Growth, water and nitrogen relations in grassland model ecosystems of the semi-arid Negev of Israel exposed to elevated CO2

Are ecosystems in dry regions particularly responsive to atmospheric CO2 en richment? We studied responses of semi-arid grassland assemblages from the northern Negev (Israel) to CO2 concentrations representative of the pre-ind ustrial era, and early and mid to late 21st century (280, 440, and 600 mul l(-1), respectively). Communities of 32 mostly annual species were grown fo r a full season in large containers (ca 400 kg each) on native soil and und er a simulated winter climate of the northern Negev. Ecosystem water relati ons were monitored weekly by wheeling containers onto a large electronic fr eight balance. Evapotranspiration was lower and soil water content was high er at elevated atmospheric CO2. Deep soil drainage was increased, thus redu cing the amount of applied rainwater that was effectively captured by the m odel ecosystems at elevated CO2. At peak season, midday net ecosystem CO2 e xchange increased with rising CO2 concentration, whereas nighttime exchange was not significantly affected. Aboveground biomass was 7% greater at 440 mul l(-1) and 17% greater at 600 mul l(-1) compared to 280 mul l(-1) CO2 Re productive output at the end of the season was increased by 10% and 24% at the two elevated CO2 concentrations. Shoot nitrogen concentration was sligh tly reduced (significantly for grasses), but the total plant nitrogen pool reflected the biomass gain and was increased. While some responses, such as water savings and plant nitrogen pool, were more pronounced across the hig her (440-600 mul l(-1)) than across the lower CO2 (280-440 mul l(-1)) inter val, total plant biomass (above- plus below-round) was already CO2 saturate d at 440 mul l(-1) (14% increase over biomass at 280 mul l(-1)). Surprising ly, the biomass, reproduction, and nitrogen responses at the community leve l were largely caused by a single legume species (Onobrychis crista-galli), with the other five legume species contributing less, and most grasses, no n-leguminous forbs, and geophytes hardly responding to elevated CO2. Overal l, responses were relatively small, despite the fact that we compared eleva ted to pre-industrial concentrations of CO2. This contrasts with our origin al assumption that ecosystems in seasonally dry regions will be particularl y responsive to elevated CO2. Impacts of CO2 enrichment on soil moisture de pletion and biomass production in semi-arid ecosystems will largely depend on the net effect of reduced water use (evapotranspiration) versus increase d water loss (deep drainage and runoff), and on the presence of certain spe cies. In this case, I out of 32 species was responsible for most of the eff ects at the community level.