Nutrient concentrations in foliar and soil samples collected from a rainfall gradient on 20kyr basaltic substrate
Water inputs from precipitation affect foliar nutrient concentrations by altering plant-available nutrient pools in soil via weathering of solid phases and transporting plant nutrients into and out of the system. Here we analyze patterns of foliar and soil pools of nitrogen (N), phosphorus (P), and other nutrients along a precipitation gradient. We collected canopy leaves and soil samples from sites along a rainfall gradient where mean annual precipitation varied from 600-5500 mm/yr. Our sites were located on 20kyr basaltic substrate on the eastern side of Mauna Kea Volcano, HI. Both plant and soil samples were analyzed for concentrations of N, P, carbon (C), and a variety of cations. Additionally, soil samples were analyzed for exchangeable cations, cation exchange capacity, base saturation, and resin extractable P.
Foliar and soil N peak at intermediate annual rainfall (2000-3000 mm/yr), with decreasing concentrations in drier or wetter sites. Soil N increases from 0.1% to 1.7% over the interval 600-2000 mm/yr, and falls back to 0.6-1.2% for rainfall above 3500 mm/yr. Average foliar N (relativized across species) increases from 0.6% to 1.6% over 600-3000 mm/yr, then decreases from 1.6% to 1.0% over 3000-5500 mm/yr. In dry sites, low N concentrations in soils and plants likely result from low N inputs; in wet sites, increased leaching pressure likely diminishes overall soil N and therefore plant-available N. Foliar P is positively correlated with rainfall up to 2500mm/yr, and correlates negatively with rainfall above this point. Average foliar P concentrations (relativized across species) increase from .06% at dry sites to a peak of .10%, and then decline to .08%. Resin P remains very low across the entire gradient. Low resin P but high foliar P at sites of intermediate rainfall indicate that P could be taken up and tightly held by plants.