Alpine wetland ecosystem carbon sink and its controls at the Qinghai Lake

Wetland ecosystems play an important role in regulating the Earth’s climate. This paper presents the results of a study on the alpine wetland ecosystem CO2 fluxes at the Qinghai Lake at different vegetative development stages and CO2 controlling factors measured by eddy covariance system. The results showed that the alpine wetland ecosystem at the Qinghai Lake area was a carbon sink on the whole year basis. The average net ecosystem CO2 exchange (NEE) amount was −904.42 g CO2/m2; the average amount of the ecosystem reparation (Re) was 1450 g CO2/m2; the average gross primary productivity (GPP) was 2354.42 g CO2/m2 from July 2011 to June 2013 year. The NEE, Re and GPP amounts were the maximum during the growing stage of the whole year. There were significant negative linear correlations between monthly NEE amount and monthly average total solar radiation (SR) (R 2 = 0.27, p < 0.05), incident photosynthetically active radiation (PAR) (R 2 = 0.28, p < 0.05), ambient vapor pressure deficit (VPD) (R 2 = 0.38, p < 0.01), monthly total precipitation (P) (R 2 = 0.65, p < 0.0001), monthly mean leaf area index (LAI) (R 2 = 0.85, p < 0.0001) and enhanced vegetation index (EVI) (R 2 = 0.84, p < 0.0001). Extreme significant polynomial nonlinear negative correlations were found between NEE amounts and monthly average air temperature (T a ) (R 2 = 0.84, p < 0.0001), soil temperature at 0–5 cm depth (T s ) (R 2 = 0.90, p < 0.0001) and air relative humidity (RH) (R 2 = 0.76, p < 0.0001). The monthly Re amount of alpine wetland ecosystem at the Qinghai Lake at different stages showed significant nonlinear positive correlations with all the identified affecting factors except for monthly mean EVI, which showed a linear relationship (R 2 = 0.72, p < 0.0001). The monthly GPP amount at different vegetative development stages showed a significant exponential correlation with monthly average T a (R 2 = 0.92, p < 0.0001), a power function with VPD (R 2 = 0.24, p < 0.05), quadratic polynomial ones with monthly average T s (R 2 = 0.93, p < 0.0001), RH (R 2 = 0.74, p < 0.0001) and monthly P (R 2 = 0.55, p < 0.05). Extremely significant linear correlations of the monthly GPP amount at different developing stages with monthly average LAI (R 2 = 0.85, p < 0.0001) and EVI (R 2 = 0.92, p < 0.0001) were also found.