9700929 Matson This research proposal is designed to determine the effect of water on biological processes involved in carbon cycling in high precipitation, non-wetland ecosystems. While the influence of water on carbon dynamics is well studied in dry to mesic terrestrial systems, it remains poorly defined in mesic to wet ecosystems although patterns of soil carbon storage suggest that it is important. The experimental approach uses a naturally occurring rainfall gradient in Hawaii; all other state factors that can affect carbon dynamics are held remarkably constant among sites, while rainfall alone changes systematically from 2500 mm to 6000 mm. This study focuses on the effect of water on decomposition and net primary productivity as the primary mechanisms of soil carbon storage. The main hypotheses are that net primary productivity and decomposition are negatively affected by water in these terrestrial systems; that the effect on decomposition rates is stronger relative to the effect on net primary productivity; and that water directly suppresses both oxygen and nutrient availability. The effect of increasing water will be evaluated across the gradient by directly measuring changes in net primary productivity and decomposition rates, by using 13C mass spectroscopy of soils and plant tissue as integrated measures of carbon fluxes, and by analysis of the soil atmosphere and nutrient availability. Understanding the mechanistic controls over soil carbon storage is important for predicting the transient response of the global soil carbon pool to climate change; though soil carbon storage increases linearly across the axis of precipitation, this masks real changes in the processes that control storage. This research will provide a robust empirical test of the effect of water on production and decomposition in high precipitation ecosystems, and will link changes in these carbon cycling processes to patterns of soil carbon storage. NSF Form 1358 (1/94)
