Soil moisture effects determine CO₂ responses of grassland species

It has been suggested that positive biomass responses of grassland to elevated CO₂ result from moisture savings in the soil as opposed to direct photosynthetic stimulation. In order to test this hypothesis for calcareous grassland we subjected experimental communities consisting of two important graminoid components of such grasslands (Carex flacca and Bromus erectus) on natural substrate to a fully factorial treatment of ambient (360 ppm) and elevated (600 ppm) CO₂ concentration and four irrigation regimes (9 mm, 18 mm, 27 mm and 36 mm week^-1). Biomass stimulation under elevated CO₂ was higher the lower the irrigation rate was. Superimposed on the effects of irrigation on soil moisture, elevated CO₂-induced higher soil water contents in all irrigation treatments via reduced plant water consumption (on average one-third lower stomatal conductance). This led to eight different soil moisture regimes instead of the intended four. When growth parameters were plotted against the effective soil water content rather than irrigation treatment, the "pure" CO₂ effect on total biomass and other traits became much smaller and completely disappeared for biomass per tiller, leaf area per ground area, leaf mass fraction (LMF) and root mass fraction (RMF). We conclude that the CO₂ response observed in this graminoid system consisted of a small primary CO₂ effect and a large secondary, CO₂-induced, soil moisture effect. Thus, it is difficult to use responses to CO₂ from experiments in which CO₂-induced soil moisture savings occur to predict CO₂ effects as long as future soil moisture regimes are not defined. We suggest that direct and indirect (moisture driven) CO₂ effects should be strictly separated, which requires data to be tested against soil moisture.