Increased precipitation during the vegetation periods was observed in and further predicted for Inner Mongolia.The changes in the associated soil moisture may affect the biosphere-atmosphere exchange of greenhouse gases.Therefore,we set up an irrigation experiment with one watered(W) and one unwatered plot(UW) at a winter-grazed Leymus chinensis-steppe site in the Xilin River catchment,Inner Mongolia. UW only received the natural precipitation of 2005(129 mm),whereas W was additionally watered after the precipitation data of 1998(in total 427 mm).In the 3-hour resolution,we determined nitrous oxide(N20),methane(CH4) and carbon dioxide (C02) fluxes at both plots between May and September 2005,using a fully automated,chamber-based measuring system.N2O fluxes in the steppe were very low,with mean emissions(±s.e.)of 0.9±0.5 and 0.7±0.5 μg N m-2 h-1 at W and uw,respectively.The steppe soil always served as a CH4 sink,with mean fluxes of-24.1±3.9 and-31.1±5.3μg Cm-2h-1 at W and UW. Nighttime mean C02 emissions were 82.6±8.7 and 26.3±1.7 mg C m-2 h-1 at W and UW.respectively, coinciding with an almost doubled aboveground plant biomass at W.0ur results indicate that the ecosystem C02 respiration responded sensitively to increased water input during the vegetation period,whereas the effects on CH4 and N20 fluxes were weak,most likely due to the high evapotranspiration and the lack of substrate for N20 producing processes.Based on our results,we hypothesize that with the gradual increase of summertime precipitation in Inner Mongolia,ecosystem C02 respiration will be enhanced and CH4 uptake by the steppe soils will be lightly inhibited.