Soil aggregation, and total and particulate organic matter following conversion of native forests to continuous cultivation in Ethiopia Y. Ashagrie a, * , W. Zech a , G. Guggenberger a,1 , T. Mamo b a Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany b Ministry of Agriculture and Rural Development, Addis Ababa, Ethiopia Received 16 June 2005; received in revised form 6 July 2006; accepted 10 July 2006 Abstract Conversion of native forests to cultivation is usually accompanied by a decline in soil organic carbon (SOC) and nutrients, and deterioration of soil structure. The objectives of this study were: (i) to assess the effect of cultivation on soil aggregation and aggregate stability and (ii) to determine the effect of cultivation on total SOM, and the quantity and quality of POM in a Rhodic Nitisol at Munesa, in southeastern Ethiopia. Samples were collected from a cropland cultivated for 26 years and an adjacent natural forest. After cultivation, the proportion of water-stable macroaggregates was significantly reduced from >70% in the natural forest soil to 50% in the cultivated soil, being more pronounced in the >1 mm size aggregates. Cultivation also induced significant losses of OC and N both in bulk soil and water-stable aggregates. The amounts of free light fraction (free LF) C and N were more affected by cultivation than the amounts of intraparticulate organic matter (iPOM) C and N. The effect of cultivation was more pronounced on macroaggregate associated total and particulate organic matter than on microaggregate-associated fractions. The losses of POM C and N due to cultivation were highest in comparison to the total aggregate and whole soil OC and N suggesting that POM constitutes a more sensitive SOM fraction to the effects of cultivation. Soil aggregate stability and SOM could be used as indicators to apply the more appropriate management practices for increasing soil sustainability or productivity. # 2006 Published by Elsevier B.V. Keywords: Aggregate stability; Cultivation; POM; Nitisol; Ethiopia 1. Introduction Maintenance of SOM is important for sustainable use of soil resources due to the multiple effects of SOM on soil nutrient status and soil structural stability. Conversion of native forests to cultivation is usually accompanied by a decline in SOC and nutrients, and deterioration of soil structure (Detwiler, 1986; Bales- dent et al., 1998; Spaccini et al., 2001). Cultivation effects on SOM are caused by complex interactions of the physical, chemical and biological soil processes including reduced inputs of plant residues and increased soil disturbance, but the exact nature of the changes induced by cultivation depend on the particular agronomic practices adopted and on the properties of the virgin soil (Christensen, 1992). Identification of the magnitude of such management induced changes in soil aggregate stability, and SOM and associated nutrients is needed to select appropriate management options. www.elsevier.com/locate/still Soil & Tillage Research 94 (2007) 101–108 * Corresponding author. Tel.: +49 921 55 21 46; fax: +49 921 55 22 46. E-mail address: yeshanew@hotmail.com (Y. Ashagrie). 1 Present address: Institute of Soil Science and Plant Nutrition, Martin Luther University, D-06099 Halle, Germany. 0167-1987/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.still.2006.07.005