The Double Agent of Carbon Input From Roots: Contribute to Soil Carbon and Accelerate Soil Carbon Loss through Priming.

The size of the soil organic carbon pool is twice as large as the atmospheric carbon pool. Any appreciable change in either the rate of soil carbon loss through decomposition or the rate of soil carbon accumulation through net carbon sequestration can significantly influence the concentration of atmospheric carbon dioxide, a major greenhouse gas. The role of plant roots in regulating soil carbon dynamics has been controversial. Root carbon input to soils in some cases has been shown to be positively correlated to total soil organic carbon levels. Paradoxically, in other studies, root carbon input has also been shown to significantly accelerate the loss of soil organic carbon through a phenomenon called “rhizosphere priming effect.” These two aspects of root carbon input need reconciliation. The present analysis offers two mechanistic hypotheses that may explain the paradox: (1) root carbon input stimulates microbial decomposition of relatively labile soil carbon fractions through priming effect which causes the apparent increased loss of soil carbon in the short term, and simultaneously enhances the level of humification which contributes to long-term soil carbon gain; and (2) root carbon input provides the substrates for rhizosphere priming which accelerates nutrient release and enhances net primary production, in turn, this increased net primary production ultimately contributes to a higher level of soil organic carbon.