Ozeas da Silva Costa , W. Berry Lyons, Russell S. Harmon, Anne E. Carey, Susan A. Welch, Helena Mitasova

We present chemical analyses from rivers and streams in five partly to near-fully forested watersheds in the humid tropics of central Panama. Contrary to the situation observed for temperate watersheds in the Northern Hemisphere, the concentration of dissolved inorganic nitrogen in the Panamanian watersheds is low (mostly <2.6 µmol L-1), whereas concentrations of organic nitrogen are several times higher, mostly >10 µmol L-1. We provide evidence that almost all NH4+ and much of the NO3- from precipitation are being converted to DON as nitrogen is cycled from the rainforest ecosystem to watershed rivers and streams, and that nitrogen loss from these pristine watersheds occurs mainly via dissolved organic N compounds. Based on this information we conclude that these Panamanian forests are not nitrogen saturated and are sinks for inorganic N. These Panamanian streams have lower DIN concentrations and higher DON concentrations than comparable montane forested streams in the Caribbean and Central America. Dissolved soluble phosphate concentrations are also very low (<1 µmol L-1). The TDN, DON and SRP yields are 4.45, 4.28 and 0.26 kg ha-1 yr-1, respectively. These are similar to those calculated for Costa Rican tropical rain forest streams, with the Panama sites having lower TDN and SRP yields but higher DON yields. The central Panama TDN yield estimate is within the range determined in other tropical undisturbed watersheds (0.57 – 9.40 kg ha-1 yr-1), with the DON yield for Panama at the higher end of the range for tropical watersheds.