Long‐term optimization of crop yield while concurrently improving soil quality

Sustainably feeding the growing human population requires improvements in both soil quality and nitrogen (N) management. However, in response to N fertilizer addition, whether soil quality will be optimized at N application rates that maximize yields is rarely explored from a long‐term perspective. We conducted a 9‐year field experiment to examine agronomic and soil quality indices in a wheat (Triticum aestivum L.)/maize (Zea mays L.) double cropping system. Optimal nitrogen rates (ONR) were determined by in‐season soil NO3−‐N testing. Other treatments included control plots, 50–70% ONR, 130–150% ONR, and local farmers' N practices (FNP). The ONR increased grain yield by 10.5% in comparison with the 50–70% ONR treatment and achieved yields comparable to the 130–150% ONR treatment. The ONR treatment reduced the applied N fertilization rate by 38.5% in comparison with FNP levels but without any yield losses. After N fertilization for 9 years, soil organic carbon (SOC) stocks, C sequestration rates, and soil microbial biomass C were the highest with the ONR treatment. Relative to the control, the ONR treatment significantly increased the weight diameter of soil water‐stable aggregates, reduced the richness and diversity of fungi, and reduced the richness of bacteria in comparison with the FNP treatment. Furthermore, the soil microbial community structure had a significant relationship with the SOC, organic C input, and soil inorganic N content, which was ascribed to the driving of N management strategy. These results highlight the importance of the optimization of N management to produce high grain with less N fertilizer input in agricultural systems while concurrently promoting soil quality and providing benefits to the microbial community.