Aggregation and organic matter fractions of three Nigerian soils as affected by soil disturbance and incorporation of plant material Durodoluwa Joseph Oyedele a , Per Schjùnning b,* , Erik Sibbesen 1,b , Kasia Debosz b a Obafemi Awolowo University, Department of Soil Science, Ile-Ife, Nigeria b Danish Institute of Agricultural Sciences, Department of Crop Physiology and Soil Science, Research Centre Foulum, PO Box 50, DK-8830, Tjele, Denmark Received 23 December 1997; accepted 9 December 1998 Abstract Soil management involving organic matter incorporation and reduced tillage intensity has often been reported to improve soil structure. However, the mechanisms and roles of soil organic matter (SOM) fractions in the process of soil structural stabilisation are not well documented. Furthermore, documentation of their positive effects on soils in the tropics is scarce. This study examined the changes in soil structure in response to SOM fractions dynamics in an incubation experiment. Three Nigerian soils: Iwo series (Oxic Tropudalf), Itagunmodi series (Tropeptic Euthrothox) and Owode series (Oxisol) were incubated with ground barley straw (Hordeum vulgare L.) or green ryegrass (Lolium perenne L.) with and without disturbance of the soil during incubation. The incorporation of plant material signi®cantly increased the light fraction organic matter (LFOM) with density <1.7 g cm 3 but had no effects on the heavy fraction polysaccharides (HFPS), i.e. polysaccharides attached to heavy fraction soil minerals with density >1.7 g cm 3 . A signi®cant decline of LFOM was recorded with time of incubation (0±41 days: 21%, 30% and 36% decrease for the no plant material treatment, the barley straw treatment, and the green ryegrass treatment, respectively). This was associated with a signi®cant rise of HFPS. Furthermore, the added plant material clearly decreased the soils' fraction of dispersible clay even before incubation, which was interpreted as a direct effect of the LFOM on the clay. Therefore, the use of dispersible clay as an indicator of macro-aggregate stability is questioned. After 41 days of incubation the concentration of water-stable aggregates was on an average 9% higher for soils with added plant material relative to soils not receiving plant material. When packing the post-incubation soil in metal cores, the volume of pores with equivalent neck diameter >10 mm was from 0.024 to 0.033 m 3 m 3 higher and the saturated hydraulic conductivity 18±24% higher for soils with added plant material than for soils without. Disturbance of the soil during incubation reduced the concentration of water-stable aggregates by 7% relative to undisturbed. Also the volume of pores with a neck diameter >10 mm and the saturated hydraulic conductivity were signi®cantly reduced (by 6% and 19% relative, respectively) when the soil was disturbed during incubation. The results demonstrate that rapid changes of SOM fractions can occur over a short incubation period and that these changes have a pronounced positive effect on the structure of these tropical soils. It also suggests that incorporated plant material may correct tillage induced degradation of soil physical properties over a relatively short time. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Plant material; Soil organic matter; Light fraction; Heavy fraction; Extracellular polysaccharides; Aggregate stabilisation; Dispersible clay; Hydraulic conductivity Soil & Tillage Research 50 (1999) 105±114 *Corresponding author. Tel: +45-8999-1766; fax: +45-8999-1619; e-mail: per.schjonning@agrsci.dk 1 Erik Sibbesen died on 23 October 1998. 0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0167-1987(98)00200-1