Seasonal and spatial patterns of S, Ca, and N dynamics of a Northern Hardwood forest ecosystem

Seasonal dynamics of S, Ca and N were examined at the Huntington Forest, a northern hardwood ecosystem in the central Adirondacks of New York for a period of 34 months (1985-1988). Solute concentrations and fluxes in bulk precipitation, throughfall (TF) and leachates from the forest floor, E horizon and B horizon were quantified. Both above and below-ground elemental fluxes mediated by vegetation (e.g. uptake, litter inputs, and fine roots production) were also determined. The roles of abiotic and biotic processes were ascertained based on both changes in solute concentrations through the strata of the ecosystem as well as differences between dormant and growing seasons. Concentrations of SO42-, NO3-, NH4+ and Ca2+ were greater in TF than precipitation. Forest floor leachates had greater concentrations of SO42-, NO3-+NH4+ and Ca2+ (9, 6 and 77 μeq L-1, respectively) than TF. There were differences in concentrations of ions in leachates from the forest floor between the dormant and growing seasons presumably due to vegetation uptake and microbial immobilization. Concentrations and fluxes of NO3- and NH4+ were greatest in early spring followed by a rapid decline which coincided with a demand for N by vegetation in late spring. Vegetation uptake (44.7 kg N ha-1 yr-1) could account for the low leaching rates of NO3-. Within the mineral soil, changes with soil depth and the absence of seasonal patterns suggest that cation exchange (Ca2+) or anion sorption (SO42-) are primarily responsible for regulating solute concentrations. The increase in SO42- concentration after leachates passed through the mineral soil may be attributed to desorption of sulfate that was adsorbed during an earlier period when SO42- concentrations would have been greater due to elevated S inputs.