Laboratory Assessment of Nostoc 9v (Cyanobacteria) Effects on N₂ Fixation and Chemical Fertility of Degraded African Soils

Abstract

The potential of Nostoc 9v for improving the nitrogen (N)₂–fixing capacity and nutrient status of semi‐arid soils from Tanzania, Zimbabwe, and South Africa was studied in a laboratory experiment. Nostoc 9v was inoculated on nonsterilized and sterilized soils. Inoculum rates were 2.5 mg dry biomass g⁻¹ soil and 5 mg dry biomass g⁻¹ soil. The soils were incubated for 3 months at 27 °C under 22 W m² illumination with a photoperiod of 16 h light and 8 h dark. The moisture was maintained at 60% of field capacity. In all soils, Nostoc 9v proliferated and colonized the soil surfaces very quickly and was tolerant to acidity and low nutrient availability. Cyanobacteria promoted soil N₂ fixation and had a pronounced effect on total soil organic carbon (SOC), which increased by 30–100%. Total N also increased, but the enrichment was, in most soils, comparatively lower than for carbon (C). Nitrate and ammonium concentrations, in contrast, decreased in all the soils studied. Increases in the concentration of available macronutrients were produced in most soils and treatments, ranging from 3 to 20 mg phosphorus (P) kg⁻¹ soil, from 5 to 58 mg potassium (K) kg⁻¹ soil, from 4 to 285 mg calcium (Ca) kg⁻¹, and from 12 to 90 mg magnesium (Mg) kg⁻¹ soil. Positive effects on the levels of available manganese (Mn) and zinc (Zn) were also observed.

Keywords:

• Cyanobacteria
• N₂ fixation
• Nostoc 9v
• nutrient status
• semi‐arid soils
• soil inoculation

ACKNOWLEDGMENTS

This research was supported by the INCO‐DEV program (UE), project ICA4‐CT‐20001‐10058. We thank Dr. C. Ascaso and Mr. F. Pinto (CCMA, CSIC) for assistance with the microscopic analysis.