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.