CO₂ emission and microbial community dynamics in soybean farmland under different organic material returning

Fertilization not only influences crop productivity but also plays a pivotal role in regulating soil CO₂ emissions within agricultural ecosystems. This study aimed to elucidate the mechanisms governing soil CO₂ emissions under different fertilization regimes in the Northeast China Black Soil Region. Field experiments were conducted using four treatments: mineral fertilizer (NPK), and NPK combined with biochar (NPK + BC), organic fertilizer (NPK + OF) or straw (NPK + SF). A comprehensive assessment was performed, including measurements of soil CO₂ flux, physicochemical properties, microbial community composition, plant physiological traits and crop yield. The results showed that, compared with the CK, soybean yields significantly increased by 10.00%, 8.15% and 3.70% under NPK + BC, NPK + OF and NPK + SF, respectively. Regarding mature stage soil properties, NPK + BC, NPK + OF and NPK + SF significantly elevated soil organic matter (SOM) by 16.30%, 23.86% and 1.39%, meanwhile available phosphorus (AP) by 19.76%, 24.00% and 6.78%, respectively. CO₂ flux was reduced by 24.66% in NPK + BC but increased by 41.34% in NPK + OF. Furthermore, fertilization significantly altered microbial diversity; the ACE index increased by 7.82% under NPK + BC in the mature stage. Specifically, Proteobacteria abundance rose by 41.30% under NPK + OF, while Basidiomycota-associated with carbon sequestration-increased by 163.88% under NPK + BC. Structural equation modelling indicated that variation in soil CO₂ flux was associated with SOM and microbial metabolic responses. DNDC model projections suggest that adopting biochar-based fertilization could reduce CO₂ emissions in Heilongjiang Province by approximately 586,465 tons annually. These findings provide quantitative evidence for developing low-carbon, yield-enhancing fertilization strategies in sustainable soybean production.