Phosphorus and cation losses as a function of soil age and climate in the Hawaiian Islands

The depletion of rock-derived nutrients (particularly phosphorus) from ecosystems is often assumed to be most widespread in the lowland tropics, regions with high precipitation and putatively old soils. Nevertheless, the effects of rainfall and soil age have not been simultaneously investigated where it was possible to hold other soil forming factors constant. We determined the loss of rock-derived nutrients from three, minimally eroded, basaltic lava flows (10, 170, and 350 ky old) in close proximity to each other on the Island of Hawai'i. Each flow crosses a precipitation gradient from ~ 600 to ~ 2,500 mm/yr, so we were able to assess the relative influence of climate and substrate age on the chemical depletion of phosphorus, calcium, magnesium and potassium (P, Ca, Mg, K). Using a backhoe, we sampled the entire weathering zone in 7-14 locations on each flow, and calculated mass loss of each element by comparison to an immobile element (niobium). There was virtually no P lost from the 10 ka flow at any rainfall, but cations showed a progressive depletion with increased rainfall across the flow. On the two older flows, P losses averaged ~ 30% at dry to mesic sites (<1500 mm/yr) flow, and increased ~ 60% at any rainfall amount > 1500 mm/yr. In contrast, Ca, Mg and K were lost progressively with increasing precipitation along the older flows, and with increasing flow age at a given rainfall. Cation losses reached almost 100% at > 1500 mm/yr on the older flows. The different sensitivity of P vs. cation losses to soil age and precipitation suggest that depletion of rock-derived elements other than P may play a role in determining the biogeochemical properties of some tropical ecosystems.