Effects of increasing rainfall on organic carbon release and transport processes in permafrost on the central Tibetan Plateau

Rainfall can alter the hydrothermal state of permafrost, subsequently affecting organic carbon decomposition and CO₂ transport. However, the mechanisms by which rainfall influences organic carbon decomposition and carbon dioxide transport processes in permafrost remain unclear. In this study, a coupled permafrost water-heat-vapor-carbon model, based on the surface energy-water balance theory, is employed to explore the effects of increased precipitation on permafrost moisture, temperature, organic carbon decomposition, and carbon dioxide transport through numerical simulations. The results are as follows: (1) with increased rainfall, surface latent heat flux rises while surface sensible heat flux declines, leading to a reduction in surface heat flux. The annual mean surface heat fluxes for the three precipitation conditions of no change in precipitation (ΔP = 0 mm), 50 mm increase in precipitation (ΔP = 50 mm) and 100 mm increase in precipitation (ΔP = 100 mm) are −0.1 W/m², −0.2 W/m² and −0.4 W/m² respectively; and (2) as rainfall increases, soil moisture content increases significantly, but the impact of rainfall on soil moisture content diminishes with increasing soil depth; and (3) increased rainfall results in a decrease in soil carbon fluxes, soil organic matter decomposition rates, and CO₂ concentrations. Compared to the case of constant precipitation, the surface carbon fluxes decreased by 0.04 $\mu \text{mol}·{\text{m}}^{\text{-2}}{\text{s}}^{\text{-1}}$ and 0.08 $\mu \text{mol}·{\text{m}}^{\text{-2}}{\text{s}}^{\text{-1}}$ under ΔP = 50 mm and ΔP = 100 mm, respectively. Additionally, the decomposition rate of soil organic matter at 10 cm depth decreased by 3.2 E-8 $\text{mol}·{\text{m}}^{\text{-2}}{\text{s}}^{\text{-1}}$ and 6.3 E-8 $\text{mol}·{\text{m}}^{\text{-2}}{\text{s}}^{\text{-1}}$, respectively, while the soil carbon concentration decreased by 3 $\mu mol/mol$ and 5 $\mu mol/mol$, respectively.