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

Abstract

Carbon dioxide enrichment can alter grassland ecosystem functioning directly and through indirect, soil-specific effects on moisture, nitrogen availability and species composition. Now research shows that change in aboveground net primary productivity (ANPP) with carbon dioxide enrichment depends strongly on soil type; indicating that soils could cause spatial variation in carbon dioxide effects on ANPP and other ecosystem attributes. Grasslands are structured by climate and soils1,2, and are increasingly affected by anthropogenic changes, including rising atmospheric CO2 concentrations3,4. CO2 enrichment can alter grassland ecosystem function both directly and through indirect, soil-specific effects on moisture, nitrogen availability and plant species composition5,6,7,8, potentially leading to threshold change in ecosystem properties9. Here we show that the increase in aboveground net primary productivity (ANPP) with CO2 enrichment depends strongly on soil type. We found that the ANPP–CO2 response of grassland was 2.5× greater on two soils with higher plant-available soil moisture and where direct CO2 effects on ANPP were accompanied by indirect CO2 effects on ANPP mediated through an increase in soil moisture or increased dominance of a productive C4 grass. Indirect CO2 effects on ANPP were absent on a third soil that was less responsive to CO2 (1.6×). Unexpectedly6,10, soil N availability changed little with CO2 and did not seem to drive responses in ANPP. On the more responsive soils, the more productive grass C4 was favoured with CO2 enrichment because of greater photosynthetic efficiency. Our results enhance present models of the controls on ecosystem responses to CO2 (refs 7, 8, 11) and demonstrate mechanisms by which soils could cause spatial variation in CO2 effects on ANPP and other ecosystem attributes.