Composition changes of eroded carbon at different spatial scales in a tropical watershed suggest enrichment of degraded material during transport

Abstract. In order to assess whether eroded carbon is a net source or sink of atmospheric CO₂, characterisation of the chemical composition and residence time of eroded organic matter (EOM) at the landscape level is needed. This information is crucial to evaluate (1) how fast EOM can be decomposed by soil microbes during its lateral transport and (2) its impact at deposition sites. This study considers a continuum of scales to measure the composition of EOM across a steep hillslope landscape of the Mekong basin with intense erosion. We sampled suspended sediments eroded during rainfall events from runoff plots (1 and 2.5 m²) and the outlets of four nested watersheds (0.6 × 10⁴ to 1 × 10⁷ m²). Here we show that changes in the chemical composition of EOM (measured by nuclear magnetic resonance spectroscopy) and in its ¹³C and ¹⁵N isotope composition from plot scale through to landscape scale provide consistent evidence for enrichment of more decomposed EOM across distances of 10 km. Between individual soil units (1 m²) to a small watershed (10⁷ m²), the observed 28% decrease of the C/N ratio, the enrichment of ¹³C and ¹⁵N isotopes as well as O-alkyl C in EOM is of similar magnitude as changes recorded with depth in soil profiles due to soil organic matter "vertical" decomposition. Radiocarbon measurements indicated ageing of EOM from the plot to the watershed scale. Therefore transport of EOM may lead to enrichment of stabilised soil organic matter compounds, eventually being subject to export from the watershed.