DISSOLVED INORGANIC NITROGEN COMPOSITION, TRANSFORMATION, RETENTION, AND TRANSPORT IN NATURALLY PHOSPHATE-RICH AND PHOSPHATE-POOR TROPICAL STREAMS

The composition, transformation, and transport of dissolved inorganic nitrogen (DIN) was compared in waters associated with two lowland stre ams in Costa Rica. The Salto River is enriched by geothermal-based sol uble reactive phosphorus (SRP), which raises the concentration up to 2 00 mug/L whereas Pantano Creek, an unimpacted tributary, has an SRP co ncentration <10 mug/L. Ammonium concentration in springs adjacent to t he Salto and Pantano was typically greater than channel water (1 3 of 22 locations) whereas nitrate concentration was less (20 of 22 locatio ns). Ground waters were typically high in ammonium relative to nitrate whereas channel waters were high in nitrate relative to ammonium. Sed iment slurry studies indicated nitrification potential in two sediment types, firm clay (3.34 mug N.cm-3.d-1) and uncompacted organic-rich s ediment (1.76 mug N.cm-3.d-1). Ammonium and nitrate amendments to each stream separately resulted in nitrate concentrations in excess of tha t expected after correction for dilution using a conservative tracer. SRP concentration was not affected by DIN amendment to either stream. SRP concentration in the Pantano appeared to be regulated by abiotic s ediment exchange reactions whereas DIN composition and concentration w ere regulated by a combination of biotic and abiotic processes.