Abstract
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.
Highlights
Decomposition of soil organic matter is one of the most important processes controlling the response of the global carbon cycle to climate and land use change (e.g. Lal et al, 2004)
The aim of the study was to assess the changes occurring in elemental, isotopic and biochemical composition of eroded organic matter (EOM) recovered from different scales and to compare them to changes occurring during soil organic matter stabilisation from topsoil to subsoil horizons for five major rainfall events occurring throughout the rainy season
The action of microbial decomposition on EOM at larger scales is evidenced by changes in the stable carbon and nitrogen isotopic ratios, which both increase towards larger horizontal scales and longer transport (Fig. 3)
Summary
Decomposition of soil organic matter is one of the most important processes controlling the response of the global carbon cycle to climate and land use change (e.g. Lal et al, 2004). The paradigm of one-dimensional microbial decomposition occurring only at depth in the soil profile, as adopted by all global models, is highly questionable because of lateral transport of soil organic matter during hydrologic erosion. In this context, it remains controversial whether eroded organic matter (EOM) is a source or a sink of carbon (van Oost et al, 2007, 2008; Lal and Pimentel, 2008; Kuhn et al, 2009).
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