Elevated CO2 effects on canopy and soil water flux parameters measured using a large chamber in crops grown with free-air CO2 enrichment

An arable crop rotation (winter barley-sugar beet-winter wheat) was exposed to elevated atmospheric CO2 concentrations ([CO2]) using a FACE facility (Free-Air CO2 Enrichment) during two rotation periods. The atmospheric [CO2] of the treatment plots was elevated to 550 ppm during daylight hours (T > 5 degrees C). Canopy transpiration (E-C) and conductance (G(C)) were measured at selected intervals (> 10% of total growing season) using a dynamic CO2/H2O chamber measuring system. Plant available soil water content (gravimetry and TDR probes) and canopy microclimate conditions were recorded in parallel. Averaged across both growing seasons, elevated [CO2] reduced E-C by 9%, 18% and 12%, and G(C) by 9%, 17% and 12% in barley, sugar beet and wheat, respectively. Both global radiation (Rg) and vapour pressure deficit (VPD) were the main driving forces of E-C, whereas G(C) was mostly related to Rg. The responses of E-C and especially G(C) to [CO2] enrichment were insensitive to weather conditions and leaf area index. However, differences in LAI between plots counteracted the [CO2] impact on E-C and thus, at least in part, explained the variability of seasonal [CO2] responses between crops and years. As a consequence of lower transpirational canopy water loss, [CO2] enrichment increased plant available soil water content in the course of the season by ca. 15 mm. This was true for all crops and years. Lower transpirational cooling due to a [CO2]-induced reduction of E-C increased canopy surface and air temperature by up to 2 degrees C and 0.5 degrees C, respectively. This is the first study to address effects of FACE on both water fluxes at canopy scale and water status of a European crop rotation.