Forest decomposition processes are concentrated in surficial layers of soil, with canopy litterfall adding substantial amounts of organic matter to the soil surface and rhizodeposition adding organic matter to the soil decomposer system from belowground. 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 dissolved organic matter (DOM) and nutrient 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 dissolved organic carbon (DOC), dissolved inorganic and organic nitrogen (DIN, DON), and other anionic and cationic solutes, as well as two indices of DOM quality: molecular weight and aromaticity. Here, we use multilevel regression models to examine quantitative and qualitative changes in DOC, DON, DIN compounds with depth in the ecosystem profile, with seasons, and in relation to fluctuations in temperature and precipitation.
Results/Conclusions
Solution composition differs dramatically among depths (with depth accounting for 38-83% of the variance in individual component concentrations), 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 and 7.5cm soil solution, and barely detectable in bulk precipitation and 60cm and 200cm soil solutions. Seasonality accounted for 0-28% of the variance in DOM component concentrations and qualities. 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 different among depths, and consistent among seasons, while responses to temperature changes differ greatly among seasons, but were 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.
