BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-11-3299-2014 Composition changes of eroded carbon at different spatial scales in a tropical watershed suggest enrichment of degraded material during transport Rumpel C. 1 Chaplot V. 2 Ciais P. 3 Chabbi A. 4 Bouahom B. 5 Valentin C. 6 CNRS, Institut d'Ecologie et de l'Environnement de Paris (IEES), Centre INRA Versailles-Grignon, Bâtiment EGER, Thiverval-Grignon, France Institut de Recherche pour le Développement (IRD), Laboratoire d'Océanographie et du Climat (LOCEAN), Université Pierre et Marie Curie, Paris, France Laboratoire des Sciences du Climat et de l'Environnement (LSCE), UMR8212, CEA CNRS UVSQ, Gif-sur-Yvette, France INRA, UR P3F, Centre INRA Poitou-Charentes, Lusignan, France SSLCC/NAFRI, Vientiane, LAO-PDR Centre IRD d'île de France, 32, avenue Henri Varagnat, 93143 Bondy Cedex, France 20 06 2014 11 12 3299 3305 Copyright: © 2014 C. Rumpel et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/11/3299/2014/bg-11-3299-2014.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/11/3299/2014/bg-11-3299-2014.pdf

In order to assess whether eroded carbon is a net source or sink of atmospheric CO<sub>2</sub>, 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 m<sup>2</sup>) and the outlets of four nested watersheds (0.6 × 10<sup>4</sup> to 1 × 10<sup>7</sup> m<sup>2</sup>). Here we show that changes in the chemical composition of EOM (measured by nuclear magnetic resonance spectroscopy) and in its <sup>13</sup>C and <sup>15</sup>N 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 m<sup>2</sup>) to a small watershed (10<sup>7</sup> m<sup>2</sup>), the observed 28% decrease of the C/N ratio, the enrichment of <sup>13</sup>C and <sup>15</sup>N 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.

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