Environmental controls on carbon fluxes over three grassland ecosystems in China

Abstract

This study compared the CO2 fluxes over three grassland ecosystems in China, including a temperate steppe (TS) in Inner Mongolia, an alpine shrub-meadow (ASM) in Qinghai and an alpine meadow-steppe (AMS) in Tibet. The measurements were made in 2004 and 2005 using the eddy covariance technique. Objectives were to document the different seasonality of net ecosystem exchange of CO2 (NEE) and its components, gross ecosystem photosynthesis (GEP) and ecosystem respiration (Reco), and to examine how environmental factors affect carbon exchange in the three grassland ecosystems. It was warmer in 2005 than in 2004, especially during the growing season (from May to September), across the three sites. The annual precipitation at TS in 2004 (364.4 mm) was close the annual average (350 mm), whereas the precipitation at TS in 2005 (153.3 mm) was significantly below the average. Both GEP and Reco of the temperate steppe in 2005 were significantly reduced by the extreme drought stress, resulting in net carbon release during almost the whole growing season. The magnitude of CO2 fluxes (daily and annual sums) was largest for the alpine shrub-meadow and smallest for the alpine meadow-steppe. The seasonal trends of GEP, Reco and NEE of the alpine shrub-meadow tracked closely with the variation in air temperature, while the seasonality of GEP, Reco and NEE of the temperate steppe and the alpine meadow-steppe was more related to the variation in soil moisture. The alpine shrub-meadow was a local carbon sink over the two years. The temperate steppe and alpine meadow-steppe were acting as net carbon source, with more carbon loss to the atmosphere in warmer and drier year of 2005. Annual precipitation was the primary climate driver for the difference in annual GEP and NEE among the three sites and between the two years. We also found the annual GEP and NEE depended significantly on the growing season length, which was mainly a result of the timing and amount of precipitation for the temperate steppe and the alpine meadow-steppe, but was more linked to the variation in air temperature for the alpine shrub-meadow.