Mycorrhizal Mediation of Soil Organic Carbon Decomposition Under Elevated CO2.

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. 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.