Mechanisms of organic carbon sequestration in long-term tilled soils: The primacy of microbial over active mineral protection

In current models of the soil carbon cycle, microbially and mineral-driven organic matter dynamics are central to maintaining agri-environmental sustainability. However, there may be a large gap between the importance of minerals and microorganisms in regulating SOC accumulation in agricultural soils. Here, we analysed the C use efficiency (CUE), turnover time, microbial respiration rate (qCO2), microbial necromass carbon (MNC) and mineral drivers of carbon accumulation using a cropland cultivation chronosequence soil (soil organic carbon (SOC) content of 22.1–49.8 g kg−1). The results revealed that with the loss of SOC, the amount of active mineral-bound organic carbon (bound OC) also decreased significantly, and the molar ratio of bound OC: minerals was significantly reduced. Compared with C-poor soils, C-rich soils exhibited a significantly higher microbial carbon use efficiency (CUE, 0.47 vs. 0.28) and a lower microbial metabolic quotient (qCO₂). Microbial necromass carbon (MNC, 28.9–61.6% of SOC) showed a co-variation pattern with total SOC across the degradation gradient. There was no difference in microbial uptake efficiency among soils with different SOC concentrations, but qCO2 was significantly greater in low-carbon soils. These findings suggest that the decrease in soil CUE was due primarily to the increase in qCO2. We emphasise that the reactive mineral content (Fed, Ald) was not the limiting condition for bound OC accumulation during soil cultivation and that the reduced microbially derived carbon content was the underlying cause of the reduced soil bound OC content. Overall, microbial physiological traits (CUE, qCO2) and microbially-derived carbon showed a stronger association with soil carbon accumulation than reactive minerals. For future research on carbon sequestration in agricultural soils, the carbon turnover function of microbial communities should be further explored to identify potential improvements in the soil environment that could promote their carbon accumulation rates.