Aggregate size mediates the stability and temperature sensitivity of soil organic carbon in response to decadal biochar and straw amendments

The temperature sensitivity (Q10) of soil organic carbon (SOC) decomposition governs soil-climate feedbacks, yet how soil management mediates Q10 through aggregate-scale processes remains unclear. Through a 14-year field experiment comparing biochar and maize straw amendments, we demonstrated that aggregate size critically mediated SOC stability and temperature responses. Biochar addition enhanced SOC sequestration by 49–110 % while suppressing mineralization by 4.9–14 %, primarily through preferential stabilization in small macroaggregates (SMA) and microaggregates (MA) (i.e., increased benzene polycarboxylic acids and decreased 14C age and δ13C). These small aggregates exhibited high SOC stability and low Q10 due to enhanced mineral association, and elevated microbial carbon use efficiency (+11–39 %) for microbial necromass accrual (+35–92 %). By contrast, large macroaggregates (LMA) showed limited SOC sequestration capacity due to thermal disruption of Fe-associated SOC that led to high Q10. Maize straw preferentially sequestered SOC in SMA through physical occlusion (i.e., 36 % increase in MAOM, 45 % increase in Fe-oxides) but increased bulk Q10 by 89 % due to temperature-sensitive decomposition of labile straw-derived C. The correlation analysis indicated that while mineral protection reduced SOC mineralization across all aggregates, its concurrent increase in Q10 highlighted a warming vulnerability tradeoff. Our findings establish that biochar outperforms straw in decoupling SOC turnover from warming through aggregate-specific stabilization pathways, providing critical insights for optimizing soil amendments to mitigate carbon-climate feedbacks in agricultural systems.