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DIVISION S-7—FOREST & RANGE SOILS

Soil Organic Matter Dynamics in the Subhumid Agroecosystems of the Ethiopian Highlands

Evidence From Natural ¹³C Abundance and Particle-Size Fractionation

^a Cornell Univ., Dep. of Crop and Soil Sciences, Bradfield and Emerson Halls, Ithaca, NY 14853
^b Institute of Soil Science and Soil Geography, University of Bayreuth, Universittsstr.30, D-95440 Bayreuth, Germany
^c Ethiopian Agricultural Research Organization, Debre Zeit Agricultural Research Center, P.O. Box 32, Debre Zeit, Ethiopia

* Corresponding author (ds278{at}cornell.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
We used natural ¹³C abundance coupled with particle-size fractionation to evaluate soil organic carbon (SOC) dynamics following deforestation and subsequent cultivation in the subhumid Ethiopia highlands. Surface soil (0–10 cm), leaf, root, and litter samples were collected from natural forest and fields cultivated for 25 yr (Wushwush) and from Podocarpus dominated natural forest and 30 yr cultivated fields (Munesa) and C, N and ¹³C signature were measured. Total SOC declined by 55% (32.0 Mg ha^-1) at Wushwush and by 63% (40.2 Mg ha^-1) at Munesa following cultivation, while losses of N amounted to 52% (2.8 Mg ha^-1) and 60% (3.1 Mg ha^-1) at the two sites, respectively. ¹³C values of bulk soils of natural forests at Wushwush (-24.3) and Munesa (-23.4) were significantly lower than those from the corresponding cultivated fields (-19.9, Wushwush and -15.5, Munesa). Deforestation and continuous cultivation at Wushwush and Munesa resulted in depletion of 80 and 96% of the initial forest-derived SOC in sand, while 73 and 85% of C₃ SOC was lost from silt fraction of the two sites, respectively. These results suggest that SOC in sand was a very labile component of SOM and is a more sensitive indicator to changes in soil C storage in response to land use changes. However, the substantial amount of forest-derived SOC lost from silt indicates that SOM associated with silt was also quite susceptible to management changes, and that at least in the soils under study represents a moderately labile SOM pool, which is generally not the case in temperate soils. Forest-derived SOC in clay declined by 48 and 61% at Wushwush and Munesa, respectively, suggesting that clay retained C₃ derived SOC more effectively and that SOM bound to clay was more stable than SOM associated with sand and silt fractions.

Abbreviations: ANOVA, analysis of variance • C_dnf and C_dmc, organic C derived from natural forest and maize crop residues • CEC, cation exchange capacity • C_ms and C_fs, total SOC in the cultivated and forest soils or size separates • _m, and _f ¹³C value of fresh leaves and roots of maize crop and forest vegetation • _ms and _fs, ¹³C values of bulk soils or size separates of the cultivated fields and natural forests • LSD, least significant difference • SOC, soil organic carbon • SOM, soil organic matter • V-PDB, Vienna-Pee Dee Belemnite limestone • r_b, bulk density

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SOIL ORGANIC MATTER is a large reservoir of C that can act as a sink or source of atmospheric CO₂ (Lugo and Brown, 1993). It is also an important source of inorganic nutrients for plant production in natural and managed ecosystems. Moreover, SOM governs structural stability and cation exchange capacity of soils either directly, through its chemical structure and surface properties, or indirectly as a source of energy and nutrients for soil biota (Zech et al., 1997). These effects are especially important in cultivated tropical soils, where SOM is frequently related to soil fertility and productivity.

Replacement of natural forests by agroecosystems in the subhumid and humid tropics generally increases the flux of terrestrial C to the atmosphere, reduces levels of SOM, and thereby decreases soil fertility. Therefore, development of sustainable agricultural systems requires analytical methods that can (i) accurately monitor changes in the amount, nature, and dynamics of SOM pools in natural and human-influenced systems and (ii) estimate C fluxes between the soils and atmosphere (Bernoux et al., 1998).

