Abstract
Abstract. The effect of soil erosion on global carbon cycling, especially as a source or sink for greenhouse gases, has been the subject of intense debate. The controversy arises mostly from the lack of information on the fate of eroded soil organic carbon (SOC) whilst in-transit from the site of erosion to the site of longer-term deposition. Solving this controversy requires an improved understanding of the transport distance of eroded SOC, which is principally related to the settling velocity of sediment fractions that carry the eroded SOC. Although settling velocity has already been included in some erosion models, it is often based on mineral particle size distribution. For aggregated soils, settling velocities are affected by their actual aggregate size rather than by mineral particle size distribution. Aggregate stability is, in turn, strongly influenced by SOC. In order to identify the effect of aggregation of source soil on the transport distance of eroded SOC, and its susceptibility to mineralization after transport and temporary deposition, a rainfall simulation was carried out on a silty loam. Both the eroded sediments and undisturbed soils were fractionated into six different size classes using a settling tube apparatus according to their settling velocities: > 250, 125 to 250, 63 to 125, 32 to 63, 20 to 32 and < 20 μm. Weight, SOC content and instantaneous respiration rates were measured for each of the six class fractions. Our results indicate that (1) 41% of the eroded SOC was transported with coarse aggregates that would be likely re-deposited down eroding hillslopes, rather than with fine particles likely transferred to water courses; (2) erosion was prone to accelerate the mineralization of eroded SOC, and thus might contribute more CO2 to the atmosphere than current estimates which often ignore potential effects of aggregation; (3) preferential deposition of SOC-rich coarse aggregates potentially causes an increase of SOC remaining in the colluvial system and a reduction of SOC flux to the alluvial or aquatic system. These findings identify a potential error of overestimating net erosion-induced carbon sink effects, and thus add an additional factor to consider when improving our current understanding of SOC erosion and deposition on hillslopes.
Highlights
The net effect of soil erosion as a source or sink of CO2 in the global carbon cycle has been the subject of intense debate (Lal, 2003; van Oost et al, 2007; Quinton et al, 2010; Dlugoß et al, 2012; Doetterl et al, 2012)
Our results indicate that (1) 41 % of the eroded soil organic carbon (SOC) was transported with coarse aggregates that would be likely re-deposited down eroding hillslopes, rather than with fine particles likely transferred to water courses; (2) erosion was prone to accelerate the mineralization of eroded SOC, and might contribute more CO2 to the atmosphere than current estimates which often ignore potential effects of aggregation; (3) preferential deposition of SOC-rich coarse aggregates potentially causes an increase of SOC remaining in the colluvial system and a reduction of SOC flux to the alluvial or aquatic system
This proportion strongly contrasts the approximately 11 % SOC mass associated with coarse mineral particles > 63 μm in the original soil (Table 1), and is contrary to the high SOC content (24.3 mg g−1) in sediment fraction of Equivalent Quartz Size (EQS) < 20 μm (Fig. 3a)
Summary
The net effect of soil erosion as a source or sink of CO2 in the global carbon cycle has been the subject of intense debate (Lal, 2003; van Oost et al, 2007; Quinton et al, 2010; Dlugoß et al, 2012; Doetterl et al, 2012). Erosion exposes the previously incorporated soil organic carbon (SOC), which may accelerate the mineralization of eroded SOC (Jacinthe et al, 2002, 2004; Kuhn, 2007; Mora et al, 2007; Lal and Pimentel, 2008). Deposition limits the decomposition of SOC upon burial, while input of decomposing plant material on the surface of eroding sites partially replaces the lost SOC (Harden et al, 1999; van Oost et al, 2007; Wang et al, 2010). Several recent publications showed (at least) temporary enrichment of SOC in sediment, as well as preferential deposition of aggregates with size distribution
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