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 ϫ 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 NH 4 ϩ pool. Leachate NO 3 Ϫ 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.

Slowed Biogeochemical Cycling in Sub-arctic Birch Forest Linked to Reduced Mycorrhizal Growth and Community Change after a Defoliation Event

“…This study in forest plots which have undergone almost complete defoliation in a relatively unproductive ecosystem (Myneni and others 2001;Karlsen and others 2008) shows the opposite response to more productive ecosystems, which experienced a smaller reduction in belowground C allocation. In a temperate deciduous system, belowground respiration rates increased in response to relatively mild defoliation (8% less foliage than control; (Frost and Hunter 2004)). This was similar to patterns of increased C allocation belowground by plants in response to partial Here the authors explained the increase in C flux as being due to a combination of increased root growth, turnover, activity and labile C input, all as mechanisms to recover N that was lost from leaf biomass (Frost and Hunter 2004).…”

“…In a temperate deciduous system, belowground respiration rates increased in response to relatively mild defoliation (8% less foliage than control; (Frost and Hunter 2004)). This was similar to patterns of increased C allocation belowground by plants in response to partial Here the authors explained the increase in C flux as being due to a combination of increased root growth, turnover, activity and labile C input, all as mechanisms to recover N that was lost from leaf biomass (Frost and Hunter 2004). All of these processes depend on a high flux of autotrophic C to maintain roots and associated mycorrhizal symbionts (Litton and others 2007;Ekblad and others 2013;Brzostek and others 2015); something that was not possible in the present study as the defoliated trees suffered almost complete defoliation.…”

Moth herbivory enhances resource turnover in subarctic mountain birch forests?

“…Because the growth of soil organisms is generally controlled by carbon availability (Bardgett and Wardle 2010), a major reduction of carbon flux belowground, such as the removal of canopy foliage due to herbivory, is likely to have a marked impact on soilinhabiting organisms. Intensive moth herbivory on tree canopies results in massive deposition of moth feces (frass) with high quantities of dissolved N and C compounds into the soil (Frost and Hunter 2004). Canopy herbivory has been found to change understory plant community composition as well as alter litter input and quality (Wardle et al 2004, Mitchell et al 2011.…”

Insect herbivory, organic matter deposition and effects on belowground organic matter fluxes in a central European oak forest

“…In this context, it is suggested that DOM input with throughfall is a leading promoter for decomposition processes in the forest floor, significantly affecting the C partitioning of the litter input into CO 2 , DOC and humic material and its fate in the mineral soil (Madritch and Hunter 2003;Chapman et al 2003: Frost andHunter 2004).…”