Role of a cyanobacterial cover on structural stability of sandy soils in the Sahelian part of western Niger

Microbiotic soil crusts, mostly formed by cyanobacteria, are widespread on the surface of fallow land in western Niger. They lie adjacent to completel y bare soils. We have investigated the structural stability of these soils by testing aggregate breakdown under fast-wetting, slow-wetting and mechani cal breakdown. The tests were effected on aggregates ranging from 3 to 5 mm in size. These experiments were completed by micromorphological examinatio ns under light and scanning electron microscope. For all tests, the fragment size distribution and the mean weight diameter (MWD) revealed the great stability of aggregates from soils with a cyanobac terial cover (MWD 1.82 to 3.10 mm), compared to those from soils devoid of cyanobacterial cover (MWD 0.25 to 1.26 mm). Fast- and slow-wetting of micro biotic soil crust material induced a lesser disaggregation of aggregates co mpared to mechanical breakdown. On the contrary, fast-wetting and mechanica l breakdown of aggregates from soils devoid of cyanobacterial cover induced a greater disaggregation than slow-wetting. Microscopic examination of microbiotic crusts revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions, which b inds and entraps mineral particles on the soil surface. Organo-mineral aggr egates ranging from 0.10 to 0.12 mm were observed. Below the superficial cr usts, filaments and residual organic matter bind tightly soil particles, th us giving a compact structure. The great stability of aggregates of soil wi th cyanobacterial cover is likely related to the binding and gluing effect of cyanobacteria and derived organic matter. This is consistent with the po sitive correlation between MWD values and organic carbon content. The prese nt results thereby confirm the resistance to erosion of soil with microbial cover as indicated by field measurements. (C) 2001 Elsevier Science B.V. A ll rights reserved.