Various chemical fractionation and characterization methods have been used to describe SOM dynamics (Stevenson et al., 1989; Christensen, 1992). However, these methods have not proven particularly useful in following the dynamics of organic materials in soils and in identifying specific SOM pools that diminish upon intensive management (Stevenson et al., 1989). This is in part due to the complex nature of SOM and to the enormous array of compounds that exists in the soil, ranging from recent plant materials through a continuum of metabolic products of microorganisms to components of stable humus (Stevenson et al., 1989; Zech et al., 1997). Recently, biologically meaningful fractions or pools have been obtained by methods based on physical fractionation of soil according to particle-size separates without chemical treatments, which combined with biological and chemical analysis allows further insight into the functional attributes of the size separates (Tiessen and Stewart, 1983; Bond et al., 1992; Guggenberger et al., 1995; Lehmann et al., 1998; Solomon et al., 2000). Moreover, the use of ¹³C natural abundance technique coupled with particle-size fractionation have further advanced SOM turnover studies, since these methods are well suited to study soil organic carbon (SOC) dynamics over a time scale ranging from a few to several hundred years and are relevant for understanding the consequences of natural and anthropogenic vegetation changes (Balesdent et al., 1990; Boutton et al., 1998; Koutika et al., 2000; Shang and Tiessen, 2000; Gerzabek et al., 2001). The usefullness of ¹³C isotopic tracers for SOM studies derives from the contrasting metabolic pathways of C₃ and C₄ plants (O'Leary, 1988; Farquhar et al., 1989). The isotopic composition of SOM closely resembles the isotopic composition of the vegetation from which it has been derived because the fractionation that occurs during decomposition is small compared with the original fractionation during C fixation (Ågren et al., 1996). Thus, growing C₄ plants on a soil previously supporting C₃ vegetation can be considered as an in situ labeling of newly incorporated organic matter into the soil.

Extensive deforestation and conversion of natural forests into agricultural land is the most wide spread change in land use in Ethiopian highlands. In the past 100 yr, the total area of land covered by the forest in Ethiopia has declined from about 40% to an estimated 2.4% in 1990 (Pohjonen and Pukkala, 1990; Eshetu and Högberg, 2000). Forest degradation and destruction has continued unabated especially in south and southwestern parts of the country, where most of the remnant subhumid tropical highland forests are found. This has led to extensive environmental degradation that threatens the fragile ecosystems of the region. A considerable portion of the cleared area has been put under continuous maize (Zea mays L.) cultivation. This vegetation change provided a unique opportunity to study SOM dynamics in different pools on the basis of a progressive shift in isotopic composition of the SOC following the replacement of natural forests (C₃ species) by maize crop (C₄ species) in subhumid tropical highland environments.

The objectives of this study are (i) to assess SOC turnover in bulk soils and particle-size separates (sand, silt, and clay) by means of natural ¹³C isotope tracer techniques and (ii) to quantify the total SOC and N losses following deforestation and continuous maize cultivation in the subhumid tropical highland agroecosystems of southern Ethiopia.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site Description
The study was conducted at the southwestern highlands (Wushwush) and the southeastern Rift Valley escarpments (Munesa) of Ethiopia. Wushwush is located at 7° 19' N and 36° 07' E. The altitude of the area is 1900 m above sea level. Mean annual temperature is 18°C with an average annual precipitation of 1800 mm. Geologically, the area is associated with Jimma Volcanics, with abundant rhyolites and trachybasalts. The soils of the area are classified as plinthaquic Paleudalf with clayey texture and dark reddish brown color (Soil Survey Staff, 1999). The Wushwush natural forest is mainly composed of Olea africana Mill., Syzygium guineense Guill. ex Perr., Cordia africana Lam., Croton macrostachys Hochst. ex A. Rich., and Ficus vasta Forsk. The Munesa site is located at 7° 35' N and 38° 45' E. Mean annual temperature is 19°C with an average annual precipitation of 1250 mm. Parent materials of the Munesa area are of volcanic origin, principally trachytes and basalts with ignimbrites and pumices at the rift valley floor. The escarpment extends from about 2100 m to 3200 m and the plain descends gradually to the Rift Valley lakes at about 1600 m above sea level. The soils of the area are classified as Typic Palehumults with clayey texture and very dark reddish brown color (Soil Survey Staff, 1999). The natural vegetation of the Munesa forest ranges from Arundinaria alpina K. Schun., Hagenia abyssinica Bruce ex J.F. Gmel., Croton macrostachys Hochst. ex Rich., Podocarpus falcatus Thunb. ex Mirb., Olea hochstetteri Baker. dominated forest on the escarpment to Acacia woodlands (Acacia tortilis Forssk. ex Hayne, Acacia abyssinica Hochst. ex Benth., and Acacia seyal Delile) in the semiarid lowlands.

