Environmental influences on carbon dioxide fluxes over three grassland ecosystems in China

This study compared carbon dioxide (CO₂) fluxes over three grassland ecosystems in China, including a temperate semiarid steppe in Inner Mongolia (NMG), an alpine shrub-meadow in Qinghai (HB), and an alpine meadow-steppe in Tibet (DX). Measurements were made in 2004 and 2005 using the eddy covariance technique. Objectives were to document the seasonality of the net ecosystem exchange of CO₂ (NEE) and its components, gross ecosystem photosynthesis (GEP), and ecosystem respiration (R_eco), and to examine how environmental factors affect the CO₂ exchange in these grassland ecosystems. The 2005 growing season (from May to September) was warmer than that of 2004 across the three sites, and precipitation in 2005 was less than that in 2004 at NMG and DX. The magnitude of CO₂ fluxes (daily and annual sums) was largest at HB, which also showed the highest temperature sensitivity of R_eco among the three sites. A stepwise multiple regression analysis showed that the seasonal variation of GEP, R _eco, and NEE of the alpine shrub-meadow was mainly controlled by air temperature, whereas leaf area index can likely explain the seasonal variation in GEP, R_eco, and NEE of the temperate steppe. The CO₂ fluxes of the alpine meadow-steppe were jointly affected by soil moisture and air temperature. The alpine shrub-meadow acted as a net carbon sink over the two study years, whereas the temperate steppe and alpine meadow-steppe acted as net carbon sources. Both GEP and R_eco were reduced by the summer and spring drought in 2005 at NMG and DX, respectively. The accumulated leaf area index during the growing season (LAI_sum) played a key role in the interannual and intersite variation of annual GEP and R_eco across the study sites and years, whereas soil moisture contributed most significantly to the variation in annual NEE. Because LAI_sum was significantly correlated with soil moisture at a depth of 20 cm, we concluded that the available soil moisture other than annual precipitation was the most important factor controlling the variation in the CO₂ budgets of different grassland ecosystems in China.