Inoculation effects of two South African cyanobacteria strains on aggregate stability of a silt loam soil

Abstract

Two South African cyanobacteria strains (coded 3g and 7e) of the genus Nostoc were evaluated for improvement of the aggregate stability of a silty loam soil with low organic C content and compared with Nostoc strain 9v isolated from a Tanzanian soil. The soil was either cropped with maize or non-cropped and inoculated with the three strains in a glasshouse. After 42 days, the aggregate stability based on mean weight diameter (MWD) and fragment size distribution were determined by fast wetting, wet stirring and slow wetting methods. Inoculation of the soil with strains 3g and 7e improved the soil’s MWD and increased its proportion of large aggregates, particularly in the cropped soil. The opposite was the case for aggregates in soils inoculated with the reference strain 9v. Strain 3g resulted in greater improvement of MWD estimated by fast wetting, while strains 7e and 9v improved aggregate stability estimated by wet stirring. Improvement of aggregate stability was more related to exocellular polysaccharide (EPS) content than organic C. The results suggest that indigenous strains with high potential for EPS production could improve the soil structural stability of degraded soils in South Africa. Key words : Aggregate stability, exocellular polysaccharides, indigenous cyanobacteria, mean weight diameter.