Effects of the inoculation of cyanobacteria on the microstructure and the structural stability of a tropical soil

Cyanobacteria are widespread photo- synthetic microorganisms among which some are able to fix atmospheric nitrogen. We investigated the impact of indigenous cyanobacteria strains (Nostoc) inoculation on physical characteristics of poorly aggregated soils from Guquka (Eastern Cape, South Africa). The soil aggregates (3-5 mm) were arranged into a layer of 10-20 mm thick, and sprayed with cyanobacteria solution. Subsequently the inoculated and un-inoculated samples were incubated (30°C, 80% humidity, continuous illu- mination at 100 lmol m-2 s-1). Their micromor- phological characteristics and aggregate stability were investigated, after 1, 2, 3, 4 and 6 weeks of incubation, by using high resolution Cryo-SEM and aggregate breakdown tests.Micromorpholog- ical investigations revealed that the surface of un- inoculated samples remained uncovered, while the inoculated samples were partially covered by cyanobacteria material after one week of incuba- tion. A dense superficial network of cyanobacterial filaments and extracellular polymer secretions (EPS) covered their surface after 4 and 6 weeks of incubation. Organo-mineral aggregates com- prising cyanobacterial filaments and EPS were observed after 6 weeks of incubation. The results of aggregate breakdown tests showed no signifi- cant difference between un-inoculated samples after 1, 2, 3, 4 or 6 weeks, while they revealed improvement of aggregate stability for inoculated samples. The improvement of aggregate stability appeared in a short while following inoculation and increased gradually with time and cyanobacteria growth. The increase in aggregate stability is likely related to the changes induced in micromorpho- Our study presents new data demonstrating the improvement of soil physical quality in a few weeks after cyanobacteria inoculation. The inter- action of the inocula and other biotic components is worthy of study before field application of cyanobacteria.