Riparian Nitrogen Dynamics in Tropical Forest Groundwaters: A Challenge to Our Understanding of Near-Stream Ecosystem Function

Riparian zones are widely understood to control the flux of water, nutrients, and carbon from terrestrial to aquatic ecosystems. The near-stream environment has long been noted for its ability to retain or remove nutrients from soils and aquifers in ecosystems that receive large anthropogenic inputs of nitrogen and phosphorus (e.g. agricultural, suburban landscapes), acting as a filter for hydrologic fluxes before these nutrients reach aquatic ecosystems. In forested ecosystems, examples of both sources and sinks of nutrients from riparian zones have been identified, with stream hydrology and hyporheic processes being the theorized controls.

The dominant theory of riparian nitrogen processing holds that riparian zones retain or remove nitrogen from soil and groundwater fluxes because their redox conditions are so distinct from the adjacent hillslopes that feed water to them. Converging flowpaths bring nitrogen and organic carbon from oxygenated soils to anoxic groundwaters. A heavily reducing environment favors denitrification and the formation of gaseous nitrogen compounds which can then outgas from the belowground/surface interface.

This theory is increasingly being challenged by exceptional ecosystems where the observed patterns of groundwater chemistry do not support the process described above. For tropical forests with high rainfall and massively reduced groundwaters, ammonium is the dominant form of nitrogen found in groundwater adjacent to streams. Is this ammonium being produced in the soils directly above the sampling wells? Or is it produced by the dissimilatory reduction of nitrate (DNRA)? By what process is it removed or retained by the riparian zone? Is a coupled-nitrification/denitrification process in the hyporheic zone responsible? Or is a dissimilatory pathway of ammonium oxidation responsible for producing gaseous nitrogen compounds?

This poster presents this intriguing problem using a well-studied example, the Rio Icacos watershed in the Luquillo Experimental Forest. Patterns of nitrogen chemistry across the watershed are presented, with some possible experimental approaches to determining the processes that control this distinctive nitrogen cycling environment.