NUTRIENT CYCLING IN FORESTS OF WALKER BRANCH WATERSHED, TENNESSEE - ROLES OF UPTAKE AND LEACHING IN CAUSING SOIL CHANGES

Previous studies showed that subsoil exchangeable Ca2+ and Mg2+ decreased over the period 1971 to 1982 in several plots on Walker Branch Watershed. It was hypothesized that wood accumulation (i.e., uptake and sequestering in both living and dead tree biomass) was the dominant cause of the exchangeable Ca2+ decreases, whereas atmospheric deposition-induced leaching was the dominant cause of the exchangeable Mg2+ decreases. As hypothesized, wood accumulation far exceeded leaching of Ca2+ in those plots where soil exchangeable Ca2+ decreased. the hypothesis regarding causes of exchangeable Mg2+ decrease was neither accepted nor rejected from the data on hand. Leaching exceeded wood accumulation of Mg2+ in three of the four plots studied (pine, Pinus; yellow-poplar, Liriodendron tulipifera; and chestnut oak Quercus prinus). Cation leaching in these three plots was balanced by SO42-, and input-output balances indicated that soil solution SO42- originated primarily from atmospheric deposition. Thus, the trends toward decreased subsoil exchangeable Mg2+ in those particular plots (while not all statistically significant at the 95% level), are attributable primarily to atmospheric S deposition. However, leaching was minimal in the fourth plot (oak hickory, Quercus-Carya spp.) due to very low soil solution SO42- concentrations, yet subsoil Mg2+ decreases were significant at the 95% level. In the latter case, wood uptake was the dominant mechanism of Mg2+ removal from the soil. The importance of slope position in determining nutrient cycling patterns and the long-term changes in nutrient status is also discussed.