Structural stability and carbohydrate contents of an ultisol under different management systems J.K. Adesodun a , J.S.C. Mbagwu a,*,1 , N. Oti b a Department of Soil Science, University of Nigeria, Nsukka, Nigeria b Department of Crop Production, Federal University of Technology, Owerri, Nigeria Received 4 April 2000; accepted 9 February 2001 Abstract An understanding of the dynamics of soil carbohydrate pools is necessary for assessing the impact of organic residue management in organic matter build up and structural stability in tropical ecosystems. The objectives of this study were to evaluate temporal changes in aggregate stability and cold water-soluble, hot water-soluble and acid-soluble carbohydrate fractions of a sandy soil under different organic residue management practices. The soil is an Nkpologu sandy clay loam (fine- loamy, kaolinitic, isohyperthermic, typic kandiustult) at Nsukka in southeastern Nigeria. In July 1995, it was incorporated with complete fertiliser (N:P:K ¼ 12:12:17 at 480 kg/ha) (F); rice mill wastes (RW, 10 t/ha); RW þ F; poultry manure (PM, 10 t/ ha) and RW ð5t=haÞþ PM ð5t=haÞ up to the 0–20 cm depth. A control, tilled up to the 0–20 cm depth, was also included. Surface soil samples (0–20 cm), collected at 3, 6 and 12 months after residue applications were used to measure changes in aggregate stability by mean weight diameter (MWD), total OC and carbohydrate pools. In all treatments MWD increased whereas the concentrations of acid-soluble, hot water-soluble and cold water-soluble carbohydrates decreased with sampling time. Also irrespective of the type of amendment, the carbohydrate concentrations at each sampling period varied in the order, acid-soluble > hot water-soluble > cold water-soluble. Aggregate stability correlated very poorly with all the carbohydrate fractions and OC. The correlation coefficient values were rather low and did not mean much in the physical interpretation of these results. This shows that these carbohydrate pools were not very effective in stabilizing the soil aggregates. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Aggregate stability; Soil organic matter pools; Carbohydrate concentrations; Soil management practices 1. Introduction The structural stability of a soil has an impact on a wide range of processes that influence crop growth, soil erosion, runoff and the transport of contaminants from farmlands to surface water bodies (Ketcheson, 1980; Perfect et al., 1990). Information on the form and persistence of the aggregating constituents of soil organic matter (SOM) is vital, particularly when deal- ing with fragile tropical soils which are exposed to high risk of water erosion. Considerable controversy exists from studies car- ried out in temperate regions on the actual role of organic matter in soil aggregate stability. Some work- ers (Hamblin and Greenland, 1977; Dormaar, 1983) reported that it is the fractions of organic matter rather than the amount per se that are important in modifying the structural stability of aggregates whereas others Soil & Tillage Research 60 (2001) 135–142 * Corresponding author. E-mail address: misunn@aol.com (J.S.C. Mbagwu). 1 Tel.: þ234-42-770568; fax: þ234-42-771977. 0167-1987/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0167-1987(01)00164-7