The land use systems studied at the Wushwush site were natural forest (C₃) and fields cultivated continuously with maize for 25 yr (C₄), while at the Munesa site, Podocarpus dominated natural forest (C₃) and fields cultivated continuously with maize for 30 yr (C₄) were investigated. In the cultivated fields of both sites, maize was grown without fertilizer inputs. However, during the intermittent dry periods, sorghum [Sorghum bicolor (L.) Moench] was grown at Munesa. The depth of ploughing at the Wushwush and Munesa sites varies between 10 to 12 cm. Crop residues that remain on the cultivated fields are normally collected and used as animal feed.

After considering the depth of cultivation in the region, we used a core sampler (200-cm³ core volume at each subsite) and collected soil samples in three replicates from the upper 10 cm of the different land use systems in April 1998. This helps to minimize differences, which may arise because of the dilution of SOM by mixing the surface soil with that of the subsoil through ploughing. We selected three representative fields from each land use system and collected nine subsamples in a radial sampling scheme from each site (Wilding, 1985). A composite sample was then prepared from the subsamples. The samples were air dried and sieved (<2 mm) prior to fractionation and chemical analysis. In addition, fresh leaf, root, and undecomposed and decomposed litter samples were collected from the natural forest sites, whereas only fresh leaves and roots were collected from the fields cultivated with maize.

Particle-Size Fractionation
Particle-size fractionation was done on <2-mm material (bulk soil) according to Amelung et al. (1998). After removing visible root remnants, 30 g of soil was treated ultrasonically with an energy input of 60 J mL^-1 by a probe type sonicator (Branson Sonifier W-450, Danbury, CT) in a soil:water ratio of 1:5 (w/v). Coarse sand size fraction (250–2000 µm) was isolated by wet sieving. To completely disperse the remaining material in the <250-m suspension, ultrasound was again applied with an energy input of 440 J mL^-1 in a soil:water ratio of 1:10 (w/v). The clay size fraction (<2 µm) was separated from the silt (2–20 µm) and fine sand (20–250 µm) size fractions by repeated centrifugation. The silt size fraction was separated from the fine sand fraction by wet sieving. Coarse and fine sand fractions were combined and finally all size separates were dried at 40°C before grinding them for chemical analysis. The recovery of size separates after ultrasonic dispersion, wet sieving, and centrifugation ranged from 97 to 98% of the initial soil mass.

Chemical Analysis
Carbon and N contents were analyzed by dry combustion with a C/H/N/S-analyzer (Elementar Vario EL, Hanau, Germany). The pH-H₂O and pH-KCl were determined in 1:2.5 soil:water suspension with a pH meter. Cation-exchange capacity (CEC) was determined by means of 1 M NH₄OAc (pH = 7.0) according to Avery and Bascomb (1974). Dithionite-citrate-bicarbonate extractable aluminum and iron (Al_d, Fe_d) were determined after two extractions at 70°C for 15 min as described by Mehra and Jackson (1960). Oxalate-extractable aluminum and iron (Al_o, Fe_o) were determined after extraction for 2 h by means of 0.2 M ammonium oxalate at pH = 3 in the dark (Blume and Schwertmann, 1969) (Table 1) .

