Environmental effects on net ecosystem CO2 exchange at half-hour and month scales over Stipa krylovii steppe in northern China

The objectives of this research are to quantify the magnitude of CO2 exchange and to investigate the responses of net ecosystem CO2 exchange (NEE), gross ecosystem production (GEP) and ecosystem respiration (Re) to environmental factors over a temperate Stipa krylovii steppe by an eddy covariance system in northern China. Differences in annual precipitation, soil water content (SWC), air temperature (Ta) and evapotranspiration (ET) were the most important factors which controlled patterns and variations of CO2 exchange between atmosphere and the steppe. Precipitation received at study site was 297 mm in 2004, close to the average value during 1971–2000 (±S.D., 290 ± 82 mm), whereas precipitations in 2005 (174 mm) and 2006 (215 mm) were significantly below the average. The lowest values of precipitation and SWC in 2005 resulted in much lower carbon exchange rate related to those in 2004 and 2006. In 2005, daytime NEE reached the seasonal peak about −3 μmol C m−2 s−1, this was much lower than those in 2004 and 2006 (−7 to −8 μmol C m−2 s−1). Seasonal peaks of integrated monthly NEE was the lowest in 2005 (−8.8 g C m−2 mon−1) and the highest in 2004 (−34.3 g C m−2 mon−1). Compared with 2004, the peak values of monthly GEP and Re in 2005 reduced more than 30% and 20% due to soil water stress.