Ecology Letters (2010) 13: 284-291 Atmospheric CO₂ enrichment generally stimulates plant photosynthesis and nutrient uptake, modifying the local and global cycling of bioactive elements. Although nutrient cations affect the long-term productivity and carbon balance of terrestrial ecosystems, little is known about the effect of CO₂ enrichment on cation availability in soil. In this study, we present evidence for a novel mechanism of CO₂-enhancement of cation release from soil in rice agricultural systems. Elevated CO₂ increased organic C allocation belowground and net H⁺ excretion from roots, and stimulated root and microbial respiration, reducing soil redox potential and increasing Fe²⁺ and Mn²⁺ in soil solutions. Increased H⁺, Fe²⁺, and Mn²⁺ promoted Ca²⁺ and Mg²⁺ release from soil cation exchange sites. These results indicate that over the short term, elevated CO₂ may stimulate cation release from soil and enhance plant growth. Over the long-term, however, CO₂-induced cation release may facilitate cation losses and soil acidification, negatively feeding back to the productivity of terrestrial ecosystems.