Calculation of C Derived from C₃ and C₄ Vegetation
Stable ¹³C isotope abundance is commonly expressed by notation in per mil (), as the relative deviation in the ratio of the heavy isotope to the light isotope in a sample ([¹³C/¹²C]sam) compared with that of a standard ([¹³C/¹²C]std):

The standard used for reporting ¹³C data (¹³C/¹²C = 0.0112372) is the Vienna-Pee Dee Belemnite limestone (V-PDB) (Coplen, 1995).

Organic C derived from natural forest (C_dnf) and maize crop residues (C_dmc) was expressed either as g SOC kg^-1 soil or as percentage of the total SOC (C_dnf[%] and C_dmc[%]) in the cultivated fields. These SOC pools were calculated by the following equations:

[2]

[3]

where C_ms denotes total SOC content of samples from the cultivated soils or separates, while _ms and _fs denote average ¹³C values of the respective soils or size separates from the cultivated fields and natural forests, respectively. The _m and _f represent the average ¹³C values of fresh leaves and roots of maize crop and forest vegetation, respectively.

Theoretically, after a vegetation change from forest (C₃) to maize crop (C₄), ¹³C values of a soil supporting only the original forest vegetation and ¹³C values of a soil supporting only maize crop have to be also measured to calculate the amounts (C_dnf and C_dmc) and percentages of SOC derived from forest and maize vegetation (C_dnf[%] and C_dmc[%]); Eq. [2] and [3] are used for this. The ¹³C value of the soil supporting only forest vegetation can be obtained from the ecological reference site with similar soil type and topography kept under the original forest vegetation. However, it is not possible to measure the equilibrium ¹³C value of soil supporting only maize, since there is no site that has been kept only under maize cultivation for a sufficient period that it can be used as an ecological reference site for C₄ vegetation. Since such a reference sites does not exist, Balesdent and Mariotti (1987) and Van Noordwijk et al. (1997) stated that the difference between ¹³C values of a soil supporting maize and forest vegetation [Dd = ¹³C(soil only under maize) - ¹³C(soil only under forest)] must be estimated. Thus, several researchers substituted the ¹³C value of soil supporting only the final C₄ vegetation with the ¹³C values of fresh leaves, roots, or litter derived from the final C₄ vegetation to estimate the difference (Balesdent et al., 1988; Vitorello et al., 1989; Desjardins et al., 1994). Since isotopic enrichment can occur during decomposition of organic matter (Balesdent et al., 1990), the C remaining after mineralization is generally richer in ¹³C than the original plant material. Thus, the above assumptions involve comparison of materials at different states of humification. Bernoux et al. (1998) suggested that the difference in ¹³C values of the final (C₄) and original vegetation (C₃) [Dd = ¹³C(final Veg.) - ¹³C(initial Veg.)] can be used to approximate the difference in ¹³C values of a soil supporting the initial (C₃) and final (C₄) vegetation [Dd = ¹³C(soil only under final Veg.) - ¹³C(soil only under initial Veg.)]. This approach might help to minimize the error associated with comparing organic materials at different state of humification to approximate the potential difference in ¹³C values of soils kept only under C₃ and C₄ vegetation. Thus, in the present study, the difference between mean ¹³C value of fresh leaves and roots of maize crop (_m = -12.2 at both sites) and forest vegetation (_f = -30.1 and -26.6 at Wushwush and Munesa, respectively) were used in Eq. [2] and [3] to calculate SOC derived from C₃ and C₄ vegetation (Tables 2 and 3).

[4]

where C_ms and C_fs denotes total SOC in the cultivated and forest soils or size separates.

