Distinct nitrogen application strategies and straw incorporation modulate carbon mineralization rate and SOC dynamics by regulating soil microbiome composition

The impact of conventional nitrogen (N) application on straw degradation is well-established, but how precise N placement affect decomposition kinetics, microbial community shifts, and soil organic carbon (SOC) transformations remains poorly understood. In this context, a 90-day lab incubation study was conducted to evaluate three N application strategies: uniform mixed application (MA), localized band application (BA), and targeted super granule application (TA), along with a non-fertilized control under two straw conditions (with and without straw). Straw incorporation markedly increased CO2 efflux by 85%, cumulative C mineralization (Cmin) by 111%, and microbial metabolic quotient by 77%. Kinetic modeling revealed that N application, especially with MA, enhanced the Cmin rate regardless of straw presence. Although N application methods alone had negligible impact on total SOC, straw addition markedly increased SOC accrual. Notably, MA enhanced easily oxidizable and microbial biomass C, whereas TA predominantly increased both labile (particulate and dissolved organic C) and stabilized (mineral-associated organic C) fractions. N application increased the relative abundances of Patescibacteria, Bacteroidota, and Pseudomonadota, while straw incorporation further enriched Chloroflexota. MA distinctly promoted Rhodanobacter and suppressed Chujaibacter, whereas TA caused fewer shifts in microbial composition. Fungal communities were dominated by Ascomycota (>95%) under straw addition, with MA favoring Fusarium and Leptogium, and TA promoting Fusarium, Angulomyces, and Aspergillus, indicating greater fungal diversity under TA. Collectively, co-application of N and straw strongly modulated SOC fractions by regulating soil microbial communities. Moreover, TA promoted both labile and stable C fractions under straw incorporation and supported SOC build-up primarily by reducing the Cmin rate.