Impacts of increasing water and nitrogen availability on ecosystem CO₂ fluxes in a temperate steppe of Northern China

Abstract. Changes in precipitation patterns and nitrogen (N) cycling across the globe are likely to affect ecosystem primary productivity and CO₂ exchanges, especially in the arid and semi-arid grasslands because of their co-limitation of water and N supply. To evaluate the effects of water and N availability on ecosystem CO₂ fluxes, we conducted a manipulative field experiment with water and N addition in a temperate steppe of Northern China. The growing-season CO₂ fluxes, including net ecosystem exchange (NEE), gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) were examined in 2006 and 2007 with remarkably different amount of precipitation. Net carbon uptakes were found in all of treatments over the growing season in both years. However, their magnitude had inter-annual variations which coincided with the seasonal changes of precipitation amount. During these two growing seasons, water and N addition significantly increased NEE, owing to higher stimulation of GEP than ER. Our results suggest that net primary productivity, especially dominant species' biomass, correlated closely with variations in GEP and ER. Soil moisture was the driving environmental factor controlling seasonal and inter-annual variability in GEP and ER subsequently inducing changes in NEE. Moreover, the strengths of both water and N addition effects were greatly depended on the initial water condition in this temperate typical steppe.