The total SOC stocks of the surface soils (Total SOC, Mg ha^-1) were calculated from SOC contents of samples (g kg^-1 soil), soil layer thickness (z, m), and bulk density (_b, Mg m^-3), of the samples by the following equation:

[5]

Forest clearing and cultivation usually cause compaction and consequently the bulk density of the cultivated soils increases with time. Therefore, samples of forest soils at a certain depth may not be directly comparable with samples of cultivated soils from the same depth. Since the bulk density has an important effect on C-balance calculations, Veldkamp (1994) suggested that when comparing total SOC contents of forested and cultivated or pasture soils, if sampling is based on depth, the depth of the soil layer has to be adjusted to avoid an error due to differences in bulk density. Thus, in the present study, the thickness of the cultivated soils (z) was corrected (z_corrected) as follows, assuming that the bulk density and depth of the cultivated soils were originally the same as those of the corresponding forest soils:

[6]

Statistical Analysis
Statistical analysis of the data was carried out on the replicates by one-way analysis of variance (ANOVA). If the main effects were significant at P < 0.05, a post hoc separation of means was done by univariate least significant difference (LSD) test. All statistical analyses were conducted with the software package Statistica 5.1 for Windows (1995).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SOC and N Distribution in Bulk Soils and Particle-Size Separates
Deforestation of natural forests in conjunction with continuous cultivation induced a significant depletion of SOC and N in bulk soils and particle-size separates of these subhumid tropical highland agroecosystems. Total SOC and N contents in bulk soils of the cultivated fields at Wushwush and Munesa sites were significantly lower (P < 0.05) than the corresponding natural forests (Table 4). Total SOC declined by 55% (32.0 Mg ha^-1) at Wushwush and by 63% (40.3 Mg ha^-1) at Munesa following forest clearing and continuous cultivation, while losses of N from the cultivated fields of the two sites amounted to 52% (2.8 Mg ha^-1) and 60% (3.1 Mg ha^-1), respectively, after corrections for bulk density differences were made. These values are in agreement with the results of Dalal and Mayer (1986), Sombroek et al. (1993), and Solomon et al. (2000) following clearing tropical forests and their subsequent conversion in to arable land. However, if the changes in bulk density were ignored, only 42% (24.4 Mg ha^-1) and 38% (24.2 Mg ha^-1) depletion in total SOC and 39% (2.1 Mg ha^-1) and 33% (1.7 Mg ha^-1) reduction in total N were found at Wushwush and Munesa, respectively (Table 4). These results indicate that without correction for bulk density changes between the forested and cultivated fields, losses of total SOC and N could be systematically underestimated. Veldkamp (1994) arrived at a similar conclusion in his investigation conducted at the Atlantic Zone of Costa Rica. Therefore, all subsequent further calculations in this study show values with corrections for bulk density differences. The observed losses of SOC and N in the continuously cropped fields could be attributed to rapid mineralization of SOM following cultivation, which disrupts soil aggregates, and thereby increases aeration and microbial accessibility to organic matter (Guggenberger et al., 1995; Solomon et al., 2000). Moreover, reduced input of plant residues into the surface soils may also contribute to the depletion of SOM in the cultivated fields. Compared with losses of SOC, land use changes resulted in slightly lower depletion of N both at Wushwush and Munesa. Thus, the influence of management was more pronounced in SOC than in N contents. However, C/N ratios in bulk soils of the cultivated fields and the natural forests did not reveal a significant difference in SOM composition of soils under the two land use systems.

¹³C Signature of Plant and Litter Samples

View this table: [in this window] [in a new window]   Table 6. 13C values () of fresh leaves, roots and litter samples.

¹³C Signature of Bulk Soils and Particle-Size Separates

View larger version (15K): [in this window] [in a new window]   Fig. 1. Average 13C values of bulk soil and particle-size separates in the subhumid highlands of southern Ethiopia. Each point is the mean of three replicates, and error bars represent ±1 standard deviation.

The ¹³C values of SOM associated with particle-size separates from forest vs. cultivated soils exhibited different patterns. In the natural forests, the ¹³C value increased with decreasing particle size, the highest being observed in SOM bound to the clay (-23.0 at Wushwush and -23.2 at Munesa) (Fig. 1). In contrast, particle-size separates of the cultivated fields showed higher enrichment in the sand (-19.1 and -14.5) than in the silt (-21.0 and -15.5) and clay (-21.5 and -18.3) at the Wushwush and Munesa sites, respectively.

