Abstract
Abstract. The yields of the tropical rivers of Southeast Asia supply large quantities of carbon to the ocean. The origin and dynamics of particulate organic matter were studied in the Houay Xon River catchment located in northern Laos during the first erosive flood of the rainy season in May 2012. This cultivated catchment is equipped with three successive gauging stations draining areas ranging between 0.2 and 11.6 km2 on the main stem of the permanent stream, and two additional stations draining 0.6 ha hillslopes. In addition, the sequential monitoring of rainwater, overland flow and suspended organic matter compositions was conducted at the 1 m2 plot scale during a storm. The composition of particulate organic matter (total organic carbon and total nitrogen concentrations, δ13C and δ15N) was determined for suspended sediment, soil surface (top 2 cm) and soil subsurface (gullies and riverbanks) samples collected in the catchment (n = 57, 65 and 11, respectively). Hydrograph separation of event water was achieved using water electric conductivity and δ18O measurements for rainfall, overland flow and river water base flow (n = 9, 30 and 57, respectively). The composition of particulate organic matter indicates that upstream suspended sediments mainly originated from cultivated soils labelled by their C3 vegetation cover (upland rice, fallow vegetation and teak plantations). In contrast, channel banks characterized by C4 vegetation (Napier grass) supplied significant quantities of sediment to the river during the flood rising stage at the upstream station as well as in downstream river sections. The highest runoff coefficient (11.7%), sediment specific yield (433 kg ha−1), total organic carbon specific yield (8.3 kg C ha−1) and overland flow contribution (78–100%) were found downstream of reforested areas planted with teaks. Swamps located along the main stream acted as sediment filters and controlled the composition of suspended organic matter. Total organic carbon specific yields were particularly high because they occurred during the first erosive storm of the rainy season, just after the period of slash-and-burn operations in the catchment.
Highlights
Soil is a larger terrestrial reservoir of carbon than the biosphere and atmosphere combined (e.g. Sarmiento and Gruber, 2002)
Surface and subsurface sources of particulate organic matter are best discriminated by their total organic carbon (TOC) content, which is significantly higher in surface soils
Soil-originating particles accumulated in sediments of the swamp provide 13C-enriched compositions, up to ca. −15 ‰, that are explained by the input of organic matter derived from C4 photosynthetic pathway plant tissues
Summary
Soil is a larger terrestrial reservoir of carbon than the biosphere and atmosphere combined (e.g. Sarmiento and Gruber, 2002). Huang et al (2012) estimated that the tropical rivers of Asia have the highest specific total, inorganic and organic, dissolved and particulate carbon yield in which ca. Small mountainous headwater catchments play a key role in the delivery pattern because they are characterized by high specific discharges and sediment loads (Milliman and Syvitski, 1992) In this context, processes that control organic matter export from tropical catchments should be better understood and quantified, as they contribute significantly to drawdown or emission of carbon dioxide (Lal, 2003)
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