Transformations of Pine Forest-Derived Dissolved Organic Matter in a Temperate Zone Soil.

Forest decomposition processes are concentrated in surficial layers of soil, with canopy litterfall adding substantial amounts of organic carbon to the soil surface and rhizodeposition adding carbon to the soil decomposer system from belowground.  Only a small fraction of the carbon additions are available for long-term sequestration, however, because decomposition efficiently and rapidly oxidizes the organic carbon and returns it to the atmosphere as CO₂.  Despite the central importance of decomposition to forest productivity, nutrient availability, and carbon cycling, remarkably few studies describe the trajectories by which decomposition affects the chemical quality of organic matter as it is processed and oxidized. This study seeks to describe the transformations of DOM quantity and composition, and seasonal variations therein, as precipitation water traverses depth and decomposition gradients from the pine forest canopy, through the litter layer, and into the carbon-sequestering mineral soil.  For three years, three-weekly collections of precipitation, canopy throughfall, litter leachate, and soil solution from an experimental pine forest in South Carolina, were analyzed for a variety of DOC, DON/DIN,  and other anionic and cationic solutes.  Here, we use multilevel regression models to examine quantitative and qualitative changes in dissolved carbon and nitrogen compounds with depth in the ecosystem profile, with seasons, and in relation to fluctuations in temperature and precipitation. Solution composition differs dramatically among depths, with litter leachate composition drastically different from the canopy throughfall entering from above, and also from the mineral soil solution only 7.5 cm below.  In general, C and N solute concentrations, molecular weight, and aromaticity were all highest in litter leachate, intermediate in canopy throughfall, low in 7.5cm soil solution, and barely detectable in bulk precipitation and 60 and 200cm soil solutions.  Responses of solute concentrations to fluctuations in temperature and precipitation were strongest in litter leachate and 7.5cm soil solution. Responses to precipitation were generally driven by depth, and consistent among seasons, while responses to temperature changes differ greatly among seasons, but are consistent among depths.  These results suggest that the O-horizon serves as both a bioreactor, with a large proportion of the chemical transformations of decomposition taking place in that thin organic layer, and as a capacitor, decoupling aboveground and belowground concentrations and responses to temperature and precipitation.