The Effects Of Day And Night Warming On Water-use Efficiency In A Typical Steppe In Northern China

Human activities result in global warming, which will have significant impact on terrestrialecosystem structure and function. The asymmetry between day and night warming is prevalent all over theworld. Experimental and modeling studies with diurnal warming are difficult to effectively evaluate theimpacts of climate change on terrestrial ecosystems. As one of the most important biomes, grasslandaccounts for40%of Earth’s land area and stocks about34%of terrestrial carbon （C）. Changes in grasslandecosystem C cycle will be important in influencing terrestrial ecosystem C cycle. As a critical link betweenC and water cycles in terrestrial ecosystems, water-use efficiency （WUE） has been identified as an effectiveintegral trait for assessing ecosystem. Nevertheless, it remains unclear how WUE responds to day and nightwarming at different organism scales.This study used a field manipulative experiment to examine the impacts of day and nightwarming on ecosystem C and water cycling and WUE at different organism levels in a typical steppe innorthern China. The key findings are as follows:（1） Day and night warming had different impacts on ecosystem C cycling in the typical steppe innorthern China. Day warming reduced gross ecosystem productivity （GEP） by6.84%, but not ecosystemrespiration （ER）, leading to a shift from a C sink to source in the temperate steppe. Night warmingsuppressed ER （10.44%） rather than GEP （6.96%）, thereby increasing the C sequestration capacity of theecosystem. There were no interactive effects of day or night warming on net ecosystem CO₂exchange（NEE）, GEP or ER. The differences in the response of GEP and NEE to day and night warming in the twogrowing seasons were due to significant interannual fluctuations in soil moisture. （2） Day and night warming had differential impacts on ecosystem water cycling in the typicalsteppe in northern China. There was no effect of day warming on evapotranspiration （ET）. Night warmingreduced ET （7.08%） by suppressing evaporation （E,5.62%） and canopy transpiration （Tc,7.22%）. Therewere no interactive effects of day or nights warming on ET, E or Tc. Tcplayed a major role in the change ofET in the typical steppe in northern China. The decreases of ET induced by night warming could beascribed to the regulation of other biotc and abiotic factors, especially plant community composition.（3） Day and night warming had different impacts on WUE at different organism levels in thetypical steppe in northern China. Day warming decreased ecosystem WUE （5.66%） because of the highertemperature sensitivity of GEP than ET. There was no effect of night warming or interactive effects of dayand night warming on ecosystem WUE. Canopy WUE had the similar response to day and night warmingwith ecosystem WUE. The responses of leaf level WUE to day and night warming varied with plant species.Leaf level WUE response to day and night warming differed from those of ecosystem or canopy WUE. Theresponses of canopy and ecosystem WUE to climate change cannot be simply upscaled from plant WUE.