Export of carbon, nitrogen, and major ions from three 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- ’ yr-I) 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 ’ yr-I 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. NH, +, N03-, Sod2, and K I concentrations howed little or no response to variations in discharge; Na+, Ca2-+, Mg2 ’ , Cl-, HCO,J-, and SiO, all dccrcased 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 ’ yr ‘) were large. Tropical landscapes play an important role in current efforts to characterize global ele