Reduced nitrogen fertilization enhances microbial community stability in straw-returned agroecosystems

Returning straw to the field is a common practice to increase soil organic carbon (SOC) content, but farm households often apply excessive nitrogen fertilizer to support microbial decomposition of straw, leading to nitrogen loss, soil degradation, and microbial community instability. However, microbial community stability is crucial for straw decomposition and SOC accumulation. Therefore, optimizing nitrogen fertilizer application in straw return systems is particularly important. We conducted a five-year field trial in a dryland wheat field to compare reduced nitrogen fertilization (ND, 108 kg N ha −1 ) with conventional fertilization (CK, 135 kg N ha −1 ) on SOC content and soil microbial community stability. The reduced nitrogen treatment increased dissolved, mineral-associated, particulate, and total organic carbon content by 29.6 %, 14.6 %, 20.4 %, and 18.7 %, respectively ( p < 0.05). This treatment also enhanced microbial community stability. Bacterial and fungal AVD decreased by 9.0 % and 11.5 %, respectively. Robustness of bacterial and fungal community networks increased by 4.1 % and 3.4 % ( p < 0.05). Although bacterial Shannon and Chao1 indices decreased by 2.6 % and 13.2 %, respectively, fungal Chao1 index increased by 15.4 % ( p < 0.05). PLS-PM analysis indicated that soil C:N ratio and microbial keystone taxa positively influenced microbial stability, with a higher soil C:N ratio potentially contributing to enhanced microbial community stability, while bacterial diversity had a negative effect. These results indicate that moderately reduced nitrogen fertilizer application in straw-returned agroecosystems not only enhanced soil organic carbon content but also improved microbial network stability, providing a basis for optimizing nitrogen application in straw-returned agroecosystems. • Moderate N reduction with straw return raised SOC content by 14.6–29.6 %. • Moderate N reduction and straw return raised fungal but reduced bacterial diversity. • Moderate N reduction with straw return changed keystone taxa in soil microbes. • Moderate N reduction with straw return improved soil microbial stability.