NUTRIENT LOSSES OVER FOUR MILLION YEARS OF TROPICAL FOREST DEVELOPMENT

Abstract

Biological, atmospheric, and geochemical processes interact to shape how element cycles and nutrient losses develop within newly formed landscapes. We examined losses of nitrogen (N), phosphorus (P), and base cations across a four-million-year substrate age gradient of Hawaiian montane tropical forests, with the goal of understanding how losses depend on changes in biotic demand, weathering, and atmospheric sources. We were particularly interested in whether losses of nutrients that are not subject to traditional mechanisms of biotic availability could influence ecosystem fertility and nutrient limitation over time. Over a three-year period we sampled nutrient outputs in soil solutions below the active plant–soil system and small streams, gaseous N losses (NO, N2O, and N2), and pools and transformations of N in soils. Weathering was the major determinant of ecosystem losses of P, of base cations and Si, and of ecosystem acid–base status. Sharp reductions in weathering inputs after 20 000 years of forest development caused dramatically lower outputs of P (∼65% reduction), Ca2+ (∼99%), and Si (∼94%), to rates that matched inputs from dilute sea-salt and dust aerosols. Internal production of organic acids, in combination with low weathering, caused highly acidic soil waters (pH < 5.0) with elevated Al (up to 300 μg/L) and Ca:Al ratios (<0.3) below values considered critical thresholds for many plant species. Long-term N and P interactions were more complex than predicted by the Walker and Syers model, with important influences of N and P loss pathways that were not subject to direct biotic availability. While losses of available forms of dissolved N and P, and N gases followed (as predicted) ecosystem nutrient availability, significant dissolved organic N (DON) and dissolved organic P (DOP) losses occurred independent of ecosystem N or P status. DON losses were not sufficient (relative to external inputs) to sustain N limitation beyond 20 000 years, while dissolved P losses remained large enough to maintain P limitation in older forests. Forests developed high N:P loss ratios over time (>300 on mass weight basis) due to efficient P recycling and unexpectedly high N throughputs (4–9 kg N·ha−1·yr−1) by either N fixation and/or N deposition.