Global warming and associated decrease of summer precipitation will intensify the limitation of crop growth through water unavailability in Europe. Concomitantly, the rise of atmospheric CO2 concentration ([CO2]) decreases stomatal conductance and thus transpiration as evident at the leaf level. However, knowledge about the effect of elevated [CO2] ([eCO2]) on seasonal water use of crops is rather poor. In a two year field study, winter wheat was grown under ambient [CO2] (393 ppm) and [eCO2] (600 ppm) using free air CO2 enrichment (FACE). In addition, subplots were established with three levels of nitrogen (N) supply (35, 190, 320 kg N ha−1). Soil moisture was continuously measured and wheat was irrigated when necessary to keep field capacity at between 50% to 90%. Evapotranspiration (ET) from stem elongation until maturity was calculated using a soil water balance approach. Water use efficiency (WUE) was determined from the ratio of aboveground biomass production and ET during this period. Increasing N supply increased canopy size and decreased radiation transmission to the soil surface. Moreover increasing N supply enhanced biomass production from 771–1569 g m−2, ET from 227 to 336 mm and WUE from 4.07 to 6.20 g kg−1. Biomass was increased under [eCO2] by 17% among all N levels. [eCO2] increased soil moisture especially in the upper soil layer (0–20 cm) and thus irrigation was reduced under [eCO2] compared to [aCO2]. This effect was intensified by rising N supply leading to a significant CO2×N interaction on ET and WUE. Thus, the [eCO2] effect was for ET −2, −9 and −10% and for WUE + 20,+ 30 and +29% under 35, 190, 320 kg N ha-1, respectively. Simultaneously, there seems to be a greater increase of evaporation by [eCO2] under low than high N supply.
