Variation in net CO_2 exchange,gross primary production and its affecting factors in the planted pasture ecosystem in Sanjiangyuan Region of the Qinghai-Tibetan Plateau of China

Aims Our objective was to understand seasonal and diurnal variations in CO_2 exchange of a grassland ecosystem by clarifying the carbon cycle and its affecting factors for a planted pasture ecosystem in the Sanjiangyuan Region of the Qinghai-Tibetan Plateau.Methods We used the eddy covariance method to measure net ecosystem CO_2 exchange (NEE) and environmental factors in the planted pasture (Elymus nutans) ecosystem in 2006.Important findings Daily maximum uptake and release of CO_2 were 6.56 and-4.87 g CO_2·m-2·d-1,respectively.Maximum rates of NEE uptake and release were-0.35 and 0.22 mg CO_2·m-2·s-1,respectively.Annual gross primary production (GPP) was 1 761 g CO_2·m-2·a-1,of which more than 90% was consumed by ecosystem respiration (Reco).Annual NEE was-111 g CO_2·m-2.In the growing season,maximum and minimum Reco/GPP values were 90% in May and 79% in June,respectively.The Q10 was 4.81,which is higher than in other ecosystems.The NEE was mostly influenced by photosynthetic photon flux density (PPFD),temperature and vapor pressure deficit (VPD).The Reco was mainly affected by soil temperature at 5 cm depth (Ts).