Soil Water Content Under Plants at High CO 2 Concentration and Interactions with the Direct CO 2 Effects: A Species Comparison

Abstract

Wheat, maize and cotton, grown as spaced plants in large pots of soil, differed in the way high (2 X ambient) CO2 concentration affected the time-course of soil water use. For wheat, the tendency to conserve water owing to reduction in stomatal conductance in high CO2 was largely offset by the stimulation of leaf area development as the soil column dried. However, when the soil was maintained continuously wet, soil water conservation occurred because in the absence of water stress high CO2 did not maintain a greater leaf area. For maize, which has little or no photosynthetic response to CO2 concentrations above ambient but a strong stomatal response, water was conserved and the soil profile dried more slowly. Maize leaf area and dry matter growth increased in response to damper soil under high CO2, despite no growth response to CO2 in the absence of water stress. For cotton, which has a strong photosynthetic but weak stomatal response to CO2, the soil column dried faster under high CO2. Despite this drier soil, cotton still showed the greatest response to high CO2 of leaf area and dry matter growth of the three species compared. Under wet soil conditions, cotton exhibited a very large leaf area response to CO2 leading to much greater water use per plant. This contrasts with both wheat and maize which con- served water at high CO2 when wet. Despite these contrasting transpiration and growth responses, all three species exhibited a relatively similar increase in water use efficiency under high CO2 for both wet and dry conditions. It is concluded that the secondary effect of high CO2 on soil water content exerts a strong confounding influence on growth responses to CO2. In the longer term, the changed soil water status would influence hydrology, soil microbiology, nutrient relations and species composition. From indirect evidence it is proposed that the relative enhancement of growth owing to CO2 enrichment is greater under drought conditions than in wet soil because of the effect of water deficit on the intercel-