D3397: A novel field mesocosm experiment illustrates forest management effects on soil mesofauna communities and their ecological functions

Soil mesofauna influence fungal communities and their functions, but few manipulative field studies have parsed direct and indirect contributions of mesofauna to fungal-mediated ecosystem processes. We used mesocosms of novel design to untangle relationships among mesofauna, fungal communities, and decomposition in contrasting ecological contexts: thinned/burned and untreated ponderosa pine (Pinus ponderosa) forest management units. Our mesocosms, deployed for two growing seasons, were engineered to manipulate microarthropod communities via mesh hole sizes while addressing mesh treatment side effects. We hypothesized that fungivorous microarthropods could influence decomposition indirectly by modulating the Gadgil effect (where ectomycorrhizal fungi decelerate decomposition by outcompeting saprotrophic fungi for nitrogen). We used multigroup structural equation modeling (SEM) to quantify direct and indirect effects of mesofauna on decomposition of labile (cellulose) and recalcitrant (wood) substrates. Our SEM indicated that mesofauna increased the ratio of ectomycorrhizal (EcM) fungi to litter and wood saprotrophs (LWS), but only in the thinned/burned forest. Higher ratios were correlated with reduced decomposition, consistent with the Gadgil effect. However, we found no evidence of the Gadgil effect in the untreated forest, despite higher EcM:LWS ratios and more microarthropod individuals and species there. Cellulose decomposition was unaffected by the EcM:LWS ratio. We also observed no direct effect of microarthropods on decomposition, consistent with other reports from moisture-limited systems. Our findings suggest that the Gadgil effect may play out differently in contrasting ecological theaters, with decomposition rates determined by the stage (abiotic context and substrate recalcitrance) as well as the cast (communities of microarthropods and fungi).