Moderate N fertilizer reduction with straw return modulates cropland functions and microbial traits in a meadow soil

Abstract. Nitrogen (N) fertilization has received worldwide attention due to its benefits with regard to soil fertility and productivity, but excess N application also causes an array of ecosystem degenerations, such as greenhouse gas emissions. Generally, soil microorganisms are considered to be involved in upholding a variety of soil functions. However, the linkages between soil cropland properties and microbial traits under different N fertilizer application rates remain uncertain. To address this, a 4-year in situ field experiment was conducted in a meadow soil from the Northeast China Plain after straw return with the following treatments combined with regular phosphorus (P) and potassium (K) fertilization: (i) regular N fertilizer (N + PK), (ii) 25 % N fertilizer reduction (0.75N + PK), (iii) 50 % N fertilizer reduction (0.5N + PK), and (IV) no N fertilizer (PK). Cropland properties and microbial traits responded distinctly to the different N fertilizer rates. Treatment 0.75N + PK had overall positive effects on soil fertility, productivity, straw decomposition, and microbial abundance and functioning and alleviated greenhouse effects. Specifically, no significant difference was observed in soil organic carbon (SOC), total N, P content, straw C, N release amounts, microbial biomass C, N content, and cellulase and N-acetyl-D-glucosaminidase activities, which were all significantly higher than in 0.5N + PK and PK. Greenhouse gas emissions was reduced with the decreasing N input levels. Moreover, the highest straw biomass and yield were measured in 0.75N + PK, which were significantly higher than in 0.5N + PK and PK. Meanwhile, 0.75N + PK up-regulated aboveground biomass and soil C:N and thus increased the abundance of genes encoding cellulose-degrading enzymes, which may imply the potential ability of C and N turnover. In addition, most observed changes in cropland properties were strongly associated with microbial modules and keystone taxa. The Lasiosphaeriaceae within the module-1 community showed significant positive correlations with straw degradation rate and C and N release, while the Terrimonas within the module-3 community showed a significant positive correlation with production, which was conducive to soil multifunctionality. Therefore, our results suggest that straw return with 25 % chemical N fertilizer reduction is optimal for achieving soil functions. This study highlights the importance of abiotic and biotic factors in soil health and supports green agricultural development by optimizing N fertilizer rates in meadow soil after straw return.