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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)

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

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