COS 53-6
Precipitation influences soil biogeochemistry across 20kyr old basaltic soils
Water inputs from precipitation have the potential to drive nonlinear and irreversible changes in soil biogeochemistry. We asked what patterns in soil properties can precipitation drive when the rainfall regime has been enforced on timescales of 20kyr, and how these patterns fit into the greater context of soil development in basaltic soils. To answer these questions, we sampled soils from across a rainfall gradient that ranged from 250 to 5500mm of annual rainfall. The soils are developing on a basaltic substrate that is roughly 20kyr old, on the windward side of Mauna Kea Volcano, HI. Sampling included 5 complete soil profiles and 97 surface soil samples (0-30cm depth). Samples were analyzed for bulk chemistry, exchangeable cations, cation exchange capacity, base saturation, and nutrient availability. Data from rainfall gradients on 4.1Myr and 150kyr substrates were utilized to provide a temporal context.
Results/Conclusions
Base saturation; plant relevant exchangeable cations such as calcium and potassium; and the total phosphorus remaining compared to parent material (Pr) show a similar pattern of peaks in concentration between 1000 and 1500mm of precipitation. Resin P shows high variability, ranging between 0.06 and 2 µg P/g of soil, across the entire rainfall gradient. This contrasts the 4.1Myr and 150kyr soils, where resin P peaks at rainfalls of ~650 and ~1200mm of precipitation, respectively, and then is greatly reduced by ~1000mm and ~2200mm, respectively. It should also be noted that resin P concentrations are significantly higher in the 150kyr soils, where ~50% of sites have resin P concentrations >10ug/g of soil, than either the 4.1Myr or the 20kyr soils, where ~85% and ~75% of sites have resin P concentrations <1µg/g of soil, respectively. In the 20kyr soils, low concentrations are not driven by a lack of P in the soil, as total P ranges between 1 and 5 mg/g. More likely, P leaches from the soils more rapidly than weathering can supply it from primary minerals and other recalcitrant P pools.