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.