Abstract
To investigate the potential effects of changing precipitation on forest ecosystems, the Throughfall Displacement Experiment (TDE) was established on Walker Branch Watershed, Tennessee in 1993. Three different throughfall amounts were tested: -33% (DRY), ambient (no change, AMB), and +33% (WET). Throughfall manipulations had no statistically significant effects on total C, N, exchangeable Ca2+, Mg2+, bicarbonate-extractable P, or extractable SO42- in soils after twelve years of sustained treatments. Increased K+ inputs in the WET treatment resulted in relative increases in exchangeable K+ compared to the AMB and DRY treatments. Soil C, N, and extractable P declined in all treatments over the 12-year study, and the declines in N were inexplicably large. Field observations contrasted with earlier simulations from the Nutrient Cycling Model (NuCM), which predicted greater decreases in exchangeable K+, Ca2+, Mg2+, and extractable P on the order of WET > AMB > DRY and no change in C, N, and extractable SO42-. The failure of the NuCM model to accurately predict observed changes is attributed to the lack of mechanisms for deep rooting and the transfer of throughfall K+ from one plot to another in the model. Measurements of element availability using resin membranes during the final years showed higher values in wet and lower values in dry treatments compared to ambient in for mineral N, K, Mn, Zn, and Al but the opposite for B, Ca, and Mg. In the cases of Ca and Mg, the patterns in resin values were similar to those on the soil exchange sites (greatest in the dry treatment) and appeared to reflect pre-treatment differences. This study showed that while longer term changes in soil nutrients are likely to occur with changes in precipitation, potential changes over this twelve-year interval were buffered by ecosystem processes such as deep rooting.