The ¹³C values of SOM bound to the clay fraction of the Wushwush and Munesa forests were 7.1 and 3.3 higher than average ¹³C values of the corresponding forest vegetation (-30.1 at Wushwush and -26.5 at Munesa). In contrast, the SOM associated with the sand fraction of soils from the natural forests had the lowest ¹³C values (-26.1, Wushwush and -25.2, Munesa) and generally were only 4.0 and 1.3 higher than mean ¹³C values of the original forest vegetation at the two sites, respectively. Similar patterns were observed by Desjardins et al. (1994) and Shang and Tiessen (2000) for tropical soils from Brazil and by Boutton et al. (1998) for soils collected from a subtropical savanna ecosystem in North America. The ¹³C values of SOM associated with the silt fraction of both land use systems were intermediate between those of the sand and clay fractions. The mechanisms involved for the observed enrichment of ¹³C with decreasing particle size in the forest soils are not fully understood yet. However, they might be attributed to the increasing degree of humification of the associated SOM, which could involve (i) the preferential loss of ¹²C during decomposition of organic carbon, (ii) the increasing contents of ¹³C enriched organic components such as microbial tissues (Macko and Estep, 1984), which are less available to for microbial digestion (Roscoe et al., 2000; Gerzabek et al., 2001), and (iii) the decreasing contents of ¹³C depleted organic substrates such as lignin (Benner et al., 1987) in the finer size separates.

Comparison of the two land use systems at both sites indicated that the SOM associated with particle-size separates of the cultivated soils was generally more enriched in ¹³C than the corresponding size separates of the natural forest soils. This was demonstrated by the difference in ¹³C values of the corresponding size separates of the two land use systems [i.e., (Dd = ¹³C value of sand from the cultivated fields - ¹³C value of sand from the natural forest)]. According to Fig. 1, the differences in ¹³C values of similar size separates of the cultivated and forested soils were greatest in the sand separate of both sites (7.0 and 10.5) and decreased with decreasing particle size (silt, 4.4 and 9.2 and clay, 1.5 and 5.0) at the two sites, respectively. These results indicated that ¹³C signature of organic matter input from the current vegetation (C₄) was clearly reflected in the sand followed by the silt fraction, following a management-inflicted vegetation change from C₃ forest to C₄ cropland. According to our data, however, the ¹³C signatures of the original plant community (C3 forest) were more evident in the clay fraction of the cultivated soils and only little C from the newly introduces C₄ vegetation reached the clay fractions during the decomposition process of SOM in these tropical agroecosystems.

Percentages and Amounts of SOC Derived from Maize and Forest Vegetation
The effects of C₃ to C₄ vegetation change were reflected by estimates of the amount of SOC derived from maize (C_dmc) and forest (C_dnf) residues in surface soils (0–10 cm) of the two cultivated fields. After 25 yr of continuous cultivation at the Wushwush site, on average only 25% (6.5 Mg ha^-1) of the existing total SOC was derived from maize. The remaining 75% (19.8 Mg ha^-1) of the total SOC in the cultivated soils was still attributed to the natural forest vegetation (Fig. 2 and Table 2). However, at the Munesa site, after 30 yr of continuous maize cropping, 55% (13.1 Mg ha^-1) of the existing total SOC was derived from maize, while only 45% (10.6 Mg ha^-1) of the total SOC in bulk soils of the cultivated fields originated from the preceding C₃ forest vegetation.

View larger version (34K): [in this window] [in a new window]   Fig. 2. Proportion of forest- (Cdnf) and maize-derived (Cdmc) SOC in bulk soils and particle-size separates of two cultivated soils in the southern Ethiopian highlands.

View larger version (46K): [in this window] [in a new window]   Fig. 3. Loss of forest-derived SOC (Loss(Cdnf)) in bulk soil and particle-size separates of cultivated fields at the Wushwush and Munesa sites.

