Assessment of Soil Erosion Using 137 Cs on Cultivated Fields Following Natural Forest Conversion in the Kefa Zone of Southwest Ethiopia.

Transcription

1 Assessment of Soil Erosion Using 137 Cs on Cultivted Fields Following Nturl Forest Conversion in the Kef Zone of Southwest Ethiopi. Mekuri Argw 1*, Pul L.G. Vlek 1, Soojin Prk 2, Mnfred Denich 1, Demel Tekety 3 1 Center for Development Reserch, University of Bonn, Wlter-Flex Str. 3, D Bonn, Germny 2 Geogrphy Deprtment, Seoul Ntionl University, Shilim-Dong, Kwnt-Gu, Seoul, Kore 3 Ethiopin Agriculturl Reserch Orgniztion, P.O. Box. 2003, Addis Abb, Ethiopi Abstrct Soil erosion is prime cuse of loss of productivity of lnd. Decline in lnd productivity in most cses triggers the conversion of nturl forests into griculturl lnd. The severe soil erosion in the highlnds of Ethiopi is believed to be result of griculturl conversion. The process of nturl forest conversion is growing phenomenon in the southwest region of Ethiopi. This study ws imed t ssessing the mgnitude nd rte of soil erosion in 24 km 2 sub-ctchment in the Kef Zone of southwest Ethiopi, using the 137 Cs technique. A chronosequence of continuously cultivted fields of 2, 6, 12, 16, 20, 24 nd 58 yers fter conversion were studied. A reference inventory of 2026 ± 176 Bq m -2 with coefficient of vrition of 24.6 % ws recorded indicting the pplicbility of the technique in the region. Although wek, the distribution of the 137 Cs inventories in the studied fields showed declining trend (R 2 = 0.2) with incresing yers of continuous cultivtion fter conversion. The vlues of the younger nd older fields were 1994 Bq m -2 nd 1164 Bq m -2 respectively, indicting the greter extent of soil erosion in the older fields thn in the younger fields. Estimted rtes of soil erosion using the Proportionl Model (PM) nd Mss Blnce Model 1 (MBM1) showed n incresing trend (R 2 = 0.41) with incresing yers of cultivtion. The rte of soil erosion rnged between 1 t h -1 yr -1 in the younger field nd 25.7 t h -1 yr -1 in the older field. Estimted rtes of erosion for the sub-ctchment were 11.6 ± 2.6 t h -1 yr -1 nd 17.3 ± 4 t h -1 yr -1 by the PM nd MBM1, respectively. An estimte using the Universl Soil Loss Eqution (USLE) yielded rte of 12.3 t h -1 yr -1 vlidting the results from the 137 Cs models nd the pplicbility of the technique for soil erosion studies in the Ethiopi. The results of this study showed tht soil erosion in the Kef Zone is on the verge of surpssing the tolerble level nd it should be n immedite concern to conservtionists nd development plnners t ll levels. Key words: Cesium-137, Ethiopi, forest conversion, Mss Blnce Model 1, Proportionl Model, soil erosion * Corresponding uthor. Tel ; Fx: ++49-(0) E-mil ddress: mrgwd@uni-bonn.de; m_rgw@yhoo.com 1

