Carbohydrates in water-stable aggregates and particle size fractions of forested and cultivated soils in two contrasting tropical ecosystems

Information on changes in storage and loss of soil organic carbon (SOC) whe n tropical forests are converted to cropland is needed for evaluating soil structural degradation and for selecting appropriate sustainable soil manag ement practices. We evaluated changes in SOC storage of organic carbon and acid-hydrolyzable carbohydrates content of aggregated classes and particle size fractions of adjacent forested and cultivated soils in eight agroecosy stems from Ethiopian highlands and Nigerian lowlands. In all agroecosystems , SOC content was two to four times higher in the forested than the cultiva ted soils. Higher SOC content was found in Ethiopian (20.2-47.3 g.kg(-1)) t han Nigerian (12.0-24.0 g.kg(-) (1)) forested soils. The magnitude of reduc tion in SOC and total carbohydrates with cultivation was soil-specific, bei ng generally higher in the sandy than the clayey soils. The smaller aggrega te classes (< 1.00 mm) and the sand-sized particles (2000-63 mum) of the fo rested soils were preferentially enriched in carbohydrates relative to larg er aggregates (4.75-1.00 mm). Carbohydrates were more concentrated in the c lay-size fraction of the forested than in that of the cultivated soils. Cul tivation reduced aggregate stability, increased the proportions of the smal ler size aggregates and their associated carbohydrates relative to the fore sted soils. The susceptibility of the cultivated soils to loss in structura l stability reflected this initial aggregation which was greater in the mor e stable clayey than the fragile sandy soils. The aggregate stability of ei ther the forested or the cultivated soil could not be accounted for by the levels of OC or total carbohydrates in the soil.