Fertilization and Throughfall Reduction Effects on Soil Respiration and Litter Decomposition As Mediated By Extracellular Enzyme Activity.

Heterotrophic soil respiration is one of the largest fluxes in the terrestrial carbon (C) cycle and ultimately constrains ecosystem carbon storage.  Forest management and altered precipitation regimes are expected to influence extracellular enzyme activity, decomposition, and soil respiration, thereby ultimately affecting the net C balance of forest ecosystems.  Extracellular enzymes produced by soil microorganisms control nutrient cycling and litter decomposition by hydrolyzing polymeric compounds, yet enzyme production and activity is partially dependent on N availability. Forest fertilization will alter litter quality and decomposition dynamics while simultaneously increasing litter C inputs; however, the mechanisms of decomposition responses from increased nutrient availability remain ambiguous and the combined effects of fertilization and reduced precipitation are unknown. The objectives of this study are to evaluate the interactive effects of fertilization and throughfall reduction on extracellular enzyme activity and litter quality that ultimately control litter decomposition, and understand the contribution of this flux to total soil respiration. Litter and soil samples will be collected from the VA PINEMAP Tier III treatment plots, and assayed for enzyme activity using fluorometric and colorimetric microplate analysis. Litter exclusion treatments will be used to measure total soil respiration, and by difference, litter respiration. Litter quality will be evaluated using litter nutrient content and carbon chemistry. We expect that (1) fertilization will increase the abundance of C-degrading enzymes but decrease the abundance of N- and P-degrading enzymes and (2) reduced throughfall will inhibit potential enzyme activities in unfertilized, and to a lesser extent, fertilized plots. Results from this study could be used to infer shifts in decomposition dynamics, total soil respiration, and ecosystem C sequestration.