Use of Indigenous N2-Fixing Soil Cyano-Bacteria for Low-Input, Sustainable Improvement of Overall Fertility of Degraded Lands in Semiarid Tropics

For a sustainable improvement of both, biogeochemical performance and physical fertility of degraded soils in marginal lands of three Southern Africa Countries (Tanzania, Zimbabwe and South Africa), an holistic approach was adopted to asses the possibility of locally growing and applying to soil, of biomasses of selected strains of indigenous cyanobacteria. Some strains of these photosynthetic, autotrophic micro-organisms are able to fix atmospheric Nitrogen, and to produce exo-polysacharides (EPS), capable to bind soil mineral particles, thus increasing aggregates stability. The main results of the investigations undertaken can be summarized as follows: a)Cyanobacteria identification and selection. A large number of cyanobacteria strains (about 200) have been identified. Some of these strains were found able to fix consider- able amounts of atmospheric Nitrogen and to produce abundant EPS. The most productive indigenous strains for each Country were successfully selected, purified and grown with suitable procedures and techniques specifically adapted to obtain large amounts of biomass, even under stress. These biomasses were used for laboratory and field studies and experiments aimed to increase permanently the overall fertility and productivity of degraded soils. b)Effects on chemical- fertility. Laboratory, greenhouse and field experiments demonstrated that cyanobacteria were capable to grow even under limiting conditions (low nutrients availability, low pH). In spite the metabolic cyanobacteria activity decreases the NO3- and NH4+ concentration, as they use these forms of N to grow, the overall soil C and N content increased. In soils with low nutrients content, cyanobacteria increased the amount of total and available phosphate, potassium, calcium and magnesium and also the concentration of micronutrients. In soils with higher inherent fertility in which cyanobacteria developed to larger extent, a decreased concentration of available macro- and micronutrients was found, due to sorption on the negative charged surface of the EPS produced by these micro¬organisms. Moreover, cyanobacteria modify the SOM characteristics and increase the more stable humic fraction. c)Effects on physical fertility. The results of laboratory, greenhouse, and preliminary field experiments showed an inerease of aggregates stability due to cyanobacteria inoculation on poorly structured soils. This improvement appeared shortly after inoculation and increased with time and growth of cyanobacteria biomass. The increased stability is accompanied by changes of soils surface microstructure and formation of organo-mineral aggregates. d)Effects on crop growth. Greenhouse experiments with rice and maize showed that inoculation with cyanobacteria had limited effect on the first crop, but the yield of the subsequent crop increased significantly. The effect of cyanobacteria on crop growth varied with soil types and this depended on the different extents of cyanobacteria establishment. Preliminary, short-term field experiments seem to confirm the beneficial effect of cyanobacteria on improving crops yield.