Influence of indigenous strains of Cyanobacteria on physical and biochemical properties of tropical soils

Indigenous cyanobacteria strains exhibiting a high capability of fixing atmospheric Nitrogen and producing, as a result of their metabolism, large amounts of exo-polysaccharides (EPS), have been isolated from soils in semi-arid environments of Tanzania, South Africa and Zimbabwe. The aim was to apply the purified strains to degraded soils of marginal lands and promote their proliferation, in order: a) to improve soil aggregates stability, as a result of binding of mineral particles by BPS; and b) to increase soil biochemical fertility by fixation of atmospheric Nitrogen. In preliminary experiments the selected strains of cyanobacteria were grown in water suspensions in the laboratory under appropriate conditions of temperature and light to obtain large amounts of biomass. The suspensions were then sprayed on 3-5 mm soils aggregates, arranged on layers 10-20 mm thick and incubated for 1 to 4 weeks at 30°C and 80% WHC. The micro-morphological characteristics and aggregates stability to different tests (fast and low wetting and wet stirring) were then measured and compared with that of non-inoculated samples. After one week of incubation the surface of coarse particles of inoculated aggregates showed a significant cyanobacteria cover, but the aggregates stability was not different from that of non-inoculated samples. However, after four weeks of incubation the cyanobacteria covered quite the entire surface of the inoculated aggregates and the ESP secretion formed an organic membrane binding the particles. As a result, the stability of inoculated samples to fast and slow wetting test was increased 2-3 times. In a greenhouse experiment, pots containing inoculated and non-inoculated soils were planted with rice and maize. After 52 days they were harvested and weights of dry matter determined. No significant differences were obtained on the first yield of inoculated and non-inoculated soils. However, the rootstocks remaining after harvesting were allowed to re-grow and after 6 weeks the dry matter was collected and weighted again. An increase of biomass of about 25% was measured on the inoculated samples. These preliminary experiments suggest that cyanobacteria are capable to improve both soil physical and bio-geochemical properties and their field application can represent a valuable tool for a friendly sustainable improvement of soil fertility and productivity of marginal lands in semi arid tropics.