Soil Organic Matter Dynamics in the Subhumid Agroecosystems of the Ethiopian Highlands: Evidence From Natural 13C Abundance and Particle-Size Fractionation

requires analytical methods that can (i) accurately monitor changes in the amount, nature, and dynamics of We used natural 13C abundance coupled with particle-size fractionSOM pools in natural and human-influenced systems ation to evaluate soil organic carbon (SOC) dynamics following deforand (ii) estimate C fluxes between the soils and atmoestation and subsequent cultivation in the subhumid Ethiopia highsphere (Bernoux et al., 1998). lands. Surface soil (0–10 cm), leaf, root, and litter samples were collected from natural forest and fields cultivated for 25 yr (WushVarious chemical fractionation and characterization wush) and from Podocarpus dominated natural forest and 30 yr cultimethods have been used to describe SOM dynamics vated fields (Munesa) and C, N and 13C signature were measured. (Stevenson et al., 1989; Christensen, 1992). However, Total SOC declined by 55% (32.0 Mg ha 1 ) at Wushwush and by these methods have not proven particularly useful in 63% (40.2 Mg ha 1 ) at Munesa following cultivation, while losses of following the dynamics of organic materials in soils and N amounted to 52% (2.8 Mg ha 1 ) and 60% (3.1 Mg ha 1 ) at the in identifying specific SOM pools that diminish upon two sites, respectively. 13C values of bulk soils of natural forests at intensive management (Stevenson et al., 1989). This is Wushwush ( 24.3‰) and Munesa ( 23.4‰) were significantly lower in part due to the complex nature of SOM and to the than those from the corresponding cultivated fields ( 19.9‰, Wushenormous array of compounds that exists in the soil, wush and 15.5‰, Munesa). Deforestation and continuous cultivation ranging from recent plant materials through a continat Wushwush and Munesa resulted in depletion of 80 and 96% of the uum of metabolic products of microorganisms to cominitial forest-derived SOC in sand, while 73 and 85% of C3 SOC was ponents of stable humus (Stevenson et al., 1989; Zech lost from silt fraction of the two sites, respectively. These results et al., 1997). Recently, biologically meaningful fractions suggest that SOC in sand was a very labile component of SOM and or pools have been obtained by methods based on physiis a more sensitive indicator to changes in soil C storage in response to land use changes. However, the substantial amount of forest-decal fractionation of soil according to particle-size separived SOC lost from silt indicates that SOM associated with silt was rates without chemical treatments, which combined with also quite susceptible to management changes, and that at least in biological and chemical analysis allows further insight the soils under study represents a moderately labile SOM pool, which into the functional attributes of the size separates (Tiesis generally not the case in temperate soils. Forest-derived SOC in sen and Stewart, 1983; Bond et al., 1992; Guggenberger clay declined by 48 and 61% at Wushwush and Munesa, respectively, et al., 1995; Lehmann et al., 1998; Solomon et al., 2000). suggesting that clay retained C3 derived SOC more effectively and Moreover, the use of 13C natural abundance technique that SOM bound to clay was more stable than SOM associated with coupled with particle-size fractionation have further adsand and silt fractions. vanced 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 S organic matter is a large reservoir of C that can years and are relevant for understanding the conseact as a sink or source of atmospheric CO2 (Lugo quences of natural and anthropogenic vegetation changes and Brown, 1993). It is also an important source of (Balesdent et al., 1990; Boutton et al., 1998; Koutika et inorganic nutrients for plant production in natural and al., 2000; Shang and Tiessen, 2000; Gerzabek et al., 2001). managed ecosystems. Moreover, SOM governs strucThe usefullness of 13C isotopic tracers for SOM studies tural stability and cation exchange capacity of soils eiderives from the contrasting metabolic pathways of C3 ther directly, through its chemical structure and surface and C4 plants (O’Leary, 1988; Farquhar et al., 1989). properties, or indirectly as a source of energy and nutriThe isotopic composition of SOM closely resembles the ents for soil biota (Zech et al., 1997). These effects are isotopic composition of the vegetation from which it especially important in cultivated tropical soils, where has been derived because the fractionation that occurs SOM is frequently related to soil fertility and producduring decomposition is small compared with the original fractionation during C fixation (Ågren et al., 1996). tivity. Thus, growing C4 plants on a soil previously supporting Replacement of natural forests by agroecosystems in C3 vegetation can be considered as an in situ labeling the subhumid and humid tropics generally increases of newly incorporated organic matter into the soil. the flux of terrestrial C to the atmosphere, reduces levExtensive deforestation and conversion of natural els of SOM, and thereby decreases soil fertility. Thereforests into agricultural land is the most wide spread fore, development of sustainable agricultural systems change in land use in Ethiopian highlands. In the past D. Solomon and J. Lehmann, Cornell Univ., Dep. of Crop and Soil Abbreviations: ANOVA, analysis of variance; Cdnf and Cdmc, organic Sciences, Bradfield and Emerson Halls, Ithaca, NY 14853; F. Fritzsche, C derived from natural forest and maize crop residues; CEC, cation and W. Zech, Institute of Soil Science and Soil Geography, University exchange capacity; Cms and Cfs, total SOC in the cultivated and forest of Bayreuth, Universittsstr.30, D-95440 Bayreuth, Germany; M. Tekasoils or size separates; m, and f 13C value of fresh leaves and roots lign, Ethiopian Agricultural Research Organization, Debre Zeit Agof maize crop and forest vegetation; ms and fs, 13C values of bulk ricultural Research Center, P.O. Box 32, Debre Zeit, Ethiopia. Resoils or size separates of the cultivated fields and natural forests; LSD, ceived 9 Oct. 2000. *Corresponding author (ds278 @cornell.edu). least significant difference; SOC, soil organic carbon; SOM, soil organic matter; V-PDB, Vienna-Pee Dee Belemnite limestone; rb, bulk density. Published in Soil Sci. Soc. Am. J. 66:969–978 (2002).