CO₂ enrichment and soil type additively regulate grassland productivity

Atmospheric CO₂ enrichment usually increases the aboveground net primary productivity (ANPP) of grassland vegetation, but the magnitude of the ANPP-CO₂ response differs among ecosystems. Soil properties affect ANPP via multiple mechanisms and vary over topographic to geographic gradients, but have received little attention as potential modifiers of the ANPP-CO₂ response. We assessed the effects of three soil types, sandy loam, silty clay and clay, on the ANPP response of perennial C₃ /C₄ grassland communities to a subambient to elevated CO₂ gradient over 10 yr in Texas, USA. We predicted an interactive, rather than additive, effect of CO₂ and soil type on ANPP. Contrary to prediction, CO₂ and soil additively influenced grassland ANPP. Increasing CO₂ by 250 μl l^-1 increased ANPP by 170 g m^-2 across soil types. Increased clay content from 10% to 50% among soils reduced ANPP by 50 g m^-2 . CO₂ enrichment increased ANPP via a predominant direct effect, accompanied by a smaller indirect effect mediated by a successional shift to increased dominance of the C₄ tallgrass Sorghastrum nutans. Our results indicate a large, positive influence of CO₂ enrichment on grassland productivity that resulted from the direct physiological benefits of CO₂ augmented by species succession, and was expressed similarly across soils of differing physical properties.