Climate, substrate, and soils in Hawaii and New Zealand
Department of Biology and Stanford Woods Institute for the Environment Stanford University
April 17, Psychology, Room 113, at 1:30 pm
Soil properties exhibit distinct thresholds along continuous rainfall gradients on basaltic soils in Hawaii; on one such gradient from 250 – 3250 mm/yr annual precipitation, multiple soil properties change substantially between 1700 and 2100 mm/yr, and both traditional human land use and soil microbial communities reflect that threshold. We explored the influences of climate variability and of differences in substrate (different parent materials) on this threshold through use of a simple ‘toy’ model of weathering and cation denudation, and through sampling an equally spectacular rainfall gradient on metamorphic parent material in New Zealand. We found that 1) rainfall variability from time step to time step (in the model) is central to controlling the position and dynamics of the soil threshold; 2) we observed a threshold in dynamic soil properties (base saturation, pH, exchangeable cations) on metamorphic as well as basaltic parent material, but more conservative properties (total inventories of elements derived from parent material) did not change at the same time as the more dynamic properties – raising the possibility that this threshold is not irreversible on metamorphic substrates (as opposed to basaltic substrates); and 3) a longer-term reduction in rainfall can lead to migration of the threshold (reversal of nutrient depletion) to the extent kinetic properties as opposed to equilibrium solubility determine the recalcitrance to weathering of primary minerals in simulated metamorphic substrate.