Export of carbon, nitrogen, and major ions from three tropical montane watersheds

Abstract

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. NH 4, NO 3−, SO 42−, and K+ concentrations showed little or no response to variations in discharge; Na+, Ca 2+, Mg 2, Cl −, HCO 3−, and SiO 2 all decreased at high flows. After removing the effects of discharge, residual NO 3− 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.