Abstract:

Background and Aims Many reports on global change have predicted major change in the temporal and spatial pattern of precipitation, which may have significant effects on temperate grasslands in arid and semi-arid regions. The responses of grasslands to environmental changes, especially amount and timing of precipitation, can be very different. Some studies indicate that drought may result in degradation of ecosystem function in NEE, even changing the ecosystem from a carbon sink to a carbon source.

Methods In order to quantify net ecosystem carbon exchange in Leymus chinensis steppe and its response to precipitation, we used the eddy covariance technique to measure carbon dioxide flux during the 2005 growing season in Xilin River Basin of Inner Mongolia Plateau in 2005. Only 126 mm precipitation fell during this growing season, far less than average; therefore, the steppe was in an extreme drought condition.

Key Results The daily pattern of CO₂ uptake in this drought year was consistent bimodal, with peaks at 8∶00 and 18∶00. In normal years, the bimodal pattern occurred only when soil water stress occurred. Maximum half-hourly average CO₂ uptake was -0.38 mg·m^-2·s^-1 in 2005, which was half that in typical growing seasons. Moreover, the ecosystem was a CO₂ source most of the growing season, releasing about 0.05 mg CO₂·m^-2·s^-1 at nighttime.

Conclusions The seasonal pattern of CO₂ uptake closely followed that of aboveground biomass and was strongly affected by soil temperature and soil water content. The ecosystem emitted 372.56 g CO₂·m^-2 during the growing season in 2005. The partial explanation is that much plant litter accumulated on the ground surface due to enclosure of the grassland since 1979, and this litter decomposed and resulted in a net release of CO₂ to atmosphere.

Key words: Extreme drought, Eddy covariance, CO₂ flux, Leymus chinensis steppe