SIGNIFICANCE OF EROSION-INDUCED LATERAL CARBON FLUXES FOR
THE CARBON BALANCE IN A REFORESTED CATCHMENT
Nadeu, E.a, de Vente, J. a, Martínez-Mena, M. a, Boix-Fayos, C. a
a
Grupo de Erosión y Conservación de Suelos. Centro de Edafología y Biología Aplicada del Segura
(CEBAS-CSIC), Campus Universitario de Espinardo, 30100, Murcia
1. Introduction
The assessment of the impact of soil erosion on soil organic carbon (SOC) stocks and
its significance for the carbon (C) balance requires the quantification of lateral and
vertical C fluxes associated to detachment, transport and deposition of soil and
sediment. In this context, lateral C fluxes can directly increase or decrease SOC stocks
by removing topsoil and, indirectly, can also affect C dynamics at eroding and
depositional sites by changing the magnitude and spatial distribution of vertical C fluxes
such as C input to soil and C mineralization to the atmosphere (Stallard, 1998). Here,
we estimated their impact on SOC stocks and C sequestration in a Mediterranean
catchment (50 km2) with heterogeneous land use and land cover (LULC) conditions
and where hydrological correction works, including reforestation and the construction of
check-dams, have been conducted during the last 40 years.
2. Materials and methods
Estimations of erosion-induced C fluxes were based on previous studies at the study
catchment (Rogativa catchment, NW Murcia, Spain) (i.e. Boix-Fayos et al, 2009;
Nadeu et al., 2012). The catchment is divided in 58 subcatchments delimited by checkdams with sedimentary wedges. We used each subcatchment as a unit for estimations
of sediment yield and exported C. We first calculated SOC stocks for the years 1976
(check-dam construction date) and 2003 based on a land use and land cover (LULC)
maps (Boix-Fayos et al. 2008). Then, a C budget for the subcatchments was estimated
with a bottom-up approach using field, modeled and literature data. We: (i) estimated C
stocks in the sediment wedges behind check-dams (CCD) (Mg); (ii) calculated C
exported downstream (Cexp) (Mg) by using the sediment trapping efficiency value (%)
calculated for each check-dam (Boix-Fayos et al. 2008); (iii) used output data from the
application of an erosion and C redistribution model to estimate C deposition values
over the landscape (26%) (Cred) and (iv) data from literature for an approximate C
mineralization value during transport and deposition (20%) (Cmin) (Lal et al. 2003; Van
Hemelryck et al. 2010).
3. Results and discussion
From these calculations, total C mobilized by erosion was 3850±1000 Mg (Ctot) during
the 27 years between check-dam construction and the field survey. This value
represents between 3% and 18% of the SOC stocks in the top 5 cm present in 1974
and taking into account uncertainties in C concentration in sediments and soils (67%
confidence interval). The change in SOC stocks during the 1974-1997, induced by
agricultural abandonment and reforestations, resulted in an average increase of SOC
in the drainage area estimated around 10200±8000 Mg (Figure 1). The LULC changes
resulted in a decrease in total erosion rates (Boix-Fayos et al. 2008), but not in the
potential C mobilization, given the increased SOC stocks.
When looking at the whole C balance (Figure 1), we observed that erosion-induced
lateral C fluxes in the Rogativa catchment had a non-negligible impact on SOC stocks
and on the total C budget. They significantly reduced SOC stock gains through LULC
changes (by an average 40%) and enhanced CO2 emissions of otherwise protected C.
On the other hand, however, since the loss of C from eroded soil profiles opens the
opportunity for the input of fresh organic matter into the soil matrix, in what is known as
C replacement (Harden et al. 1999), erosion processes likely contributed to an
additional C sequestration in soils, as observed in a previous study in two
subcatchments (Nadeu et al. 2012). Finally, to complete the catchment balance, the
presence of check-dams in this environment allowed for the establishment of a C sink
within the sediment wedges in the order of ~41% (CCD) of the total C eroded from
catchment soils. This large amount of deposited C indicates that, eventually, the
significance of erosion-induced lateral C fluxes in the Rogativa catchment will largely
depend on the fate of the C stored at these depositional sites, as well as the fate of the
C exported further downstream into the river network.
Fig 1. Total erosion-induced C fluxes during the studied period (27 years) and SOC stocks for
the top 5 cm.
4. Conclusions
The results presented stress the importance of quantifying erosion-induced C fluxes
and including them in evaluations of the impact of LULC changes, such as
reforestations, on ecosystem C budgets which, in most assessments, only account for
changes in standing biomass and SOC stocks.
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