INSECT CANOPY HERBIVORY AND FRASS DEPOSITION AFFECT SOIL NUTRIENT DYNAMICS AND EXPORT IN OAK MESOCOSMS

Abstract

Increased nitrogen (N) mobilization and export from terrestrial forest ecosystems following canopy herbivory have been well documented, though the mechanism behind the loss is not clear. Because carbon (C) and N dynamics are closely linked, herbivore activity may also affect C distribution. We initiated a replicated mini-ecosystem experiment to test the hypothesis that insect frass (feces) influences soil C and N dynamics following insect defoliation. One hundred and sixty red oak (Quercus rubra) saplings were transplanted to seven-gallon (26.5-L) pots with soil and litter from the Coweeta Hydrologic Laboratory (CWT) (Otto, North Carolina, USA) and overwintered in experimental pot stands. During the 2002 growing season, trees were subjected to a 3 × 2 factorial experimental design with three damage groups (herbivore, mechanical, “undamaged”) and two frass depositions (frass, no frass). Frass deposition increased soil total C, total N, and the soil NH4+ pool. Leachate NO3− export also increased following frass additions. We suggest that herbivore frass mobilizes sufficient C and N to affect soil pools and N export, though abiotic factors may influence the ultimate fate of the nutrients in frass. In addition, herbivory increased soil respiration and decreased total soil N relative to “undamaged” controls independent of frass deposition. While we discuss four possible mechanisms for this observation, we hypothesize that the increased soil respiration results from enhanced root-exudate C and subsequent microbial oxidation. This mechanism has implications for C sequestration and N retention in forest soils. In addition, the effects of mechanical damage consistently did not match those of real herbivory, suggesting that differential responses of Q. rubra to damage types also may affect soil nutrient dynamics. Our results demonstrate that the feeding activity of insect herbivores can have direct and indirect effects on the cycling of C and N within the season of defoliation.