Combined application of straw and biochar: reducing nitrogen fertilizer demand and enhancing soybean yield

Soybean (Glycine max (L.) Merr.) is an important food crop in Northeast China. However, intensive cultivation relying on long-term high nitrogen fertilizer input has led to the loss of soil organic carbon and the decline of cultivated land quality in the black soil region, compromising soil sustainability. Therefore, it is urgent to explore a cultivation mode that achieves both stable soybean yield and coordinated improvement of soil sustainability. The application of biochar and straw return presents a promising strategy to enhance crop productivity and agricultural sustainability. However, the mechanisms underlying these improvements, particularly those driven by the combined application of straw and biochar under reduced nitrogen application, remain unclear. From 2022 to 2023, a field experiment was conducted on black-calcium soil using 'Dongnong 55' soybeans. Two biomass-return methods were evaluated: straw addition and a straw-biochar mixture addition, under six treatments: CK (conventional N with full straw return), SN2 (80% N with full straw), SN3 (60% N with full straw), SBN1 (conventional N with straw - biochar mix), SBN2 (80% N with straw - biochar mix), and SBN3 (60% N with straw - biochar mix). The results revealed that straw-biochar co-application improved the soil hydrothermal environment and nutrient supply compared with straw return alone. In the 0-20 cm soil layer, soil moisture content increased by 9.0% - 14.3%, while available N, P, and K increased by 47.5%-48.9%, 56.8%-71.7%, and 28.4%-41.2%, respectively. These improvements enhanced soybean physiological performance, increasing chlorophyll content, net photosynthetic rate, and leaf intrinsic water use efficiency by 12.3%, 20.6%, and 16.9%, respectively. Notably, SBN2 increased soybean yield by 9.3%-11.0% compared with SN2, and maintained a yield statistically comparable to SBN1 despite a 20% reduction in N input. Therefore, SBN2 was identified as the optimal treatment because it maintained soybean yield while reducing N fertilizer input and improving soil water, heat, nutrient supply, and resource-use efficiency. This study provides a valuable reference for managing agricultural straw, biochar utilization, and nitrogen fertilizer.