Retention of Foliage and Root Derived Dissolved Organic Carbon in Soil: A Comparison between Aspen and Conifer.

In soils, dissolved organic carbon (DOC) can interact with silt and clay minerals and undergo microbial transformations eventually forming relatively stable organo-mineral complexes, which contain some of the most stable soil organic carbon (SOC). In Utah, conifer forests have higher soil DOC concentrations and fluxes than adjacent aspen forests. Nevertheless, aspen soils have higher and more stable SOC pools. This suggests that, instead of DOC concentration, DOC quality and differences in leachate-soil interactions drive the higher SOC pools under aspen. In this study, we compared the retention of leachates from four plant sources – aspen leaves, aspen roots, conifer needles, and conifer roots – in a batch sorption experiment. DOC retention was examined on soils sampled from aspen and conifer forests at two depths, representing differences in carbon (C) saturation, and from two sites, which differ in their oxyhydroxide concentrations.

Foliage leachates reached the null point concentration (NPC; net sorption = net desorption) at lower initial (added) DOC concentrations than did root leachates. Overall, aspen leaf leachates had the lowest NPC, and also experienced the highest retention of C in soils. While oxyhydroxide and SOC concentration affected some aspects of DOC retention, the type of SOC already present in soil – aspen vs. conifer – had the biggest effect on DOC retention from all the soil characteristics considered. When mixed with aspen soils, all leachates reached the NPC at lower initial DOC concentrations, and in total aspen soils retained more C than conifer soils. As NPC roughly relates to field DOC concentrations, this suggests that, even though conifer forests have higher field DOC concentrations, they do not necessarily retain the C in the soil. Aspen soil and aspen leachates, on the other hand, require lower DOC concentrations to retain the C from the solution in the soil.