Bacteria drive SOC sequestration within aggregates under nutrient limitation in broadleaf-modified coniferous plantations

Mixing broadleaf species with coniferous plantations alters soil nutrient stoichiometry and microbial resource acquisition strategies. However, how bacterial taxa’s resource acquisition strategies affect soil aggregate organic carbon (OC) sequestration following stand conversion remains unclear. This study investigated stand conversion in subtropical China to assess microbial resource limitation in soil aggregates and explore how bacterial taxa adaptation regulates aggregate OC sequestration. Mixing broadleaf species with coniferous plantations altered soil nutrient contents and the activities of resource-acquisition enzymes, with clear aggregate-size dependence. Across stand types, aggregate-associated microbes were generally co-limited by nitrogen (N) and phosphorus (P), with a stronger limitation by P; bacterial communities adapted to nutrient changes through taxonomic adjustment and enzyme investment, increasing their allocation toward acquisition of the most limiting nutrient. Investment in N-acquisition enzymes (NAG and LAP) showed pronounced functional differentiation among bacterial taxa; but these taxa did not fully conform to a simple r/K strategy classification and were more likely governed by substrate accessibility and microhabitats. Moreover, NAG and its associated bacterial taxa were negatively correlated with aggregate OC, supporting an “N mining” mechanism, whereas LAP and its associated taxa promoted aggregate OC accumulation. SEM further indicated that LAP can offset aggregate OC losses induced by NAG. Following stand conversion, mixed stands with higher N content and LAP (and associated bacterial taxa), but lower NAG (and associated taxa), showed substantially enhanced SOC sequestration. Overall, SOC accumulation after stand conversion is coupled with bacterial N-acquisition strategies, and increasing N availability to bacteria in coniferous plantations promote SOC sequestration. • Broadleaved species introduction did not alter soil microbial N and P co-limitation. • Bacteria enhanced limiting-resource acquisition via taxonomic shifts and enzyme investment. • Bacterial taxa showed functional partitioning in N acquisition via LAP and NAG. • Bacterial N-acquisition strategies regulated aggregate OC storage via distinct enzymatic pathways.