Monitoring the change in aggregate stability and water infiltration in tropical soil following inoculation with indigenous cyanobacteria strains.

Cyanobacteria are widespread photosynthetic organisms among which some are able to fix nitrogen. In arid environment, they form tough, entangled structures that render stability to the soil surface and protect it from erosion. They have been widely used as natural fertilizer in wetland rice crop. The present study is part of an EU project (Cyanosoils) about the application of local strains of cyanobacteria in order to improve resilience and overall soil fertility in Southern Africa. The results of laboratory experiments carried out in an investigation of cyanobateria potential in improving the physical quality of tropical soils were reported. Indigenous cyanobacteria strains were isolated and selected for inoculation on poorly aggregated soils from Guquka (Eastern Cape, South Africa). Briefly, soil aggregates of 3 to 5-mm in diameter, arranged into layer of 10-20 mm thick, were sprayed with the solution of cyanobacteria and, subsequently incubated in a climatic chamber at 30o C, 65 % humidity and under continuous illumination at 50 Μmol m-2 s-1. Aggregate stability, water infiltration measurements and micromorphological characteristics investigation were made using soil samples obtained after 1, 3, 4 and 6 weeks period of cyanobacteria growing. The aggregate stability was measured by testing aggregate breakdown under fast wetting, slow wetting and mechanical breakdown. The infiltration was measured by collecting percolating water under rainfall simulation. Micromorphological characteristics were investigated under high resolution Cryo-SEM. These experiments were completed by C, N and polysaccharides measurements. Microscopic examination of samples showed that the surface of soil aggregates is covered by cyanobacteria since the first week of inoculation. However no significant change in aggregate stability and water infiltration was recorded. After 4 weeks of incubation, a dense superficial network of cyanobacteria filaments and extracelullar polymer secretions (EPS) was formed; the samples exhibited higher aggregate stability and carbon and sugars content compare to the control (uninoculated). The change in aggregate stability following inoculation of cyanobacteria is likely related to the enmeshement of soil aggregates by their filaments and the binding and gluing effects of EPS.