Russell, C., E.A. Paul, G.P. Robertson, and D. Harris
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
Insect herbivory may regulate forest productivity through its effects on nutrient cycling. Extensive defoliation of trees represents a severe perturbation to the N cycle of an ecosystem. Defoliation results in a rapid and substantial transfer of foliar N to the forest floor, primarily as frass, and the trees are denied the opportunity to internally recycle foliar N prior to senescence. What is the fate of this N? Will it be conserved in the soils organic matter via the soils microbial biomass? Or will it be lost to the atmosphere through NH3 volatilization and/or denitrification, or to the ground water as soluble N? Or does its fate depend more on the synchronization between frass deposition and rainfall and temperature patterns?The defoliation residue (ie frass, leaf debris, and insect cadavers) is of high quality (ie low C:N ratio) and in the case of poplars low in lignin:N. We will measure the rate of formation, quality and fate of this residue in the field. This residue is analogous to animal manure, for which it is known that most of the N mineralized is lost to the atmosphere as NH3 if the manure remains on the soil surface for more than a few days. Our hypothesis is that gypsy moth defoliation will cause an immediate and substantial loss of N from a Poplar stand. This loss will result in long-term N shortages exacerbated by changes in litter-fall chemistry and soil organic matter (SOM) turnover. Much of the N loss will occur as NH3 volatilized from frass, the other portion will come from hydrological loss of SOM-derived N and decomposed poplar root-N. Hydrological losses will occur subsequent to defoliation when plant transpiration is low and warmer and wetter soils accelerate N mineralization and profile water movement. Any microbial immobilization of frass-N, SOM-derived N and Poplar root-N will lead to SOM formation and later release of this N on only a long-term basis. Furthermore, after year 1 we expect changes in foliar chemistry to result in higher C:N ratios in senesced leaves, a phenomenon which further restricts soil N availability.Apart from standard methods of analysis of total and inorganic N, further methods employed to study the N cycle in ±defoliated Poplar stands are;Furthermore, in each treatment-plot 2 trees have been labeled with 15N in situ by stem injection (Horwath et al. 1992). This method enables the direct measurement of the flux of tree-N from defoliation residues and dead roots into the soil N pools. In the fall, one of these trees will be sacrificed for a whole-tree root excavation study and for a whole-tree 15N recovery study.
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