Soil moisture, species composition interact to regulate CO2 and CH4 fluxes in dry meadows on the Tibetan Plateau

Abstract

Alpine meadows on the Tibetan Plateau are important sinks for atmosphere carbon (C). Environmental degradation is transforming the continuous sedge (e.g., Kobresia humilis) meadows into a mosaic of sedge- and forb-dominated (e.g., Potentilla leuconota) patches, and this provides an ideal system in which to examine how the change in species composition (from sedge- to forb-dominated) affects the CO2 and CH4 fluxes and their response to environmental change. Net ecosystem CO2 exchange (NEE), ecosystem respiration (Reco), and methane (CH4) fluxes were measured monthly both in sedge- and forb-dominated patches during the growing seasons of 2011 and 2012. The seasonal variations of NEE (negative values represent net uptake of CO2) were negatively affected by soil water content (SWC) both in the sedge (R=−0.46, P=0.02) and in the forb (R=−0.71, P<0.001) sites, and the SWC regulated the NEE through exerting influence on gross ecosystem photosynthesis (GEP) (P<0.001) rather than on Reco (P>0.1). The CH4 fluxes were positively correlated with SWC for both sedge (R=0.48, P=0.016) and forb sites (R=0.70, P<0.001). Nevertheless, the CH4 emission can offset only 0.05% of the net C uptake as a result of the increase in SWC. Change in species composition did not affect the rates in NEE, Reco, and GEP (P>0.05); however, it led to the NEE of the meadow being more sensitive to SWC change (P=0.042). Change in species composition significantly reduced the CH4 flux (P=0.05), which were attributed to the decrease in soil labile carbon (e.g., microbial biomass carbon) and soil enzymatic activity (e.g., protease and urease). However, it increased the soil moisture sensitivity of CH4 flux (P=0.033). We conclude that the C sink strength of the alpine dry meadow is subject to the change in soil water availability on the Plateau, and the alpine meadow will become more vulnerable to drought with the ongoing spread of forb-dominated patches. Our results highlight the role of species composition in regulating the response of the alpine meadow C cycle to environmental change.