Straw additions act as carbon sinks in typical cornfields despite bidirectional promotion of soil autotrophic and heterotrophic respiration

Understanding the drivers and mechanisms underlying variations in autotrophic (Ra), heterotrophic (Rh), and total soil respiration (Rs) is essential for improving carbon management in croplands. In this four-year field study (2018–2021) conducted in a maize system, we compared practices without straw (NS) and with straw mulch (SM). Random Forest analysis identified bulk density, soil temperature, soil inorganic carbon, and urease activity as the primary determinants of Ra, Rh, and Rs. Straw mulch improved soil biodiversity, species richness, α-diversity, and evenness. Piecewise structural equation modeling further showed that soil microenvironmental changes regulate substrate availability by altering soil biological properties, which subsequently shape bacterial communities and microbial metabolism, ultimately controlling Rs. Compared with NS, on average over the four-year study period, SM substantially enhanced Ra and Rh by 96.9% and 82.2%, respectively, and increased maize grain yield and aboveground biomass by 9.7% and 6.2%. Notably, SM shifted the net ecosystem carbon budget from negative to positive, indicating a pronounced carbon sink effect. Overall, our findings highlight the pivotal role of straw amendments in regulating Rs and its components and elucidating the pathways through which these effects occur. These results provide mechanistic evidence supporting SM as a climate-smart agricultural practice that can simultaneously enhance crop productivity and contribute to agricultural carbon sequestration strategies and carbon neutrality targets. Effects and driving mechanisms of straw additions on soil respiration and net ecosystem carbon budget. The outer brown dashed line represents the indirect effect of soil properties and bacterial structure affecting soil respiration. Rh: soil heterotrophic respiration; Rs: soil respiration; NPP: net primary productivity; NEP: net ecosystem production; NECB: net ecosystem carbon budget. NS: no straw; SM: straw mulch. * Represents the level of significance (* P < 0.05; ** P < 0.01) and ns represents non statistically significant. • Straw mulch had a bidirectional promotion effect on Ra (+96.9%) and Rh (+82.2%). • Straw mulch had a significant C sink effect. • Straw mulch increased the species richness, α-diversity, evenness, and modularity degree. • Main factors affecting Rs was soil substrate availability, followed by soil microenvironment.