Physical disturbance accelerates carbon loss through increasing labile carbon release

Labile carbon (C) is a major source of C loss because of its high vulnerability to environmental change. Yet its potential role in regulating soil organic carbon (SOC) dynamics remains unclear. In this study, we tested the effect of physical disturbance on SOC decomposition using soils from two abandoned farmlands free of management practice for more than 28 years. The soil respiration rate was measured in undisturbed and disturbed soil columns and was inversely modeled using the two-compartment model. We found that the C loss was 16.8~74.1% higher in disturbed than in undisturbed soil columns. Physical disturbance increased the total amount of labile C (C1) loss by 136~241%, while had no effect on the kinetic decomposition rate constants of both labile (k1) and stable (k2) SOC decomposition. Physical disturbance fragmented the large macroaggregates into small macroaggregates, microaggregates, and free silt and clay-sized fractions. This indicates that C loss was derived from the initially protected labile C, and there was no change of SOC fraction being decomposed. Our results give insights into the understanding of the extent of labile C loss to physical disruption and demonstrate the potential effect of physical disturbance on SOC dynamics.