Soil-mediated effects of subambient to increased carbon dioxide on grassland productivity

Grasslands are structured by climate and soils, and are increasingly affected by anthropogenic changes, including rising atmospheric CO 2 concentrations. CO 2 enrichment can alter grassland ecosystem function both directly and through indirect, soil-specific effects on moisture, nitrogen availability and plant species composition, potentially leading to threshold change in ecosystem properties. Here we show that the increase in aboveground net primary productivity (ANPP) with CO 2 enrichment depends strongly on soil type. We found that the ANPP-CO 2 response of grassland was 2.5× greater on two soils with higher plant-available soil moisture and where direct CO 2 effects on ANPP were accompanied by indirect CO 2 effects on ANPP mediated through an increase in soil moisture or increased dominance of a productive C 4 grass. Indirect CO 2 effects on ANPP were absent on a third soil that was less responsive to CO 2 (1.6×). Unexpectedly, soil N availability changed little with CO 2 and did not seem to drive responses in ANPP. On the more responsive soils, the more productive grass C 4 was favoured with CO 2 enrichment because of greater photosynthetic efficiency. Our results enhance present models of the controls on ecosystem responses to CO 2 and demonstrate mechanisms by which soils could cause spatial variation in CO 2 effects on ANPP and other ecosystem attributes.