2 1 Introduction Conversion of forests into permnent cultivtion or short-fllow systems hs been source of cultivted lnd in the tropics (Fujisk et l., 1996; Vlek et l., 1997). In the tropics, where rinfll is hevy nd intense, conversion of forests into griculturl lnd cuses severe soil erosion on cultivted fields unless pproprite preventive mesures re tken t the initil stges (Ll, 1995). The short-term effect of forest conversion is rpid decline of the orgnic mtter in the top soil while the long term effect is reduced structure nd physicl instbility of the soil (Chrmn nd Murphy, 2000). Structurl decline coupled with the bsence of mendment interventions results in reduced infiltrtion, ccelerted runoff nd ccelerted surfce soil erosion (Motvlli et l., 2000). Studies indicte tht clering of forests for griculture nd indequte erosion prevention mesures in the pst mny yers hve been the min cuses of soil degrdtion in the densely populted highlnds of Ethiopi (Feoli et l., 2002). Hurni (1988), from plotmesurements, reported men nnul soil erosion rte of 42 t h -1 yr -1 or n equivlent loss of soil depth of 4 mm yr -1 from cultivted fields in the centrl nd northern highlnds. Forest conversion in the southwestern highlnds hd remined insignificnt in the pst few decdes. The opening of inrods nd the strt of forest logging incresed movement of people to the region nd triggered the conversion forest lnd into griculturl lnd. Forest conversion nd cerel crop bsed frming expnded to new frontiers, especilly in the Kef Zone, following the resettlement of people from the centrl nd northern highlnds (Alemneh, 1990). However, reports on the stte of soil erosion in the region re scnty. The 137 Cs technique hs been used for soil erosion studies in mny prts of the world. However, there re not mny studies reported from the equtoril region, prticulrly from Afric using this technique (Collins et l., 2001; Chppell et l., 1998). This might be due to the low ssumed reference fllout, Bq m -2, for the region compred to Bq m -2 estimted for the temperte region (Collins et l., 2001). This study is pioneer contribution to the use of the technique in estern Afric, in the southwestern highlnds of Ethiopi. The min objectives re: 1) to ssess the rte of soil erosion in continuously cultivted fields tht were originlly estblished through conversion of nturl forests, nd 2) to test the pplicbility of the 137 Cs method under Ethiopin conditions. 2 Mterils nd Methods 2.1 The 137 Cs method The bsic principles of the use of 137 Cs s n erosion ssessment technique re well covered by other studies (Wlling nd Quine, 1993; Zpt, 2003). Cesium-137 is n rtificil rdioctive element with hlf-life of yers in the environment (Richie nd McCrty, 2003). Since this hlf-life is very short on geologicl time scle, it is impossible to find ny mesurble 137 Cs remins in cesium bering rocks. Thus, there re only two sources of 137 Cs in the environment: tmospheric testing of thermonucler wepons in the lte 1950s, 1970s, nd the 1986 Chernobyl ccident (Quine et l., 1999). From the tmosphere, 137 Cs flls bck to the surfce of the erth minly with rinfll. The deposited fllout is rpidly 2

3 dsorbed by fine soil prticles on the ground surfce. Once dsorbed, it is not esily detched from the soil nd moves physiclly with soil prticles tht re crried by other gents. Its mobility nd redistribution is ssocited with the mobility nd redistribution of soil prticles. This redistribution in gro-ecosystems is cumultive result of tillge, soil erosion nd deposition from the time of fllout to the time of smpling (Zpt, 2003). Loss/gin of 137 Cs from prticulr point is determined by compring to reference site. Since there is n estblished empiricl nd theoreticl reltionship between the loss nd gin of 137 Cs nd soil, the rtes of soil erosion nd deposition re redily estimted from 137 Cs mesurements using conversion models (Wlling nd He, 2001). Two conversion models, the Proportionl Model (PM) nd the Simplified Mss Blnce Model 1 (MBM1), were used for interpreting the 137 Cs mesurements into soil erosion/deposition rtes (Wlling nd He, 2001). These models re designed for cultivted soils nd re widely used (Wirnth et l., 2001; Bujn et l., 2003). Estimtions from the 137 Cs method re usully vlidted by compring results obtined from other methods such s runoff plotexperiments, rinfll simultions nd erosion models (Fuljtr, 2003). In this study n ttempt ws mde to estimte the men nnul rte of soil erosion using the dpted Universl Soil Loss Eqution (USLE) for Ethiopi (Hurni, 1985). The corresponding fctor vlues of the model prmeters were tken from Hellden (1987). 2.2 Description of the study site nd smple fields The study ws conducted in the Shomb sub-ctchment in the Gimbo District of the Kef Zone, southwest Ethiopi. The Kef Zone is situted in the northwestern prt of the Southern Ntions, Ntionlities nd Peoples Regionl Stte (SNNPRS) (Fig. 1). The subctchment covers n re of 24 km 2. Altitude rnges between 1440 m.s.l. in the vlley bottom nd 1725 m.s.l. t the upper plteus. The soils of the re belong to the Nitisols (Tfesse, 1996). Interprettion of the 1967 eril photo nd nlysis of the 2001 Lndst imge hs indicted tht more thn 80 % of the nturl forest in the sub-ctchment hs been converted to griculturl lnd between 1967 nd 2001 (Mekuri, unpublished). The smple cultivted fields, which were studied for erosion, were selected long the chronosequence of forest conversion within the sub-ctchment (Fig. 1, Tble 1). The ge of field ws counted from the first yer of conversion up to the time of smpling. Informtion from eril photo interprettion nd frmers interviews were used to verify the ge of field. Tble1. Chrcteristics of the chronosequentil smple fields Age of field Yer of conversion Are (h) Altitude (m) Men slope grdient (%) Men slope length (m)

