EXPORT OF CARBON, NITROGEN, AND MAJOR IONS FROM 3 TROPICAL MONTANE WATERSHEDS

Annual exports of suspended sediment, dissolved and particulate C and N, dissolved N and P, and major cations and anions were measured in three montane tropical rain forest watersheds in Puerto Rico during 1983-1986. Organic C was primarily exported in the form of DOC, and DOC export (33-94 kg ha-1 yr-1) was similar to values in larger tropical watersheds with similar runoff. Particulate and dissolved organic N accounted for 60-70% of the 4-9 kg ha-1 yr-1 of total N exported. Export of base cations and rates of weathering varied with bedrock geology in the three watersheds. Concentrations of suspended sediment, particulate C and N, and DOC increased as a function of discharge in all three streams. NH4+, NO3-, SO42-, and K+ concentrations showed little or no response to variations in discharge; Na-, Ca2+, Mg2+, Cl-, HCO3-, and SiO2 all decreased at high flows. After removing the effects of discharge, residual NO, concentrations in each stream were inversely related to estimated rates of leaf fall. On a watershed basis, export and accumulation of N in biomass were greater than precipitation inputs, suggesting that unmeasured inputs (8-16 kg ha-1 yr-1) were large.