Aggregate size regulates soil carbon priming through divergent microbial CN acquisition strategies

Soil aggregates create heterogeneous microenvironments that influence soil organic carbon (SOC) turnover and microbial communities. However, how aggregate size regulates priming effects (PE) through microbial life-history strategies and C N acquisition pathways remains unclear. We examined PE dynamics across macroaggregates (>2000 μm, MA), mesoaggregates (250–2000 μm, ME), microaggregates (<250 μm, MI), and bulk soil (BS) under two exogenous organic matter (EOM) inputs (0.1% and 1%) and two temperatures (15 and 25 °C). SOC and its labile fractions were enriched in MA. Larger aggregates showed higher relative abundances of r-strategist taxa, whereas MI showed increased proportions of K-strategist taxa. Cumulative PE was primarily controlled by EOM input and was lower at 25 °C than at 15 °C. Across aggregate fractions, PE generally decreased in the order MA > ME > MI, with MI showing a near-zero or slightly negative tendency under 25 °C + 0.1% EOM. In MA, positive correlations of PE rate with ΔDOC and ΔDN, coupled with balanced C- and N-acquiring enzyme responses, suggested co-metabolic decomposition of native SOC. In contrast, MI showed a negative correlation between PE rate and ΔDN and stronger relative investment in N-acquiring enzymes, suggesting an N-acquisition-constrained priming response rather than classical nitrogen mining. These findings suggest that aggregate size controls PE by shaping SOC accessibility, microbial r/K strategies, and C N acquisition pathways. Resource-rich MA and ME favor r-strategist-associated co-metabolism and stronger positive PE. In contrast, mineral-protected MI suggested enhanced microbial N-acquisition demand, but limited SOC accessibility constrained its translation into strong SOC priming.