Hydro-meteorological influences on the growing season CO2 exchange of an alpine meadow in the northeastern Tibetan Plateau permafrost region: observations using eddy covariance method

Abstract

Hydro-meteorological variations are fundamental environmental factors affecting site-level ecosystem carbon balance variations; however, how hydro-meteorological variations influence continuous CO2 exchange over alpine meadows during the growing season in permafrost regions on the Tibetan Plateau is poorly understood. Net ecosystem CO2 exchange (NEE) measured using the eddy covariance method was partitioned into its components of ecosystem respiration (ER) and gross ecosystem CO2 exchange (GPP). ER and GPP had a significant relationship with air temperature, vapor pressure deficit, and net radiation, during the growing season of an alpine meadow. There was a positive correlation between GPP and active layer soil temperatures at 5–40 cm except soil temperature at 80 cm, located in the vicinity of the bottom of the active layer. The microbial activity of underground rhizospheric microorganism, mainly distributed in in shallow soil layers at 0–20 cm in alpine permafrost regions on the Tibetan Plateau, can significantly promote plant photosynthesis. Thus, the correlation between GPP and soil temperatures at 5–20 cm was stronger than that between GPP and air temperature. The correlation between ER (GPP) and active layer soil moisture at 5–80 cm was complicated, especially for soils at 5–20 cm. With the increase of active layer soil moisture, ER (GPP) increased first and then decreased. This was related to the thawing process of the active layer, increase of rainfalls, the frozen layer acting as a weak permeable layer, and the replenishment of super-permafrost water. Our results suggested that long-term observations on how hydro-meteorological variations affect the continuous CO2 exchange of alpine meadows with different active layer thicknesses should be carried out in alpine permafrost regions.