Digging deeper: fine-root responses to rising atmospheric CO concentration in forested ecosystems.

Experimental evidence from a diverse set of forested ecosystems indicates that CO2 enrichment may lead to deeper rooting distributions. While the causes of greater root production at deeper soil depths under elevated CO2 concentration ([CO2]) require further investigation, altered rooting distributions are expected to affect important ecosystem processes. The depth at which fine roots are produced may influence root chemistry, physiological function, and mycorrhizal infection, leading to altered nitrogen (N) uptake rates and slower turnover. Also, soil processes such as microbial decomposition are slowed at depth in the soil, potentially affecting the rate at which root detritus becomes incorporated into soil organic matter. Deeper rooting distributions under elevated [CO2] provide exciting opportunities to use novel sensors and chemical analyses throughout the soil profile to track the effects of root proliferation on carbon (C) and N cycling. Models do not currently incorporate information on root turnover and C and N cycling at depth in the soil, and modification is necessary to accurately represent processes associated with altered rooting depth distributions. Progress in understanding and modeling the interface between deeper rooting distributions under elevated [CO2] and soil C and N cycling will be critical in projecting the sustainability of forest responses to rising atmospheric [CO2].