Soil moisture effects determine CO2 responses of grassland species

It has been suggested that positive biomass responses of grassland to eleva ted CO2 result from moisture savings in the soil as opposed to direct photo synthetic stimulation. In order to test this hypothesis for calcareous gras sland we subjected experimental communities consisting of two important gra minoid components of such grasslands (Carex flacca and Bromus erectus) on n atural substrate to a fully factorial treatment of ambient (360 ppm) and el evated (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 hig her the lower the irrigation rate was. Superimposed on the effects of irrig ation on soil moisture, elevated CO2-induced higher soil water contents in all irrigation treatments via reduced plant water consumption (on average o ne-third lower stomatal conductance). This led to eight different soil mois ture regimes instead of the intended four. When growth parameters were plot ted against the effective soil water content rather than irrigation treatme nt, the "pure" CO2 effect on total biomass and other traits became much sma ller and completely disappeared for biomass per tiller, leaf area per groun d area, leaf mass fraction (LMF) and root mass fraction (RMF). We conclude that the CO2 response observed in this graminoid system consisted of a smal l primary CO2 effect and a large secondary, CO2-induced, soil moisture effe ct. 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 in direct (moisture driven) CO2 effects should be strictly separated, which re quires data to be tested against soil moisture.