of mineralizable C and N. In mature and undisturbed tropical ecosystems, a balance exists between the or-Physical fractionation, degradative wet-chemical analysis and liqganic C input and output of the soil because of mineraluid-state 13 C 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 mineral-25-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 Ͻ 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 oc-govern SOM dynamics in tropical soils is essential from curred from the labile SOM associated with the sand separates concur-the viewpoint of long-term sustainability of agriculture rent with higher oxidation states of lignin. However, substantial and influence on atmospheric CO 2 concentration and amounts of these organic substrates were also lost from the stable greenhouse effect. SOM fraction. Particularly, SOM, associated with the silt-size sepa-Despite 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 13 C NMR spectra revealed larger cesses involved during decomposition and humification proportions of protonated and C-and 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. Deforesta-Information about the impact of land use changes on tion and subsequent agricultural management not only resulted in the amount and composition of organic substrate...