DIVISION S-2—SOIL CHEMISTRY Soil Organic Matter Composition in the Subhumid Ethiopian Highlands as Influenced by Deforestation and Agricultural Management

of mineralizable C and N. In mature and undisturbed tropical ecosystems, a balance exists between the orPhysical fractionation, degradative wet-chemical analysis and liqganic C input and output of the soil because of mineraluid-state 13C nuclear magnetic resonance (NMR) spectroscopy were used to assess the impact of land use changes on the amount and ization and leaching of dissolved organic matter (Zech structural composition of soil organic matter (SOM) in bulk soils and et al., 1997a). Changes in land use and soil management size separates in the subhumid highlands of southern Ethiopia. Soil can have a marked effect on the SOM stock as a result samples (0–10 cm) were collected from natural forest, tea plantations, of the interactions between detrital input and mineral25-yr cultivated fields at Wushwush (Paleudalf), Podocarpus-domiization mediated by soil microorganisms (Tate, 1987). nated natural forest, Cupressus plantations, and 30-yr cultivated fields Several studies in the past have shown that deforestation at Munesa (Palehumults) sites. Forest clearing and continuous cultivaand cultivation of native tropical soils often lead to tion led to significant depletion (P 0.05) of total soil organic C accelerated SOM turnover, and thereby to depletion of (SOC) (55% and 63%) and N (52% and 60%) in the surface soils, nutrients (N, P, and S) present as part of complex orrespectively. Compared with the cultivated fields, lower proportions ganic polymers. Therefore, understanding processes that of SOC (51 and 27%) and N (49 and 13%) were lost from the tea and Cupressus plantations, respectively. The largest depletion ocgovern SOM dynamics in tropical soils is essential from curred from the labile SOM associated with the sand separates concurthe viewpoint of long-term sustainability of agriculture rent with higher oxidation states of lignin. However, substantial and influence on atmospheric CO2 concentration and amounts of these organic substrates were also lost from the stable greenhouse effect. SOM fraction. Particularly, SOM, associated with the silt-size sepaDespite the importance of SOM in subhumid and rates, decreased suggesting that the SOM in silt was quite susceptible humid tropical soils, very little information is available to land use changes and represents a moderately labile SOM pool on the complex biological, chemical, and physical proin the soils under study. Solution 13C NMR spectra revealed larger cesses involved during decomposition and humification proportions of protonated and Cand O-substituted aryl-C in the silt of organic substrates in physically separated-size fracthan in clay-size separates. In contrast, O-alkyl-C structures were tions and on the factors influencing these processes. more prominent in the clay than in silt-size separates, coinciding with the lignin distribution obtained by wet-chemical analysis. DeforestaInformation about the impact of land use changes on tion and subsequent agricultural management not only resulted in the amount and composition of organic substrates such SOM depletion but also markedly altered the chemical composition as lignin, carbohydrates, and amino sugars in particleof SOM in the subhumid highland ecosystems. size separates can eventually improve the knowledge of organic matter dynamics in tropical ecosystems. Johansson (1986) indicated that a considerable proS organic matter is a physically and chemically portion of the terrestrial C is present in the form of heterogeneous mixture of organic compounds of lignin and carbohydrates and that SOM is predomiplant, animal, and microbial origin, and has components nantly derived from their decomposition. Lignin decomat different stages of decomposition. The total mass of orposes slowly and represents a recalcitrant fraction in ganic C stored in soils has been estimated at 1576 1015 g soil and litter (Guggenberger et al., 1994). During deC, of which about one third is found in the tropics composition of lignin, intramolecular bonds between (Mahieu et al., 1999). Thus, the SOM in tropical soils phenylpropanoid components of the lignin are severed represents an important pool of the terrestrial C reand oxidized, and phenolic derivatives are released. As serves and its transformation is an important element biodegradation progresses, the fractions derived from in the global C cycle. lignin in soil become increasingly acidic (carboxylic) Soil organic matter plays a major role in soil fertility (Haider, 1992; Kögel-Knabner, 1993). Thus, acid to alby affecting physical and chemical properties, and also dehyde ratio of phenolic moieties can be used to esticontrols soil microbial activity by serving as a source 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; AND. Solomon, F. Fritzsche, J. Lehmann, and W. Zech, Institute of Soil OVA, analysis of variance; CEC, cation-exchange capacity; (Fuc Science and Soil Geography, Univ. of Bayreuth, Universitätsstr.30, Rha)/(Ara Xyl), the ratio of fucose rhamnose to arabinose D-95440 Bayreuth, Germany; M. Tekalign, Ethiopian Agricultural xylose; (Gal Man)/(Ara Xyl), the ratio of galactose mannose Research Organization, Debre Zeit Agricultural Research Center, to arabinose xylose; GlcN/GalN, the ratio of glucosamine to galacP.O. Box 32, Debre Zeit, Ethiopia; D. Solomon, current address: Coltosamine; GlcN/MurA, the ratio of glucosamine to muramic acid; lege of Agriculture and Life Sciences, Dep. of Crop and Soil Sciences, LSD, least significant difference; NMR, nuclear magnetic resonance; Cornell Univ., Bradfield and Emerson Halls, Ithaca, NY 14853. ReSOC, soil organic C; SOM, soil organic matter; S/V, the ratio of ceived 18 Dec. 2000. *Corresponding author (ds278 @cornell.edu). syringyl to vanillyl; VSC, the sum of vanillyl, syringyl and cinnamyl units. Published in Soil Sci. Soc. Am. J. 66:68–82 (2002).