A salute to roots: Microbes, rhizodeposits and soil physical processes drive preferential stabilization of root carbon
The fate of plant inputs to soil remains contentious, including the decomposition dynamics of above and belowground plant biomass and their contribution to soil organic matter. Recent analyses of in situ decomposition studies found that twice as much soil organic matter (SOM) is derived from roots than from shoots. Roots are produced belowground in close proximity to soil microbes and mineral surfaces that stabilize C long-term. Roots are also chemically distinct from shoots and unique in their production of C that is both very labile (e.g. rhizodeposits including exudates and sloughed off cells) and recalcitrant (e.g. older root tissues high in waxes and lignin). Further, root-derived rhizodeposits are quickly incorporated into microbial biomass and may help promote both higher microbial growth efficiency and higher rates of biomass production. Here we examine the transfer of agricultural cover crop root and shoot biomass through microbial communities and into different SOM pools, and characterize the mechanisms underlying these transformations.
We traced 13C in situ labelled rye cover crop root and shoot biomass into soil carbon pools and microbial biomass. Our findings show roots are more quickly incorporated into microbial communities and that more soil carbon comes from plant roots than from plant shoots. Root source carbon was 2.5 - 3 times more abundant in bulk soil and in microbial biomass five months after rye residue incorporation. Moreover, root source carbon inputs were present in microbial biomass immediately following the first labelling event. Although root carbon did not accumulate in microbial biomass, probably due to the fast turnover of the microbial community, root carbon did accumulate in the bulk soil during the growing season. Increased root carbon storage was facilitated by preferential incorporation into the more stable dense carbon fraction. Because the presence of roots has been shown to alter microbial communities and soil properties such as aggregation and compaction that may affect root and shoot carbon storage, we designed exclusion cores to isolate the effects of actively growing plants on root decomposition. Greater stabilization of root carbon compared to shoot carbon and stabilization of short term inputs such as rhizodeposits can have important implications for the fate and management of C, particularly in agricultural systems.