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DIVISION S-2 - SOIL CHEMISTRY

Soil Organic Matter Composition in the Subhumid Ethiopian Highlands as Influenced by Deforestation and Agricultural Management

^a Institute of Soil Science and Soil Geography, Univ. of Bayreuth, Universitätsstr.30, D-95440 Bayreuth, Germany
^b Ethiopian Agricultural Research Organization, Debre Zeit Agricultural Research Center, P.O. Box 32, Debre Zeit, Ethiopia

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

Physical fractionation, degradative wet-chemical analysis and liquid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy were used to assess the impact of land use changes on the amount and structural composition of soil organic matter (SOM) in bulk soils and size separates in the subhumid highlands of southern Ethiopia. Soil samples (0–10 cm) were collected from natural forest, tea plantations, 25-yr cultivated fields at Wushwush (Paleudalf), Podocarpus-dominated natural forest, Cupressus plantations, and 30-yr cultivated fields at Munesa (Palehumults) sites. Forest clearing and continuous cultivation led to significant depletion (P < 0.05) of total soil organic C (SOC) (55% and 63%) and N (52% and 60%) in the surface soils, respectively. Compared with the cultivated fields, lower proportions of SOC (51 and 27%) and N (49 and 13%) were lost from the tea and Cupressus plantations, respectively. The largest depletion occurred from the labile SOM associated with the sand separates concurrent with higher oxidation states of lignin. However, substantial amounts of these organic substrates were also lost from the stable SOM fraction. Particularly, SOM, associated with the silt-size separates, decreased suggesting that the SOM in silt was quite susceptible to land use changes and represents a moderately labile SOM pool in the soils under study. Solution ¹³C NMR spectra revealed larger proportions of protonated and C- and O-substituted aryl-C in the silt than in clay-size separates. In contrast, O-alkyl-C structures were more prominent in the clay than in silt-size separates, coinciding with the lignin distribution obtained by wet-chemical analysis. Deforestation and subsequent agricultural management not only resulted in SOM depletion but also markedly altered the chemical composition of SOM in the subhumid highland ecosystems.

Abbreviations: (Ac/Al)_V, mass ratios of acid to aldehyde for the vanillyl • (Ac/Al)_S, mass ratios of acid to aldehyde for syringyl units • ANOVA, analysis of variance • CEC, cation-exchange capacity • (Fuc + Rha)/(Ara + Xyl), the ratio of fucose + rhamnose to arabinose + xylose • (Gal + Man)/(Ara + Xyl), the ratio of galactose + mannose to arabinose + xylose • GlcN/GalN, the ratio of glucosamine to galactosamine • GlcN/MurA, the ratio of glucosamine to muramic acid • LSD, least significant difference • NMR, nuclear magnetic resonance • SOC, soil organic C • SOM, soil organic matter • S/V, the ratio of syringyl to vanillyl • VSC, the sum of vanillyl, syringyl and cinnamyl units

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