Elevated CO_2 effects on canopy and soil water flux parameters measured using a large chamber in crops grown with free-air CO_2 enrichment

摘要

An arable crop rotation (winter barley-sugar beet-winter wheat) was exposed to elevated atmospheric CO_2 concentrations ([CO_2]) using a FACE facility (Free-Air CO_2 Enrichment) during two rotation periods. The atmospheric [CO_2] of the treatment plots was elevated to 550 ppm during daylight hours (T > 5 ℃). Canopy transpiration (E_C) and conductance (G_C) were measured at selected intervals (> 10% of total growing season) using a dynamic CO_2/H_2O 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 [CO_2] 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 [CO_2] enrichment were insensitive to weather conditions and leaf area index. However, differences in LAI between plots counteracted the [CO_2] impact on E_C and thus, at least in part, explained the variability of seasonal [CO_2] responses between crops and years. As a consequence of lower transpirational canopy water loss, [CO_2] 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 [CO_2]-induced reduction of E-C increased canopy surface and air temperature by up to 2 ℃ and 0.5 ℃, 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.