Soil carbon quality determined the responses of respiration components to nitrogen fertilization and straw return

Abstract

The responses of soil respiration (Rs) and its heterotrophic (Rh) and autotrophic (Ra) components to straw return remain equivocal, with microbial-enzymatic mechanisms under co-applied nitrogen (N) fertilization poorly characterized, limiting rational straw and N managements for soil carbon (C) sequestration in agroecosystems. Here, we quantified annual fluxes of Rs, Rh, and Ra in paired northeast China croplands with equivalent soil organic C quantity but distinct C quality (high vs. low: HCQ vs. LCQ), under four treatments: no N fertilization/straw return (Control), N fertilization-only (NF), straw return-only (SR), and co-applications (SRF). Annual Rs emissions were consistently higher in HCQ than in LCQ soils, primarily driven by Rh elevation from greater substrate C bioavailability. Crucially, N fertilization induced C quality-dependent divergence: suppressing Rs in HCQ soils through stoichiometric constraints that diverted C flux from Rh to microbial assimilation, while maintaining negligible impacts in LCQ soils due to persistent microbial N mining rather than metabolic suppression. Straw return universally stimulated Rs (26–31 %) via hydrolytic enzyme-mediated Rh amplification following C amendments. Notably, HCQ soils exhibited significantly faster straw-induced mineralization than LCQ soils (75 % vs. 43 % efficiency), attributed to HCQ’s superior fungal-oxidase capacity targeting recalcitrant C. Regarding interactive effects, N fertilization dampened straw-induced Rh in HCQ by diverting residue-C from CO₂ release to stabilization, reducing mineralization efficiency to 16 %, whereas in LCQ, SRF maintained Rh at SR levels under persistent substrate constraints. Conversely, Ra consistently depended on N supplementation, mechanistically evidenced by increased plant biomass and chlorophyll content. Under straw return and N fertilization interactions, annual Ra increased only in LCQ soils, consistent with improved mineral N availability and plant N status, enhancing belowground C allocation, whereas HCQ showed little Ra response. Collectively, our findings establish soil C quality as the pivotal regulator dictating microbe-plant resource partitioning. Precision management must implement C quality-stratified straw and N coordination to synchronize climate mitigation with sustainable productivity.