Effects of climate change and grazing on soil organic carbon stock of alpine wetlands on the Tibetan Plateau

Tibetan Plateau has been experiencing profound warming and slight wetting over recent decades, which have contradictive effects on soil organic carbon by enhancing plant growth and thereafter carbon input into the soil and increasing the soil organic carbon (SOC) decomposition rate. In this study, we developed a SOC model (WetlandC model) for wetlands, considering also the process of litterfall decomposition and parameterizing the effect of grazing on SOC accumulation. We also established a modelling framework to combine WetlandC model with TEM (Terrestrial Ecosystem Model) model to simulate the changes in SOC of the alpine wetlands on the Tibetan Plateau from 2000 to 2018. Results showed that spatially, the soil organic carbon density (SOCD) of alpine wetlands was higher in the southeast and lower in the northwest, ranging from 1358.22 to 22571.81 g C m^-2. The SOCD spatial pattern coincided with the northernmost and southernmost northern boundary of Asian summer monsoon. The SOCD was higher in region with precipitation ranging from 450 to 900 mm, suggesting that the precipitation played an important role in regulating the spatial heterogeneity of SOCD. The temporal trends of SOCD varied from -55.84 to 407.59 g C m^-2 yr^-1 over the plateau, and 97.98% of the wetland area was accumulating SOC. Temperature, precipitation and actual livestock carrying capacity, as the top influencing factors of the temporal trend of SOCD, accounted for 35.06%, 34.52% and 30.41% of the area in the alpine wetlands, respectively. The 0–30 cm SOC stock of the alpine wetlands on the Tibetan Plateau increased from 518.06 Tg C in 2000 to 607.67 Tg C in 2018. Surface soil in the alpine wetlands acts as a carbon sink of 4.98 Tg C yr^-1. Our results indicated that in the context of climate change, additional soil carbon sequestration in the alpine wetlands was facilitated by enhanced plant growth, in spite that grazing consumed the above-ground biomass. Future climate warming and wetting is likely to benefit the SOC accumulation in the alpine wetlands on the Tibetan Plateau if not overgrazed.