Abstract
Abstract. The role of fluvial sedimentary areas as organic carbon sinks remains largely unquantified. Little is known about mechanisms of organic carbon (OC) stabilization in alluvial sediments in semiarid and subhumid catchments where those mechanisms are quite complex because sediments are often redistributed and exposed to a range of environmental conditions in intermittent and perennial fluvial courses within the same catchment. The main goal of this study was to evaluate the contribution of transport and depositional areas as sources or sinks of CO2 at the catchment scale. We used physical and chemical organic matter fractionation techniques and basal respiration rates in samples representative of the three phases of the erosion process within the catchment: (i) detachment, representing the main sediment sources from forests and agricultural upland soils, as well as fluvial lateral banks; (ii) transport, representing suspended load and bedload in the main channel; and (iii) depositional areas along the channel, downstream in alluvial wedges, and in the reservoir at the outlet of the catchment, representative of medium- and long-term residence deposits, respectively. Our results show that most of the sediments transported and deposited downstream come from agricultural upland soils and fluvial lateral bank sources, where the physicochemical protection of OC is much lower than that of the forest soils, which are less sensitive to erosion. The protection of OC in forest soils and alluvial wedges (medium-term depositional areas) was mainly driven by physical protection (OC within aggregates), while chemical protection of OC (OC adhesion to soil mineral particles) was observed in the fluvial lateral banks. However, in the remaining sediment sources, in sediments during transport, and after deposition in the reservoir (long-term deposit), both mechanisms are equally relevant. Mineralization of the most labile OC (the intra-aggregate particulate organic matter (Mpom) was predominant during transport. Aggregate formation and OC accumulation, mainly associated with macroaggregates and occluded microaggregates within macroaggregates, were predominant in the upper layer of depositional areas. However, OC was highly protected and stabilized at the deeper layers, mainly in the long-term deposits (reservoir), being even more protected than the OC from the most eroding sources (agricultural soils and fluvial lateral banks). Altogether our results show that both medium- and long-term depositional areas can play an important role in erosive areas within catchments, compensating for OC losses from the eroded sources and functioning as C sinks.
Highlights
Soil erosion, a complex process that causes the transport and deposition of sediments with accompanying soil organic carbon (SOC) (Gregorich et al, 1998; Wang et al, 2014), affects the dynamics of the terrestrial carbon (C) cycle and Published by Copernicus Publications on behalf of the European Geosciences Union.M
A decrease in OC with decreasing aggregate size was found in the forest and agricultural soils, while no differences in the OC associated with different aggregate sizes were observed in the fluvial lateral banks (Fig. 3b)
In the deep layers of the depositional areas, the relatively lower OC, mineralization rates, and Mpom were less favorable for aggregate formation and the process unfolded very slowly compared to the upper layers, which confirms the results reported by other authors (Xie et al, 2017)
Summary
A complex process that causes the transport and deposition of sediments with accompanying soil organic carbon (SOC) (Gregorich et al, 1998; Wang et al, 2014), affects the dynamics of the terrestrial carbon (C) cycle and Published by Copernicus Publications on behalf of the European Geosciences Union.M. The fate of the redistributed organic carbon (OC) depends on multiple factors: (i) the nature of the soil organic matter being detached from different “sources” within a catchment (Nadeu et al, 2011, 2012; Kirkels et al, 2014); (ii) its turnover rates during transport; (iii) the type of erosion processes (selective or nonselective); (iv) the connectivity and distance of travel between eroding sources and the streambed (Boix-Fayós et al, 2015; Wang et al, 2010); and (v) the microenvironmental conditions under which the OC is stored in sedimentary settings (Van Hemelryck et al, 2011; Berhe and Kleber, 2013) All these factors, which affect the protection of OC against decomposition through physical and chemical mechanisms, remain considerably uncertain. Despite the fact that a combination of different techniques (isotopic, spectroscopic, and traditional wet chemistry) has been used (Wang et al, 2014; Kirkels et al, 2014; Liu et al, 2018) to determine if the eroded OC is lost after erosional redistribution, a full understanding of the dynamics and interactions between OC sources and sinks, in relation to soil erosion and redistribution, is still absent (Doetterl et al, 2016; Hoffman et al, 2013)
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