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In the semi-arid part of northern Tanzania, the native tropical woodland is undergoing a rapid conversion into agricultural land. This has resulted in drastic ecological changes in the region. This study was undertaken to investigate the effects of these changes in land use systems on the amount and composition of SOM in bulk soil samples and size separates. Samples were collected from the upper 10cm of a native tropical woodland, a degraded woodland, 3 and 15 years cultivated fields and home-stead fields where animal manure was regularly applied. Carbon, N, lignin derived phenols and non-celulosic carbohydrates were determined in the samples. Clearing and cultivation of the tropical woodland resulted in a decline of SOM contents in bulk soils and in all size separates. A 56% reduction of C and a 51% reduction of N contents were observed in bulk soils of the cultivated fields. A rapid decline in C and N in coarse and fine sand fractions occurred during the first 3 years of cultivation The reduction of stable SOM, i.e., the SOM which was in intimate association with the silt and clay sized fractions was relatively small. These results show the importance of organo-mineral associations in stabilization of SOM in soils of the semi-arid tropics. However, a more pronounced decline of stable SOM, especially from the clay, was observed in the 15 years than in the 3 years cultivated fields. These indicate that SOM losses due to cultivation of the native soil may not level off in the near feature. A decline in labile fractions of SOM was found in soils of the degraded woodland. Application of manure increased SOM in bulk soil and in the stabile fractions. An increasing degree of SOM humification with decreasing particle size separates was indicated by C/N ratios and by lignin and carbohydrate signatures. The carbohydrate signatures indicated a direct input of fresh plant residue into the mineral soils of the cultivated fields compared to the native woodland. Therefore, there is a need to develop sustainable soil management and cropping practices to combat the ongoing soil degradation and improve soil fertility in the region. (C)2000 Published by Elsevier Science B.V. All rights reserved.