Aggregation and organic matter fractions of three Nigerian soils as affected by soil disturbance and incorporation of plant material

Soil management involving organic matter incorporation and reduced tillage intensity has often been reported to improve soil structure. However, the m echanisms and roles of soil organic matter (SOM) fractions in the process o f soil structural stabilisation are not well documented. Furthermore, docum entation of their positive effects on soils in the tropics is scarce. This study examined the changes in soil structure in response to SOM fractions d ynamics in an incubation experiment. Three Nigerian soils: Iwo series (Oxic Tropudalf), Itagunmodi series (Tropeptic Euthrothox) and Owode series (Oxi sol) were incubated with ground barley straw (Hordeum vulgare L.) or green ryegrass (Lolium perenne L.) with and without disturbance of the soil durin g incubation. The incorporation of plant material significantly increased t he light fraction organic matter (LFOM) with density <1.7 g cm(-3) but had no effects on the heavy fraction polysaccharides (HFPS), i.e. polysaccharid es attached to heavy fraction soil minerals with density >1.7 g cm(-3). A s ignificant 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 significant rise of HFPS. Furthermore, the added plant ma terial clearly decreased the soils' fraction of dispersible clay even befor e incubation, which was interpreted as a direct effect of the LFOM on the c lay. Therefore, the use of dispersible clay as an indicator of macro-aggreg ate 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 packin g the post-incubation soil in metal cores, the volume of pores with equival ent neck diameter >10 mu m was from 0.024 to 0.033 m(3) m(-3) higher and th e 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 und isturbed. Also the volume of pores with a neck diameter >10 mu m and the sa turated hydraulic conductivity were significantly reduced (by 6% and 19% re lative, respectively) when the soil was disturbed during incubation. The re sults demonstrate that rapid changes of SOM fractions can occur over a shor t incubation period and that these changes have a pronounced positive effec t on the structure of these tropical soils. It also suggests that incorpora ted plant material may correct tillage induced degradation of soil physical properties over a relatively short time. (C) 1999 Elsevier Science B.V. Al l rights reserved.