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

Systematic investigation of carbon and nitrogen isotopic compositions of su spended particulate matters, plants, soils, sediments, and bed rocks was co nducted during 1993-1994 in the Lanyang-Hsi watershed, which is a typical s mall Oceania river. The dramatic increase in particle concentration and cha nges in nitrogen isotopic composition from tributaries to main stem indicat e a shift in the major sources of particulate matter from soil and plant de tritus (biogenic) for tributaries to poorly weathered rock fragments (litho genic) and soil for the main stem. Using a two end-member mixing of nitroge n 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 th e 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 yi eld in Oceania islands, more than half of the POC flux from these islands c ould have been humanly induced. The flux-weighted mean delta(15)N and delta (13)C values of exported particles were +3.6 parts per thousand and -25.3 p arts per thousand. The delta(13)C value is similar to those observed elsewh ere, whereas the delta(15)N value is considerably heavier, apparently attri butable to the high contribution of lithogenic PN induced by human disturba nce.