4 Figure 1. Loction of the study re, study sub-ctchment, smple fields, reference sites nd sketch of the smpling trnsects nd points. 2.3 Soil smpling, nlysis of 137 Cs ctivity nd dt nlysis The generl geomorphology of the fields ws liner nd soil smples were tken from the upper, middle nd lower slope positions of ech field. Bulk core soil smples were collected long two prllel trnsects using 5 cm dimeter nd 40 cm long Eijkelkmp (model 04.17) undisturbed Split-Tube-soil smpler. To determine the locl 137 Cs reference fllout inventory, four uncultivted reference sites (R 1, R 2, R 3, nd R4) were smpled t the middle nd lower plteus of the sub-ctchment (Fig. 1). From ech reference site, two bulk core soil smples were collected t two corners of 30 m x 30 m grid re. Six incrementl depth smples (10 cm ech) from two of the reference sites were collected for depth distribution nlysis. Soil smples were ir-dried, lightly disggregted, nd ground using mortr nd pssed through 2 mm wire mesh in order to seprte corse nd fine frctions (< 2 mm). Totl fine frctions of ech smple were weighed nd recorded for determining the 137 Cs inventory. Activity of 137 Cs in the soil smples ws nlyzed in the isotope lbortory (ISOLAB) of the University of Göttingen, Germny. Activity of 137 Cs ws mesured by gmm spectrometry with n HP detector of 26 % reltive efficiency nd PC-bsed multi-chnnel 4

5 nlyzer with totl spectrum re of 2000 Kiloelectronvolt (KeV). Cesium-137 ws represented by pek on the electricl signl spectrum centered t 662 Kev. Inventory (Bq m -2 ) of ech smple point ws determined by using the totl fine weight of the smple, the horizontl surfce re (surfce re of the smpling device) nd the 137 Cs concentrtion (Bq kg -1 ) mesured for the smple (Wlling nd Quine, 1993). One-wy ANOVA ws used to ssess vritions in 137 Cs inventories nd rtes of soil erosion mong the different yers of cultivtion nd mong the slope positions. Mens were seprted by LSD. Regression nlysis ws used to determine the reltionship between the 137 Cs inventories, rtes of soil erosion with yers of cultivtion. Dt were nlyzed using the Sttisticl Pckge for Socil Sciences (SPSS) (Brymn nd Crmer, 2001). 3 Results nd Discussion 3.1 Reference 137 Cs inventory nd its depth distribution The minimum nd mximum reference 137 Cs inventories from the bulk core profile smples were 1292 nd 2996 Bq m -2 in the first nd third reference sites, respectively (Tble 2). The estimted men reference inventory, which represents the totl fllout inventory t the study re, ws 2026 ± 176 Bq m -2 with coefficient of vrition (CV) of 24.6 %. This vribility is within the rnge of the moderte ctegory %, which is n cceptble rnge for CV of 137 Cs reference sites (Sutherlnd, 1996; Pennock, 2000). Tble 2. Reference 137 Cs inventory from bulk core nd incrementl depths Core profiles Incrementl depths Smple points (Bq m -2 ) Smple points R R R R R R R R R R Men 2026 Men S.D. 499 S.D S.E. 176 S.E The depth distribution showed tht 69 % of the 137 Cs inventory ws found in the upper 10 cm of the soil surfce with 19 % nd 12 % in the 20 nd 30 cm lyers, respectively. The pttern shows n exponentil decrese with incresing depth (Fig. 2). Lrge 137 Cs concentrtion on surfce soil nd exponentil decline with depth re typicl chrcteristics of n undisturbed reference sites in mny 137 Cs relted studies (Wlling nd Quine, 1993). These chrcteristics ffirm the relibility of the reference inventories for estimting loss or gin of 137 Cs in the study fields. 5

