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 enrichment? We studied responses of semi-arid grassland assemblages from the northern Negev (Israel) to CO₂ concentrations representative of the pre-industrial era, and early and mid to late 21st century (280, 440, and 600 μl 1^-1, respectively). Communities of 32 mostly annual species were grown for a full season in large containers (ca 400 kg each) on native soil and under a simulated winter climate of the northern Negev. Ecosystem water relations were monitored weekly by wheeling containers onto a large electronic freight balance. Evapotranspiration was lower and soil water content was higher at elevated atmospheric CO₂. Deep soil drainage was increased, thus reducing the amount of applied rainwater that was effectively captured by the model ecosystems at elevated CO₂. At peak season, midday net ecosystem CO₂ exchange increased with rising CO₂ concentration, whereas nighttime exchange was not significantly affected. Above-ground biomass was 7% greater at 440 μl 1_-1 and 17% greater at 600 μl 1^-1 compared to 280 μl 1^-1 CO₂. Reproductive output at the end of the season was increased by 10% and 24% at the two elevated CO₂ concentrations. Shoot nitrogen concentration was slightly 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 higher (440-600 μl 1^-1) than across the lower CO₂ (280-440 μl 1^-1 interval, total plant biomass (above- plus belowground) was already CO₂ saturated at 440 μl 1^-1 (14% increase over biomass at 280 μl 1^-1). Surprisingly, the biomass, reproduction, and nitrogen responses at the community level were largely caused by a single legume species (Onobrychis crista-galli), with the other five legume species contributing less, and most grasses, non-leguminous forbs, and geophytes hardly responding to elevated CO₂. Overall, responses were relatively small, despite the fact that we compared elevated to pre-industrial concentrations of CO₂. This contrasts with our original assumption that ecosystems in seasonally dry regions will be particularly responsive to elevated CO₂. Impacts of CO₂ enrichment on soil moisture depletion and biomass production in semi-arid ecosystems will largely depend on the net effect of reduced water use (evapotranspiration) versus increased water loss (deep drainage and runoff), and on the presence of certain species. In this case, 1 out of 32 species was responsible for most of the effects at the community level.