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.

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We examined the effects of grazing on aboveground biomass (AGB), vegetation constitution, and gross primary productivity (GPP) at midday in midsummer on a Mongolian steppe with mixed vegetation consisting of C3 and C4 plants. A grazed area (GA) and a nongrazed area (NGA) created by an exclosure were established in 2002 on land with a history of conventional grazing management. AGB of the mixed C3-C4 vegetation was determined in the GA and NGA from 2003 to 2006. Nongrazing resulted in increased AGB and a decreased ratio of C4 AGB to total AGB. In 2003 and 2006 a dynamic closed-chamber method was used to measure midday GPP; there was no significant difference between the GA and NGA in either year. Partitioning analysis of midday GPP to C3 and C4 photosynthesis indicated that exclosure exerted little influence on the midday GPP in midsummer, because the enhancement of GPP by increased AGB was offset by the reduced AGB of C4 plants.