To investigate the potential effects of changing precipitation on a deciduous forest ecosystem, an experiment was established on Walker Branch Watershed, Tennessee that modified the amount of throughfall at -33 , ambient (no change), and +33 using a system of rain gutters and sprinklers. We hypothesized that the drier treatments would cause: 1) disproportionate changes in soil water flux, 2) increased total ionic concentrations in soil solution that would in turn cause 3) decreased SO42-/Cl- ratios, 4) decreased HCO3- concentrations, and 5) increased ratios of Al to (Ca2+ + Mg2+) and of (Ca2+ + Mg2+) to K+. Hypothesis 1 was supported by simulation results. Hypotheses 2 and 3 were supported in part by field results, although interpretation of these was complicated by pre-treatment biases. Hypotheses 4 and 5 were not supported by the field results. Comparisons of field data and Nutrient Cycling Model (NuCM) simulations were favorable for most ions except Cl- and K+. The disparities may be due to underestimation of soil buffering in the case of Cl- and overestimation of soil buffering in the case of K+ in the model. Long-term simulations with NuCM suggest that reducing water inputs will slow the rate of soil acidification and P loss, but will not materially affect growth or ecosystem N status. References: 15
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