Carbon dioxide exchange processes over the grassland ecosystems in semiarid areas of China

Based on the carbon fluxes measured over the grassland ecosystems in Inner Mongolia (UG79 site), Loess Plateau (SACOL site), and Tongyu, Jilin Province (TY site) in the semiarid areas from 2007 to 2008 with the eddy covariance method, we have investigated the carbon exchange processes over semiarid grassland ecosystem and its main affecting environmental variables. The precipitations at UG79 and TY sites in 2007 were below the historical average, especially for TY site, which was 50% below the historical average annual precipitation. The precipitation in SACOL site was close to average in 2007 but below average in 2008. The variation of monthly diurnal average NEE showed that the diurnal mean NEE decreased in the order of TY site, UG79 site, and SACOL site. However, a longer net carbon uptake period was observed at SACOL site. The diurnal course of NEE at UG79 site was similar between 2007 and 2008. The diurnal average NEE remained large during July and August in growing season (May to September) at UG79 site, with maximum values approaching 0.08 mg C m(-2) s(-1) in August of 2008. The diurnal average NEE of 2007 was larger than 2008 at SACOL site, with maximum values of 0.07 mg C m(-2) s(-1) in September of 2007. A shorter carbon uptake period was recorded in 2007 at TY site, lasting from July to August. A larger diurnal average NEE occurred in 2008 at TY site, with maximum values of 0.12 mg C m(-2) s(-1). The ecosystem respirations of three sites were controlled by both soil temperature and soil volumetric water content (at a depth of 5 cm below the land surface). Both UG79 site and SACOL site acted as a carbon sink during the growing periods of 2007 and 2008. Annual NEE in the growing seasons of 2007 and 2008 ranged from -68 to -50 g C m(-2) at UG79 site and from -109 to -55 g C m(-2) at SACOL site. Alternation between carbon source and carbon sink was found at TY site, with respective values of annual NEE in the growing seasons of 0.32 g C m(-2) and -73 g C m(-2) in 2007 and 2008. The magnitude and duration of carbon uptake depended mainly on the amount and timing of precipitation and the timing of the first effective rainfall during the growing season in semiarid grassland ecosystems.