Frequent Droughts Reduce Carbon Stabilisation in Organo-Mineral Soils

Climate change is increasing drought frequency, threatening the stability of soil carbon sinks. While droughts are known to accelerate soil organic matter decomposition and enhance CO₂ emissions, the long-term effects of recurrent droughts on soil remain unclear. We addressed this pressing issue by modelling long-term drought events in a temperate heathland on organo-mineral soil using the ECOSSE biogeochemical model and developing new metrics to assess changes in soil organic carbon (SOC) sequestration and stabilisation. Across all scenarios, drought events decreased the size of microbial (BIO) and humified (HUM) SOC pools by up to 15% and 8% respectively. Short-interval droughts weakened the BIO-to-HUM transfer, leading to incomplete recovery after rewetting, whereas prolonged droughts increased decomposition of stable pools at depth but allowed only partial re-equilibration during recovery. These changes were mirrored by contrasting responses in the carbon use efficiencies of labile (CUE_I) and stable (CUE_S) pools. During frequent droughts, CUE_I remained relatively stable, while the contribution of CUE_S increased indicating a higher contribution of stable SOC pools under soil moisture stress. The carbon sequestration efficiency (CSE = CUE_I/CUE_S) declined by up to 15% under prolonged droughts compared with more frequent drought-rewetting cycles, signalling a progressive reduction in soil carbon sequestration. The stabilisation efficiency (SE = ΔHUM/ΔBIO) declined to about 40%, implying that recurrent droughts reduced the efficiency with which microbial carbon was stabilized into the HUM pool. Collectively, these metrics revealed a reversal in the CSE-water relationship: CSE increased with soil water during drought but declined after rewetting, indicating a persistent post-drought decoupling between decomposition and stabilisation processes. Recurrent droughts thus reshape SOC dynamics reducing CSE and altering the balance between decomposition and stabilisation with depth. Drought frequency rather than duration, emerges as the dominant control on long-term soil carbon stability in organo-mineral systems.