Fluvial sedimentary deposits as carbon sinks: organic carbon pools and stabilization mechanisms across a Mediterranean catchment
<p>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 re...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-03-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/16/1035/2019/bg-16-1035-2019.pdf |
| Summary: | <p>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 <span class="inline-formula">CO<sub>2</sub></span> 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
(<span class="inline-formula"><i>M</i><sub>pom</sub></span>) 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.</p> |
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| ISSN: | 1726-4170 1726-4189 |