Species-Specific Exudation Responses to Root Herbivory and Implications for SOM in a Temperate Hardwood Forest

Root-derived exudates are major precursors of soil organic matter (SOM), yet the influence of belowground herbivores on these inputs is poorly quantified. We tested whether root herbivory by root-feeding nematodes alters root exudation, root morphology, and downstream effects on SOM fraction pools. In an end-moraine hardwood forest in southern Indiana, USA, we installed field microcosms around intact fine roots of mature maple (Acer spp.) and hickory (Carya spp.) and manipulated herbivory pressure by varying densities of the root-feeding nematode Pratylenchus penetrans (obtained from laboratory cultures). Treatments consisted of a defaunated soil control, and defaunated soils inoculated with 300 (ambient), 600, or 900 individuals per 600 g fresh soil, with 8 replicates per treatment. After 8 weeks, we collected root exudates in situ and quantified carbon exudation rates, specific root length, and particulate and mineral-associated SOM pools. Initial results indicate that carbon exudation responses diverged strongly between genera: maple exhibited a hump-shaped response, with carbon exudation elevated at low–moderate herbivory but suppressed at the highest density where root damage was greatest. In contrast, hickory exudation increased linearly with herbivory pressure. Ongoing analyses test responses of exudate composition (i.e., metabolite profiles) and whether these contrasting exudation responses translate to changes in mineral-associated organic matter formation and SOM persistence. Our findings highlight root herbivory as an underappreciated, species-specific control on root carbon inputs and SOM stabilization in mature hardwood forests.