This paper presents results of 1 year (from March 25, 2003 to March 24, 2004 of continuous measurements of net ecosystem CO 2 exchange (NEE) above a steppe in Mongolia using the eddy covariance technique. The steppe, typical of central Mongolia, is dominated by C 3 plants adapted to the continental climate. The following two questions are addressed: (1) how do NEE and its components: gross ecosystem production (GEP) and total ecosystem respiration (R eco ) vary seasonally? (2) how do NEE, GEP, and R eco respond to biotic and abiotic factors? The hourly minimal NEE and the hourly maximal R eco were À3.6 and 1.2 lmol m À2 s À1 , respectively (negative values denoting net carbon uptake by the canopy from the atmosphere). Peak daily sums of NEE, GEP, and R eco were À2.3, 3.5, and 1.5 g C m À2 day À1 , respectively. The annual sums of GEP, R eco , and NEE were 179, 138, and À41 g C m À2 , respectively. The carbon removal by sheep was estimated to range between 10 and 82 g C m À2 yr À1 using four different approaches. Including these estimates in the overall carbon budget yielded net ecosystem productivity of À23 to 1 20 g C m À2 yr À1 . Thus, within the remaining experimental uncertainty the carbon budget at this steppe site can be considered to be balanced. For the growing period (from April 23 to October 21, 2003), 26% and 53% of the variation in daily NEE and GEP, respectively, could be explained by the changes in leaf area index. Seasonality of GEP, R eco , and NEE was closely associated with precipitation, especially in the peak growing season when GEP and R eco were largest. Water stress was observed in late July to early August, which switched the steppe from a carbon sink to a carbon source. For the entire growing period, the light response curves of daytime NEE showed a rather low apparent quantum yield (a 5À0.0047 lmol CO 2 lmol À1 photons of photosynthetically active radiation). However, the a values varied with air temperature (T a ), vapor pressure deficit, and soil water content.
