Transport-distance specific SOC distribution: Does it skew erosion induced C fluxes?

The net effect of soil erosion by water, as asink or source of atmospheric carbon dioxide (CO2), isdetermined by the spatial (re-)distribution and stability oferoded soil organic carbon (SOC), and the dynamicreplacement of eroded C by the production of newphotosynthate. The depositional position of eroded SOCis a function of the transport distances of soil fractionswhere the SOC is stored. In theory, the transport distancesof soil fractions are related to their settling velocitiesunder given flow conditions. Yet, very few field investigationshave been conducted to examine the actualmovement of eroded soil fractions along hillslopes,let alone the re-distribution pattern of SOC fractions.Eroding sandy soils and sediment were sampled after aseries of rainfall events along a slope on a freshly seededcropland in Jutland, Denmark. All the soil samples werefractionated into five settling classes using a settling tubeapparatus. The spatial distribution of soil settling classesshows a coarsening effect immediately below the erodingslope, followed by a fining trend at the slope tail. Thesefindings support the validity of the conceptual modelproposed by Starr et al. (Land Degrad Dev 11:83–91,2000) to predict SOC redistribution patterns alonghillslopes. The d13C values of soil fractions were morepositive at the footslope than on the slope shoulder or atthe slope tail, suggesting enhanced decomposition rate offresh SOC input at the footslope during or after erosioninducedtransport. Pronounced CO2 emission rates at theslope tail also suggest a higher potential for decompositionof the eroded SOC after deposition. Overall, ourresults illustrate that immediate deposition of fast settlingsoil fractions and the associated SOC at footslopes, andpotential CO2 emissions during or immediately aftertransport, must be appropriately accounted for in attemptsto quantify the role of soil erosion in terrestrial Csequestration. A SOC erodibility parameter based onactual settling velocity distribution of eroded fractions isneeded to better calibrate soil erosion models.