Reconciling agronomic and environmental effects in a maize cropping system through straw and manure integration: A 15-year field study

Soil microorganisms play a pivotal role in regulating nitrogen (N) and carbon (C) cycle, nutrient transformations, and greenhouse gas (GHG) emissions. However, the synergistic effects of straw return and manure substitution on these processes, particularly through alterations in microbial community structure and composition, and their implications for crop yield and GHG emissions remain insufficiently understood. Based on a long-term field experiment established in 2010 with seven treatments: no fertilizer application (CK); conventional fertilizer application with N, phosphorus, and potassium fertilizer (NPK); 30% substitution of NPK with cow manure (NPKM1); 50% substitution of NPK with cow manure (NPKM2); NPK combined with straw return (NPK+S); NPKM1 combined with straw return (NPKM1 + S); NPKM2 combined with straw return (NPKM2 +S). In this study, all field data were collected and measured during the 2023–2024 growing seasons. Compared to conventional NPK fertilization, the combined application of straw and manure (NPKM1 +S and NPKM2 +S) significantly increased soil organic C (SOC) by 14.0–31.2%, with NPKM2 + S achieved the highest maize yield (14.2 Mg ha⁻¹). The sole application of manure (NPKM1 and NPKM2) markedly reduced nitrous oxide (N₂O) emissions, whereas the combination of manure with straw return also mitigated emissions, albeit to a lesser extent. Compared to NPK, NPKM1 and NPKM2 increased carbon dioxide (CO 2 ) emissions by 6.2%-23.6%, whereas NPKM1 + S and NPKM2 + S resulted in larger increases of 26.7%-55.4%. Random forest analysis revealed that soil C-cycling enzyme activity was the strongest predictor of maize yield, while the composition of the N cycle microbial community best explained variations in N₂O emissions. Overall, integrated straw and manure management enhanced soil fertility and crop productivity by promoting microbial-mediated coupled N and C cycle. However, these benefits came with increased GHG emissions and associated environmental costs, highlighting a clear trade‑off between agricultural productivity and environmental sustainability. To determine the optimal fertilization strategy that balances this trade‑off, we applied the Entropy‑TOPSIS method. The results identified NPKM2 + S as the comprehensive optimal strategy, achieving a balance between long-term productivity and environmental sustainability. These findings underscore the need to balance productivity gains with environmental impacts when optimizing straw and manure management practices, thereby guiding the development of more sustainable cropping systems that harness microbial processes for efficient resource utilization.