Losses and redistribution of organic carbon by erosion in fragile agricultural

and restored catchmentsBoix-Fayos C, de Vente J, Nadeu, E. , Almagro M, Pérez-Cutillas P, Navas A,

Gaspar L, Martínez-Mena M CEBAS-CSIC, Spanish Research Council, Murcia; University of Murcia; EEAD-CSIC,

Spanish Research Council, Zaragoza; University of Northern British Columbia

Fluvial systems constitute largesediment stores and have thepotential to store large amounts oforganic carbon associated to thesediments.However, redistribution oforganic carbon by lateral flows incatchments can be affected byfluvial processes, complex erosionpatterns (gully, channel and bankerosion) and by transport andpost-depositional processes insediments. Although large effortsare being made to understand theflow paths of OC at the catchmentscale, studies quantifying globalredistribution of organic carbonby lateral flows at this scale arestill scarce. Particularly, little isknown on how organic carbon isredistributed in fragileenvironments with a variety oflithologies, land uses patterns(large agricultural areas adjacentto large reforested areas) andephemeral hydrological andsedimentological processes,typical of Mediterraneanconditions.The catchment scale research onthis matter could identify organiccarbon sinks providing insight onopportunities for carbonsequestration through sedimentsmanagement.

The objective of this work was toestimate the organic carbonredistributed by lateral flows in 2representative Mediterraneancatchments, highly disturbed byagricultural terraces, landlevelling for agriculture,reforestation and construction ofcheck-dams, with erodiblelithologies and shallow soils.Research was carried out in twocatchments with differentclimatological conditions(Cárcavo, precipitation of 285mm year-1 and Rogativa, 530 mmyear-1) in SE Spain, representingmedium mountainMediterranean environmentswith a variety of land uses. Acarbon budget within theerosional cycle was estimated foreach catchment taking intoaccount carbon mineralizedduring erosion processes, carbondeposited at channel sinks andcarbon exported downstream.

In both catchments, organiccarbon concentration ofsediments in the fluvial system issignificantly lower than the soilorganic carbon concentration offorest and shrubland soils (Table1). On average, sediments in bothcatchments contain a 43% of theorganic carbon of forest soils(first 5 cm). However, organiccarbon concentration is higher insediments than in agriculturalsoils. The specific soil carbonyield estimated at the catchmentscale is similar for both areas.Total organic carbon removed byerosion processes represented a9,68 % of the superficial 5 cm soilstock in 21 years for the Cárcavobasin, and a 9.09 % of that for theRogativa basin in 27 years. Fromthe organic carbon redistributedby lateral fluxes, 76.6 % and 67.4% (alluvial wedges + footslopes)was redeposited within the fluvialsystem in Cárcavo and Rogativa,respectively.

In fragile Mediterraneanenvironments with activegeomorphological processes,despite restoration activities(reforestation and hydrologicalcontrol works), erosionprocesses caused the loss ofmore than 10% of the superficialsoil carbon stock during thestudied period (20-30 years). Atthis catchment scale, sedimentsflowing within the fluvial systemhave similar concentrations oforganic carbon than agriculturalsoils of the catchments. Lossesof soil organic carbon due onlyto lateral fluxes represent a rateof at least 0.4% per year in bothstudied catchments, withouttaking into account losses oforganic carbon due to otherprocesses (as land useconversions and landmanagement practices). Thoseresults point out the need toprotect soil organic carbonresources in active fluvialenvironments of those fragileecosystems, for instance withmeasures for stabilizing organiccarbon in soils and sedimentsand decreasing connectivity ofrich-carbon soils with fluvialchannels.

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Tab. 1: Indicators of the Total Organic Carbon concentration, stored and exported in the Cárcavo and Rogativacatchments.

Catchment TOC forest soils (g kg-1)

TOC agricultural soils

(g kg-1) Cárcavo 15.2 ± 9.35 4.4±1.06 Rogativa 18.82 ± 5.37 8.12 ± 3.20 TOC Sediments

0-100 cm (g kg-1)

Soil TOC stock 5 cm

(tn ha-1) Cárcavo 6.6 7.3 Rogativa 11.7 9.31 TOC buried

check-dams (21 years)

(tn)

TOC exported downstream

check-dams (27 years)

(tn) Cárcavo 677.65 45.156 Rogativa 1654.18 502.29 Specific soil

carbon yield (SCY)

catchment (tn ha-1 yr-1)

Cárcavo 0.040±0.047 Rogativa 0.037±0.045

INTRODUCTION OBJECTIVES MAIN RESULTS

Fig. 2: High connectivity betweenagricultural fields and channels at theCárcavo catchment.

Fig. 4: Upper Taibilla catchment andsubcatchment of La Rogativa withsubhumid climatological conditions

Fig. 5: Cárcavo catchment withsemiarid-arid climatological conditions.Both have suffered reforestation of theircatchment areas and land abandonmentin the last 40 years

Fig. 1 : Project location

Fig. 4: Photos of sampling

Fig. 3: The alluvial wedge behind thecheck-dam creates a sediment sink andthus an opportunity to sequester carbon.Protection of soils through propermanagement is important to decreaseconnectivity

CONCLUSION