6 137 Cs (Bq m -2 ) Soil Depth (cm) Figure 2. Depth distribution pttern of 137 Cs in the reference profiles. The men reference vlue in this study is in good greement with the work of Chppell et l. (1998) in Niger, who found men reference inventory of 2066 ± 125 Bq m -2 from 11 uncultivted sites in soil depth of 60 cm. In northern Ghn, Pennock (2000) reported reference vlue of Bq m -2 (55 % less thn in this study), in n uncultivted site with CV of 21.3 % from 12 core profiles in soil depth of 25 cm. However, the vilble evidence is insufficient to explin the vribility of fllouts in the region. 3.2 Distribution of 137 Cs inventories in the cultivted fields The men inventory for the cultivted fields ws 1649 ± 81 Bq m -2 with CV of 22.5 %. This vlue is lower by 19 % thn the locl men reference inventory, indicting net loss of 137 Cs from cultivted fields in the sub-ctchment. The negtive residuls in Tble 3 indicte tht ech field hd experienced net loss of 137 Cs in the course of cultivtion fter conversion. Since the removl nd redistribution of 137 Cs is ssocited with the physicl processes of soil erosion, the net losses from the fields re indictors of soil erosion (Guimres, 2003). The high men vlue tht ws recorded from the field cultivted for 6 yers might be result of deposition of 137 Cs contining soil from the upper slope res s result of wter erosion. For instnce, Zhng et l. (2003) in Chin reported tht distribution of 137 Cs inventory on cultivted slopes ws cused by wter erosion nd tillge prctices. Tble 3. Men (± S.E) nd residuls of 137 Cs inventories (Bq m -2 ) in the different slope positions of the cultivted fields (n = 21). Age of Upper Middle Lower Men Residuls field slope slope slope ± ± ± ± ± ± 140 b ± men vlues with similr letters re not significntly different (LSD α = 0.1 ) 6

7 The increse in 137 Cs inventories in the middle nd lower slopes of the younger fields (2, 6 nd 16 yers) might indicte tht redistribution of soil in the erly yers of cultivtion fter conversion ws more under the influence of soil tillge thn wter erosion. This is becuse tillge redistributes ploughed soil within the cultivted field, wheres wter erosion tkes wy soil from the cultivted field (Zhng et l., 2003). The 137 Cs inventory ws negtively correlted with yers of cultivtion. A generl declining trend in the totl mount of 137 Cs inventories ws observed with incresing yers of continuous cultivtion fter conversion (R 2 = 0.2) (Fig. 3). This reflects n incresing degree of soil erosion with incresing yers of continuous cultivtion. Low inventory of 137 Cs in the upper slopes of the cultivted fields (Fig. 3b) suggests tht soil ws removed nd either deposited in the middle nd lower slopes, or tken wy entirely/prtilly from the fields b Cs (Bq m -2 ) y = -24.2x R 2 = 0.22 p < Cs (Bq m -2 ) Yers of cultivtion Upper Middle Low er Slope positions 1400 Figure 3. Reltionship between 137 Cs inventories nd yers of continuous cultivtion () nd men inventories in the slope positions (b). Sme letters indicte no significnt difference. 3.3 Estimted rtes of soil erosion/deposition The men rte of soil erosion/deposition estimted by both the PM nd MBM1 showed net loss of soil from ll the fields (Tble 4). The rtes of soil erosion were generlly higher thn the rtes of depositions. Soil deposition ws observed in the middle nd lower slopes of the younger fields (2, 6, nd 16 yers of cultivtion, Tble 4). There were no soil depositions in ny of the fields cultivted for more thn 16 yers. This my show tht in the erly yers of cultivtion, soil is ccumulted within the slopes. These processes continue with incresing yers of cultivtion nd culminte in complete removl of the soil from the fields. 7

