Soil Use and Management (2002) 18, 404±411 DOI: 10.1079/SUM2002152 In¯uence of the addition of organic residues on carbohydrate content and structural stability of some highland soils in Ethiopia R. Spaccini 1 , A. Piccolo 1 *, J.S.C. Mbagwu 2 , A. Zena Teshale 1 & C.A. Igwe 2 Abstract. When over exploited and coupled with climatic conditions, tropical soils are subject to increased erosion and a loss of soil organic matter. Countermeasures include the incorporation of organic materials such as crop and animal residues. We studied the effect of adding crop residues and manure to soil, at ®ve sites in Ethiopia, on carbohydrate properties, aggregate stability and the C and N distribution within water- stable aggregates. The effects of organic amendments varied between sites. The largest content of carbo- hydrates was obtained in the control treatment at Holeta, Ginchi (90 kg ha ±1 mustard meal), Jimma (5 t ha ±1 cow dung + 9 t ha ±1 coffee husk), Awassa (forested soil), and Sirinka (soil alley-cropped with Leuceanae). The aggregate stability of these soils was highly correlated with the OM content but not with carbohydrates. The smaller aggregates (<1.00 mm) accumulated more carbohydrates than the larger (>1.00 mm), thereby suggesting a protecting effect within the ®ner soil fractions. A protecting role played by humi®ed OM com- ponents was also indicated by the C and N distribution as well as the C:N ratios which showed preferential accumulation in small rather than in large aggregates. The isotopic 13 C-OC values of carbohydrate extracts were generally low, suggesting that OM was from plants with C3 photosynthetic pathways. Soil treatments with maize alone or combined with coffee husks at Jimma decreased the d 13 C½ values slightly, revealing that maize contributed a share of the labile OM. Despite the improvement in the soil OM content, neither the carbohydrate content nor the aggregate stability were increased to the level of the forested sites, suggest- ing that the additions of crop residues and manure were not alone suf®cient to restore the soil physical quality. Keywords: Soil amendments, manures, crop residues, soil structure, stability, soil organic matter, Ethiopia INTRODUCTION I n the tropics, natural forests are converted to intensive cropping and pasture with generally minimal fertilizer inputs. This low-input agricultural system, which involves cultivation with hand hoes and planting without mechanical tillage, leads to a reduction in the overall fertility of soils (Lal 1986). Decrease in soil organic matter (SOM) due to decomposition and mineralization processes, together with removal of the above-ground biomass, accompanies soil quality degradation (Zech & Guggenberger 1996). Losses of SOM are even more severe in tropical soils characterized by nutrient-poor and highly weathered conditions (Mbagwu & Piccolo 1998). The recognition of the importance of SOM to maintain sustainable agricultural systems has increased attention on improved farming options devoted to soil OM management, especially in the tropics (Fernandes et al. 1997). Some properties of particle and aggregated fractions of Ethiopian soils have been related to changes in land use (Spaccini et al. 2001). This showed a reduction in aggregate stability and increased proportions of the smaller-sized aggregates and their associated carbohydrates, following conversion of forests to arable. However, the aggregate stability of both the forested and cultivated soils could not be explained by the levels of organic carbon (OC) or total carbohydrates in the soils. This points to the possibility that the aggregate stability of these soils is mediated by other aggregate-stabilizing agents such as humic substances and/ or iron oxides. Organic amendments have been used to improve the structural stability of tropical soils. Some of them involve the use of agroforestry systems such as alley crops, some the incorporation of organic residues and/or a mixture of the two. The extent to which they improve the structural stability of soils remains to be evaluated. This is because the sources of OM act differently in different soils and their longevity in soils may be related to cultivation intensity, type of soil, texture, dominant mineralogy, ambient temperature, 1 Dipartimento di Scienze del Suolo, della Pianta e dell'Ambiente, Universita Á di Napoli Federico II, Via Universita Á 100, 80055 Portici, Italy. 2 Department of Soil Science, University of Nigeria, Nsukka, Nigeria. *Corresponding author. E-mail: alpiccol@unina.it The in¯uence of organic residues on highland soils of Ethiopia 404