Stable carbon and nitrogen isotope systematics in a human-disturbed watershed (Lanyang-Hsi) in Taiwan and the estimation of biogenic particulate organic carbon and nitrogen fluxes

Abstract

Systematic investigation of carbon and nitrogen isotopic compositions of suspended particulate matters, plants, soils, sediments, and bed rocks was conducted during 1993-1994 in the Lanyang-Hsi watershed, which is a typical small Oceania river. The dramatic increase in particle concentration and changes in nitrogen isotopic composition from tributaries to main stem indicate a shift in the major sources of particulate matter from soil and plant detritus (biogenic) for tributaries to poorly weathered rock fragments (lithogenic) and soil for the main stem. Using a two end-member mixing of nitrogen isotopes and the load-runoff relationship, we estimated the mean yield of biogenic particulate nitrogen (PN) in this watershed to be 0.41 ± 0.19 g N m-2 yr-1, which comprised only 9% of the total PN yield. Assuming the mean atomic C/N ratio (13.6) of soil samples as that for the biogenic end-member, we estimated the yield of biogenic particulate organic carbon (POC) to be 4.9 ± 2.2 g C m-2 yr-1. If this represents the natural POC yield in Oceania islands, more than half of the POC flux from these islands could have been humanly induced. The flux-weighted mean δ15N and δ13C values of exported particles were +3.6‰ and -25.3‰. The δ13C value is similar to those observed elsewhere, whereas the δ15N value is considerably heavier, apparently attributable to the high contribution of lithogenic PN induced by human disturbance.