Biofertilization of Degraded Southern African Soils with Cyanobacteria Affects Organic Matter Content and Quality

We studied, under laboratory conditions, the effects of Nostoc 9v (cyanobacteria) on the soil organic matter (SOM) content and quality of different soils from Tanzania, Zimbabwe, and South Africa. Soils were inoculated with Nostoc 9v at a rate of 5 mg dry biomass g−1 soil and incubated for 3 months at 27°C under 22 W m−2 illumination for 16 h and 8 h under darkness. Moisture content was maintained at 60% of field capacity. The composition of the SOM and the structural features of the humic acid (HA) fraction were studied by visible and infrared (IR) spectroscopies. Nostoc 9v proliferated and colonized the surfaces of all soils very quickly. The results obtained showed significant changes in the quantitative and qualitative characteristics of the SOM due to cyanobacterial growth. Inoculation and further incubation with Nostoc 9v promoted increases in organic carbon (OC) that ranged from 0.4 g C kg−1 soil to 9.0 g C kg−1 soil. Cyanobacterial growth also affected SOM characteristics through the incorporation of free, extractable, or particulate biomass with a predominantly aliphatic character. Important descriptors of the extent to which cyanobacterial metabolism modified the characteristics of the native SOM were the changes in the observed amounts of the two humin types, the increase in lipid concentration, and the changes in the optical density of the HAs.