Effects of Nitrogen Supply and Nitrogen Form on Intrinsic Water-Use Efficiency in Temperate, Semi-Natural Grasslands under Rising Atmospheric CO\u3csub\u3e2\u3c/sub\u3e

Carbon uptake and water loss of plants are controlled by the regulation of carbon assimilation rate (A) and stomatal conductance (gs). Any changes in these parameters under rising atmospheric CO2 concentration (ca) influence plant water-use efficiency and can affect carbon and water relations in grassland ecosystems. Time-series analyses of intrinsic water-use efficiency (Wi) of C3 grassland vegetation during the 20th century have shown that carbon uptake relative to water loss generally increased (Köhler et al. 2010, 2012; Barbosa et al. 2010). Short-term experiments suggest that nitrogen (N) supply (Stitt and Krapp 1999) and N form (Bloom et al. 2011) also play a role. We hypothesize that any increase in Wi should be stronger in swards with high N availability and with ammonium (NH4)-N instead of nitrate (NO3)-N fertilization