N100) to affect soil properties, enzyme activities, and the chitin-degrading ChiA functional gene. Both SD and BC significantly increased soil nutrients (total carbon, ammonium nitrogen, available potassium, and soil organic matter) and reduced bulk density. They also enhanced the activities of key carbon—and nitrogen-cycling enzymes, including nitrate reductase and N-acetylglucosaminidase. Straw return method and nitrogen rate both influenced ChiA abundance, but straw management was the primary driver of ChiA community structure according to principal component analysis. Actinobacteria and Proteobacteria dominated the bacterial phyla. Correlation analysis identified bulk density, total nitrogen, C/N ratio, available potassium, soil organic matter, and NAG activity as key factors shaping the ChiA community. Both straw return strategies improved soil fertility and chitin degradation potential. Among them, direct straw return combined with conventional nitrogen application (SD N100) showed the most balanced performance in sustaining soil health and agricultural productivity in the Mollisol region.