Examination of size separates indicated that rapid decline of SOC derived from the native forest vegetation occurred in size separates of the cultivated soils. Twenty-five and 30 yr of continuous cultivation at the Wushwush and Munesa sites resulted in depletion of 80 and 96% of the initial forest-derived SOC associated with the sand fraction, while 73 and 85% of the forest-derived SOC was lost from the silt separates of the two sites, respectively (Fig. 3). Forest-derived SOC in the clay separates declined only by 48 and 61% at Wushwush and Munesa for the same period of cultivation, suggesting that the clay fraction retained SOC derived from the previous vegetation more efficiently than did the silt and sand fractions.

Natural ¹³C abundance technique clearly showed that in these tropical soils, individual size separates represent SOM pools that have different turnover rates. As already observed in other temperate (Christensen, 1987) and tropical soils (Vitorello et al., 1989; Bonde et al., 1992), a rapid turnover and substitution of SOC derived from forest vegetation by SOC derived from maize crop residue was observed in the macroorganic matter associated with sand fraction. Thus, the SOC in the sand fraction was a very labile component of the SOM and is a more sensitive indicator of changes in soil carbon storage in response to changes in agricultural management practices. However, a substantial amount of forest-derived SOC was also lost from the silt fraction, suggesting that SOM associated with the silt was also susceptible to land use changes and, at least in the soils under study, represents a moderately labile SOM pool, which is generally not the case in temperate soils (Christensen, 1987). The lower depletion of forest-derived SOC from clay fractions indicates that the SOM bound to the clay was more stable than that found in the sand and silt fractions of the studied soils. The relative stability of clay-bound SOM in these tropical soils could be attributed to physical and chemical stabilization of SOM against biodegradation by clay minerals and clay associated sesquioxides (Zech et al., 1997).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Natural ¹³C abundance technique coupled with particle-size fractionation clearly demonstrated that the soil ecosystems of the southern Ethiopian highlands are highly sensitive to changes in vegetation cover caused by land use changes. Deforestation followed by continuous maize cropping resulted in significant reduction of forest-derived SOC (C_dnf) in bulk soils and in all size separates (sand, silt, and clay) in these subhumid tropical highland agroecosystems. Moreover, losses of forest-derived SOC were not fully compensated by SOC input from the maize crop residues (C_dmc). It is not clear if present SOC stocks have achieved a new equilibrium, or if loss may continue in these maize-cropping systems. Net losses of SOC and slower C accretions from maize cropping systems are likely due to the effects of tillage coupled with reduced organic matter inputs and removal of residues from cultivated fields. Stable C isotopes clearly indicated that, in these tropical soils, individual size separates encompass SOM pools that differ in turnover rates. As already observed in other temperate and tropical soils, turnover and substitution of SOC derived from the natural forest vegetation (C_dnf) by SOC derived from maize crop residue (C_dmc) occurred more rapidly in the macroorganic matter associated with sand than in SOM associated with the silt and clay fractions. Thus, the SOC in the sand fraction was a very labile component of the SOM and is a more sensitive indicator of changes in soil carbon storage in response to land use changes. However, substantial amount of forest-derived SOC was also lost from the silt fraction, suggesting that SOM associated with the silt was also susceptible to management changes and that, at least in the soils under study, represents a moderately labile SOM pool, which is generally not the case in temperate soils. The lower depletion of forest-derived SOC from clay fractions indicates that the SOM bound to the clay was more stable than that found in the sand and silt fractions, indicating the importance of organomineral associations in stabilization of SOM in these tropical soils.

	ACKNOWLEDGMENTS

 
We wish to express our gratitude for the German Academic Exchange Service (DAAD) for providing fellowship for D. Solomon. We also thank Dr. G. Gebauer and A. Wetzel for their assistance during isotope measurements, B. Alemu for his help during site location and sample collection, and the Wushwush Tea Plantation and Munesa Forest Enterprise for their cooperation in providing the research sites. This work was completed while the senior author was a research associate at the Institute of Soil Science and Soil Geography, University of Bayreuth.

Received for publication October 9, 2000.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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