Abstract
Soils are the largest terrestrial carbon (C) reservoir and play a crucial role in climate regulation; yet, sustaining soil organic carbon (SOC) stocks in agricultural systems remains a challenge. Although advances have been made in understanding SOC dynamics, the mechanisms controlling C stabilisation and decomposition remain uncertain, especially regarding the interplay of SOC content, soil texture, microbial community composition and environmental stressors such as drought. In this study, we addressed the key question whether high-SOC, high-clay soils support greater C decomposition of added litter due to additional breakdown of native SOC, or whether these soils have smaller C decomposition due to more C stabilisation. Additionally, the legacy effect of 120-day reduced precipitation was studied. We conducted a six-month incubation study using soils collected in a barley field along a natural gradient of SOC, clay and pH, with precipitation reduced by rainout shelters. Soils were amended with 13C-enriched plant litter to trace fresh C inputs into different C pools. We found that litter-derived CO2 production was highest in coarse-textured, low-SOC soils, supporting our hypothesis that clay content controls C stabilisation and decomposition. Accordingly, high-SOC, high-clay soils supported increased formation of mineral-associated organic carbon (MAOC), indicating rapid stabilisation and protection of fresh C by minerals. MAOC formation efficiency correlated positively with clay content and native SOC, suggesting that increased C input could further enhance SOC storage. Reduced precipitation did not affect litter decomposition or C stabilisation, indicating that short-term precipitation reduction has little effect on soil C turnover in cool-humid upland soils. Metagenomic and amplicon analyses revealed that microbial community structure and functional potential were largely stable across the gradient and unaffected by reduced precipitation. Our findings suggest that clay content, rather than native SOC or microbial community composition, is the primary factor shaping litter-C turnover and stabilisation on a field scale.