Abstract: Based on an entire year continuous measurement of surface fluxes by eddy covariance (EC) tower and microclimate gradient observation system, the diurnal and seasonal dynamics of net ecosystem fluxes including carbon, water, and heat fluxes over Stipa krylovii steppe in Inner Mongolia were investigated. The results indicated that the diurnal pattern of carbon fluxes during growing season could be expressed as U curve. S. krylovii steppe ecosystem emitted CO₂ before the sunrise and absorbed CO₂ after the sunrise, with the maximum CO₂ uptake around noon. The ecosystem had weaker CO₂ uptake after the noon, and turned to emit CO₂ after sunset. The CO₂ uptake by S. krylovii steppe ecosystem reached the maximum in September, followed in August, and got the minimum in October. The diurnal dynamic patterns of sensible heat flux (H_s) and latent heat flux (LE) could be expressed as inverse U curves. The H_s and LE over S. krylovii steppe ecosystem were positive during the daytime, while H_s was negative and LE was close to zero during the nighttime. The ecosystem had the highest H_s and LE in May and September, respectively. In winter, the steppe acted as a weak carbon source, with the CO₂ flux being small; while in summer, it became an obvious carbon sink.

Key words: fine root morphology, root bag method, plastic response, nutrient foraging, nitrogen deposition