Precipitation change and soil leaching: Field results and simulations from walker branch watershed, Tennessee

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 SO₄^2-/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.