Responses of canopy and soil climate in a six year free-air CO2 enrichment study with spring crops

Besides increased growth, plants cultivated under elevated carbon dioxide (CO2) show reduced transpiration and improved water use efficiency due to decreased stomatal conductances. While growth profits from the longer availability of soil water under CO2 enrichment, increased canopy temperature may counteract these positive effects. Here we report on time series of soil temperatures and moistures from six years in which spring crops were cultivated in free-air CO2 enrichment (Mini-FACE) experiments. Besides air and soil climate, temperature and relative humidity were determined in wheat canopies. Measurements rested on five replicates per treatment, representing a control (CON), an ambient air (AMB) and a FACE treatment. While the CON and AMB plots did not receive additional CO2, concentrations were moderately elevated by 150 μl l−1 in the FACE plots. Plant growth differed among years due to the different climate and duration of individual experiments. Total biomass production was increased in the FACE treatments but significant effects were found only in one out of six years. In most of the years, soil temperatures tended to be reduced and soil moistures remained higher under elevated CO2. Because the observed differences recurred during the growing season, we conclude that CO2 enrichment was responsible for changes of the soil microclimate. At the same time vapour pressure deficit in the canopy significantly differed between the treatments for some days. While canopy heating due to CO2 enrichment occurred in the early growing season these effects disappeared later suggesting that the stronger increase in leaf area index in the FACE treatments mitigated heating effects over time. The results support the supposed effects of CO2 enrichment on the canopy climate and indicate a ‘microclimatic paradox’ with higher soil water availability due to the reduced transpiration and stronger canopy heating in FACE plots at least early in the season.