8 Tble 4. Soil erosion (-) nd deposition (+) rtes (t h -1 yr -1 ) in the slope positions nd men vlues for ech field by the PM nd MBM1 (n = 21). Age Upper slope Middle slope Lower slope Men (yers) PM MBM1 PM MBM1 PM MBM1 PM MBM Men The net loss of soil ws positively correlted with yers of cultivtion (R 2 = 0.41) nd showed n incresing trend with incresing yers of continuous cultivtion fter conversion (Fig. 4). More thn 46 % of the net loss of soil ws recorded from the upper slopes of the fields. The men rte of soil erosion in the slopes generlly decreses from the upper to the lower slopes (Fig. 4b). This pttern is consistent with the generl chrcteristics of erosion dynmics on liner slopes. On liner slopes, the effect of runoff is considerbly higher in the upper slopes, wheres t the middle nd lower slopes runoff nd mgnitude of erosion could be hindered by the ccompnying depositions of sediments from the upper slopes (Bewket nd Sterk, 2003). 15 y = -0.69x R 2 = P < b 0 Estimtion models Pm MBM1 Erosion rte (t h -1 yr -1 ) Erosion rte (t h -1 yr -1 ) Yers of cultivtion -35 Upper Middle Low er Figure 4. Reltionship between the rtes of soil erosion nd yers of continuous cultivtion () nd men rtes of soil erosion in the different slope positions (b). Sme letters on error brs indicte no significnt difference. 8

9 The estimted men rtes of soil erosion for cultivted field in the sub-ctchment were ± 2.6 nd ± 4 t h -1 yr -1 by the PM nd MBM1, respectively. The dpted USLE yielded men nnul rte of 12.3 ± 1.5 t h -1 yr -1. The result from the USLE nd reports from similr studies support the estimted rtes from the 137 Cs models. The results in this study re in good greement with the findings of Gunten (1993) nd Eysu (2002) in the Gununo (Wolyit) re of the southern region of Ethiopi. The rinfll, soil nd frming systems in the Gununo re re similr to the present study re. Gunten (1993) reported rte of 13 t h -1 yr -1 for cultivted fields from plot-experiments. From the sme region, Eysu (2002) reported rnge of 6-13 t h -1 yr -1 from cultivted fields using the USLE. The estimted rtes from the 137 Cs models nd the USLE indicte tht soil erosion in the Kef Zone is on the verge of surpssing the tolerble level, which is 2-18 t h -1 yr -1 (Hurni, 1988). 4 Conclusions The estimted reference 137 Cs inventory in this study is considerbly high compred to vlues reported from similr studies in Afric nd elsewhere. Thus, the results suggest tht there is sizeble mount of 137 Cs fllout in the equtoril est Afric region, specificlly in southwest Ethiopi so tht the 137 Cs technique cn be pplied for long-term nd mediumterm soil erosion studies. The results lso indicte tht soil erosion in cultivted fields increses with incresing yers of cultivtion fter conversion. This will impir the productivity of the lnd in the long run, nd my hve spillover effect on the further conversion of the remining nturl forests into griculturl lnd. Thus, soil erosion needs to be given due policy considertion so tht the efforts to conserve the remining nturl forests nd plnt genetic resources in the Kef Zone would become effective. Acknowledgments We thnk the Germn Acdemic Exchnge Service (DAAD) for supporting the study of the first uthor. The Germn Federl Ministry for Eduction nd Reserch (BMZ) nd the Germn Technicl Coopertion (GTZ) re grtefully cknowledged for supporting the field reserch. References Alemneh, D., (1990). Environment, Fmine nd Politics in Ethiopi: A view from the Villge. Lynne Rienner Publishers Inc, Colordo. Bewket, W. nd Sterk, G., (2003). Assessment of soil erosion in cultivted fields using survey methodology for rills in the Chemog wtershed, Ethiopi. Agriculture, Ecosystems nd Environment, 97: Brymn, A. nd Crmer, D., (2001). Quntittive Dt Anlysis with SPSS Relese 10 for Windows: A Guide for Socil Scientists. Routledge, London. Bujn, A., Sntntogli, O.J., Chgs, C., Mssobrio, M., Cstiglioni, M., Ynez, M., Cillell, H. nd Fernndez, J., (2003). Soil erosion, evlution in smll bsin through the use of 137 Cs technique. Soil & Tillge Reserch, 69: Chppell, A., Wrren, A., Oliver, M.A. nd Chrlton, M., (1998). The Utility of 137 Cs for mesuring soil redistribution rtes in southwest Niger. Geoderm, 81:

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