Sources and export of particle-borne organic matter during a monsoon flood in a catchment of northern Laos

[摘要] The yields of the tropical rivers of Southeast Asia supply large quantities of carbon tothe ocean. The origin and dynamics of particulate organic matter were studiedin the Houay Xon River catchment located in northern Laos during the firsterosive flood of the rainy season in May 2012. This cultivated catchment isequipped with three successive gauging stations draining areas rangingbetween 0.2 and 11.6 km² on the main stem of the permanent stream, andtwo additional stations draining 0.6 ha hillslopes. In addition, thesequential monitoring of rainwater, overland flow and suspended organicmatter compositions was conducted at the 1 m² plot scale during astorm. The composition of particulate organic matter (total organic carbonand total nitrogen concentrations, δ¹³C and δ¹⁵N) wasdetermined for suspended sediment, soil surface (top 2 cm) and soilsubsurface (gullies and riverbanks) samples collected in the catchment (n= 57, 65 and 11, respectively). Hydrograph separation of event water wasachieved using water electric conductivity and δ¹⁸O measurementsfor rainfall, overland flow and river water base flow (n = 9, 30 and 57,respectively). The composition of particulate organic matter indicates thatupstream suspended sediments mainly originated from cultivated soils labelledby their C₃ vegetation cover (upland rice, fallow vegetation and teakplantations). In contrast, channel banks characterized by C₄ vegetation(Napier grass) supplied significant quantities of sediment to the riverduring the flood rising stage at the upstream station as well as indownstream river sections. The highest runoff coefficient (11.7%),sediment specific yield (433 kg ha⁻¹), total organic carbon specificyield (8.3 kg C ha⁻¹) and overland flow contribution (78–100%)were found downstream of reforested areas planted with teaks. Swamps locatedalong the main stream acted as sediment filters and controlled thecomposition of suspended organic matter. Total organic carbon specific yieldswere particularly high because they occurred during the first erosive stormof the rainy season, just after the period of slash-and-burn operations inthe catchment.

[学科分类] 地球化学与岩石