Termite-induced change in soil structure after mulching degraded (crusted) soil in the Sahel

A morphological approach was used to study the influence of termite activity on crusted soil. The structure of the top layer (0-10 cm) of these soils is degraded (no functional voids and no aggregates). Composite mulch (woody material + straw) was applied at the rate of 4 mg ha(-1) to trigger the activity of termites, Termite-induced features in soil structure were described and quantified by means of field observations, observation with polarizing microscope and computerized image analysis using Quantimet 970. The termite activity after the application of the mulch resulted in a change from a compact grain structure (original structure) to a chamber and channel structure. The channels and chambers accounted for over 60% of the macroporosity in the 0-10 cm layer. Unmulched plots (i.e. bare plots) mostly had packing voids, very few macropores, no voids with equivalent circle diameter (ECD) greater than 2 mm and one-third the number of voids with ECD > 100 mu m compared with the plots with termite activity in the 0-10 cm layer. In the mulched plots the sealed surface was perforated by termites, resulting in many visible open voids. These plots were covered by sheetings that consisted of fine soil material transported to the soil surface and linked up by termites. In the deeper layers, termite-induced change of soil structure after mulching was difficult to confirm since both on mulched and bare plots there are many termite-mediated features from previous termite activity. These features are numerous voids, open or infilled, generally with crumbs. The latter are the only type of aggregate found in these soils, suggesting that termites play an important role in soil aggregation in such environments. This study provides information about the influence of termites in the improvement of a sealed/crusted topsoil structure. It shows that soil surface management such as mulching which enhances termite activity is an option to consider when improving degraded soil structure. (C) 1997 Elsevier Science B.V.