Living roots have been shown to have conflicting effects on carbon (C) retention in soil organic matter (SOM). Roots form aggregates which make SOM less accessible to decomposers and disrupt aggregates which can increase SOM decomposition. In addition, roots stimulate SOM decomposition through the exudation of labile compounds to soil microbes. This enhanced decomposition can increase the production of microbial compounds that preferentially form mineral associated SOM but can also prime SOM losses. Even further, root exudates both efficiently form mineral associated SOM and disrupt organo-mineral associations. To answer the question of whether roots drive a net increase or decrease in the formation of new SOM, we performed an isotope-tracing experiment in the field to quantify the impact of living roots on litter decomposition and incorporation into SOM pools. We used the bioenergy feedstock, Miscanthus x giganteus, as a model system because it produces an extensive root system to overcome nutrient limitation. To isolate the effects of roots, fungi, and free-living microbes, we followed the fate of a 13C and 15N-labelled litter substrate in ingrowth cores that allowed root and fungal ingrowth, excluded roots, or excluded both roots and fungal hyphae for an entire growing season.
We found that the degree of root ingrowth was the primary driver of differences in the extent to which litter inputs became stabilized in SOM. When we analyzed the fate of the litter substrate as a function of treatment and of microbial and root biomass, we found that increasing root ingrowth was positively correlated with the retention of litter C in the SOM pool largely consisting of stable microaggregates. However, this retention was balanced by greater microbial decomposition leading to more litter C being respired. Despite this enhanced microbial activity, litter C recovered in mineral associated SOM was negatively correlated with root ingrowth, suggesting that roots did not enhance the formation of mineral-stabilized soil C. Although root ingrowth promoted both litter C stabilization and destabilization in distinct SOM pools, the greater reductions in mineral associated SOM drove net litter C losses from soil. Collectively, these results suggest that at least in this system root priming can outweigh root stabilization of new C inputs.