Heterogeneity in decomposition rates and annual litter inputs within fine-root architecture of tree species: Implications for forest soil carbon accumulation

Abstract

Fine roots (⩽2mm in diameter) of tree species, traditionally regarded as homogenous entities, are expected to be major contributors of forest soil organic carbon (C), but their contributions need to be reevaluated given the heterogeneity in many root characteristics within fine-root architecture. Here, literature data were synthesized to analyze whether there existed heterogeneity of decomposition rates within fine-root architecture of tree species and quantify the relative contributions of roots of each branching order to forest soil C storage by combining variations in annual litter inputs among fine-root branching hierarchies. Results showed the presence of heterogeneity in decomposition rates within fine-root architecture, and this pattern was influenced by classification methods (root order or diameter class). Specifically, the heterogeneity in decomposition rates was shown only when fine roots were classified by root order, but not when fine roots were classified by diameter class. The slower decomposition rates of lower-order roots (the first two orders) were related to their higher acid-unhydrolyzable residues. Furthermore, lower-order roots were found to account for 57–71% of annual root litter inputs of the first five orders. Given the slow decomposition rates and high annual litter inputs of lower-order roots of tree species, our findings suggest that lower-order roots should be important contributors to forest soil organic C.