This project will investigate fundamental processes that are likely to influence the behavior of nitrogen in tropical rain forest streams. Very little is known about hydrologic interactions between stream water and soil water or the potential influences such interactions could have on the nitrogen cycle in tropical ecosystems. Preliminary data collected in montane rain forest watersheds in Puerto Rico indicae that soil water carries large amounts of dissolved nitrogen as ammonium into the riparian zone. Very little ammonium or nitrate is found in stream water, however, suggesting that nitrogen is removed from solution before it enters bulk stream flow. It is important to determine whether the nitrogen removed from solution is exported from the ecosystem in a gaseous form by nitrification or denitrification or whether it is recycled internally by plant uptake. It is hypothesized that in these tropical streams: 1) available stream-side inorganic nitrogen supports an active nitrification- denitrification couple and that 2) this couple has a major influence on in-stream nitrogen chemistry. Furthermore, it is hypothesized that flow paths through the stream-side zone will have a predictable influence on nitrogen dynamics. To investigate these hypotheses work will be performed: 1) to established transect of observation wells on stream reaches with differing stream-side hydraulic characteristics and measure initial concentrations of inorganic and organic nitrogen at these sites; 2) test the effects of nitrogen enrichments on nitrogen chemistry at these sites; and 3) estimate the potential for nitrification and denitrification in laboratory soil and sediment systems, then quantify specific mechanisms of nitrogen loss with nitrogen-15 tracer techniques. This combination of field measurements, field manipulations, and laboratory experiments will enable us to examine the relative importance and spatial characteristics of nitrification, denitrification, and plant uptake in typical tropical stream ecosystems.
