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Nitrogen supply - a determinant in water use efficiency of winter wheat grown under free air CO2 enrichment

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 336mm 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.

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