PLANTS, CLIMATE AND SOIL AGE: INTERACTIONS THAT SHAPE NUTRIENT AVAILABILITY AND RETENTION

We analyzed changes in nutrient availability and elemental losses from the entire weathering zone at 35 sites arrayed across climatic and soil-age gradients on the island of Hawai‘i. The sites are located on three basaltic lava flows (ages 10, 170, and 350 kyr) each of which crosses a precipitation gradient from ≈500 to 2500 mm/yr. By comparing the loss of nutrient (potassium, phosphorus) and non-nutrient (e.g., sodium) rock-derived elements, we identify a mesic climatic zone where the retention of plant nutrients in the upper soil is most pronounced. We further show that there are several abiotic constraints on plant-driven retention of nutrients. At the dry sites (<750 mm/yr and highly negative water balance), plants slow the loss of nutrients, but the effect (as measured by the difference between K and Na losses) is small, perhaps because of low plant cover and productivity. At intermediate rainfall (750–1400 mm/yr and slightly negative water balance) plants substantially enrich both nutrient cations and P relative to Na in the surface horizons, an effect that remains strong even after 350 kyr of soil development. In contrast, at high rainfall (>1,500 mm/yr and highly positive water balance), the effect of plants on nutrient distributions diminishes with soil age as leaching losses overwhelm the uplift and retention of nutrients by plants after 350 kyr of soil development. The effect of plants on soil nutrient distributions can also be mediated by the movement of iron (Fe), and substantial Fe losses at high rainfall on the older flows are highly correlated with P losses. Thus redox-driven redistribution of Fe may place a further abiotic constraint on nutrient retention by plants. In combination, these data indicate that the effects of soil aging on plant uplift and retention of nutrients differ markedly with water balance, and we view this as a potentially fruitful area for future research.