CO enrichment and soil type additively regulate grassland productivity.

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