A great effort in global change research has recently been directed towards assessing the potential of soil as a C sink under future CO₂ scenarios. Considerable attention has been focused on the impact of elevated CO₂ on mycorrhizae, particularly arbuscular mycorrhizae (AM), assuming that stimulation of mycorrhizae by elevated CO₂ will enhance soil C sequestration through facilitating soil aggregation and reducing C losses. However, the role of AM fungi in organic matter decomposition in response to global change factors has not been carefully assessed.

Results/Conclusions

Here we present results from two independent but complementary experiments illustrating that CO₂-enhancement of AM fungi increases organic carbon decomposition, positively feeding back to atmospheric CO₂. Results from ¹⁵N tracer measurements also indicated that elevated CO₂ considerably increased mycorrhizally-mediated plant N acquisition from decomposing residues. These findings directly challenge the current view that increasing mycorrhizal fungi with rising CO₂ could increase soil C sequestration. They also suggest that mycorrhizal fungi might shortcut soil N cycle by increasing plant N acquisition from decomposing residues under elevated CO₂, especially in N-poor soils, thereby alleviating N limitation on ecosystem responses to elevated CO₂.