Fertilization and Throughfall Reduction Effects on Controls of Decomposition.

Heterotrophic soil respiration is among the largest fluxes in the terrestrial carbon (C) cycle and ultimately constrains ecosystem carbon storage. Forest nutrient management, altered precipitation regimes, and their interaction are expected to affect heterotrophic respiration and the net C balance of forest ecosystems by influencing litterfall, decomposition, and microbial processes. The objectives of this study were to evaluate how leaf litter quality, extracellular enzyme activity, microbial biomass, and soil respiration are influenced by the interactive effects of forest fertilization and throughfall reduction in a mid-rotation loblolly pine stand. Leaf litter (Oi and Oe horizons) and mineral soil samples were collected from the VA PINEMAP Tier III treatment plots in July 2014. Treatments beginning in 2012 consisted of a factorial combination of 30% throughfall reduction and fertilization (224 kg N ha^-1, 28 kg P ha^-1, 56 kg K ha^-1). Leaf litter and mineral soil were assayed for extracellular enzyme activity using fluorometric and colorimetric microplate analysis. Leaf litter quality was evaluated using nutrient content, and lignin and cellulose content as determined by extraction and acid hydrolysis. Microbial biomass was determined using the chloroform fumigation-extraction method. The results show that fertilization caused a shift in extracellular enzyme activity from mineral soil N and P mineralizing enzyme activity to litter C mineralizing activity, and improved litter quality by decreasing lignin:N. Throughfall reduction decreased litter quality by increasing lignin:N, and increased C mineralizing enzyme activity. Results from this study may help identify mechanisms responsible for potential shifts in decomposition dynamics, microbial nutrient demand, and ecosystem C storage associated with forest management and environmental change.