Greenhouse gas emissions as influenced by wetland vegetation degradation along a moisture gradient on the eastern Qinghai-Tibet Plateau of North-West China

摘要

Vegetation loss and plant diversity decline in wetlands affect carbon and nitrogen cycling and consequently influence gas fluxes. Although extensive grazing by livestock and climate change have caused significant physical degradation of wetlands on the Qinghai-Tibet Plateau (QTP), and created a clear drainage gradient, the impact on greenhouse gas (GHG) emissions associated with this change has rarely been reported. A 3-year study (2013-2015) was conducted to examine the effect of vegetation change and seasonality on ecosystem respiration, methane (CH4) and nitrous oxide (N2O) fluxes in four classes of wetlands with distinct magnitudes of vegetation degradation: healthy vegetation (HV), slightly degraded (SD), moderately degraded, and heavily degraded (HD). We used the dark static chamber-chromatography method to measure the gas fluxes. Highly degraded wetlands were larger C and GHG sources than HV, despite lower methane emissions, due to the loss of gross primary production. SD and HD exhibited the highest cumulative mean annual ecosystem respiration and N2O emissions, respectively. Ecosystem respiration and CH4 fluxes were much higher during the growing seasons than in the non-growing seasons. Ecosystem respiration and N2O fluxes were positively correlated with soil and air temperatures. This points at a potential effect of global warming on GHG emissions from the QTP wetlands. Top soil (0-20cm) moisture content significantly correlated positively with CH4 fluxes. Vegetation loss led to a reduced C uptake and increased global warming potential. Therefore, we recommend soil conservation measures and reduced livestock grazing in the wetlands in order to conserve their role as carbon sinks.