Downstream alteration of the composition and biodegradability of particulate organic carbon in a mountainous, mixed land-use watershed
Abstract
Despite increasing recognition of storminduced
organic carbon (C) export as a significant loss
from the terrestrial C balance, little is known about the
biodegradation and chemical transformation of particulate
organic carbon (POC) in mountainous river
systems. We combined analyses of C isotopes, solution-
state 1H NMR, and lipid biomarkers with biodegradable
dissolved organic C (BDOC) measurements
to investigate downstream changes of POC composition
and biodegradability at a mountainous, mixed
land-use watershed in South Korea. Water and
suspended sediment (SS) samples were collected in a
forested headwater stream, a downstream agricultural
stream, and two downstream rivers during peak flow
periods of four storm events, using either sequential
grab sampling from the headwater stream to the most
downstream river within a few hours around the peak
flow or sediment samplers deployed during the whole
storm event. DOC concentrations exhibited relatively
small changes across sites, whereas POC concentrations
were highest in the agricultural stream, and
tapered along downstream reaches. The d13C and d15N
of SS in the agricultural stream were distinct from upand
downstream signatures and similar to those for
erosion source soils and lake bottom sediment,
although increases in radiocarbon age indicated continuous
compositional changes toward the lake. 1H
NMR spectra of SS and deposited sediment exhibited
downstream decreases in carbohydrates and lignin but
enrichment of organic structures related to microbial
proteins and plant wax. The downstream sediments
had more microbial n-alkanes and lipid markers
indicating anthropogenic origin such as coprostanol
compared to the forest soil n-alkanes dominated by
plant wax. While the BDOC concentrations of filtered
waters differed little between sites, the BDOC concentrations and protein- to humic-like fluorescence
ratios of DOC leached from SS during a 13-day
incubation were higher in downstream rivers, pointing
to contribution of labile POC components to the
enhanced biodegradation. Overall, inputs of microbial
and anthropogenic origin, in interplay with deposition
and mineralization, appear to substantially alter POC
composition and biodegradability during downstream
transport, raising a question on the conventional view
of mountainous river systems as a passive conduit of
storm pulses of POC.