Microbial populations in heated soils inoculated with cyanobacteria

Abstract

 The composition of soil microbiota in four heated (350  °C, 1 h) soils (one Ortic Podsol over sandstone and three Humic Cambisol over granite, schist or limestone) inoculated (1.5 μg chlorophyll a g^–1 soil or 3.0 μg chlorophyll a g^–1 soil) with cyanobacteria (Oscillatoria PCC9014, Nostoc PCC9025, Nostoc PCC9104, Scytonema CCC9801, and a mixture of the four) was studied by cultural methods. The aims of the work were to investigate the potential value of cyanobacteria as biofertilizers for accelerating soil recolonization after fire as well as promoting microbiotic crust formation and to determine the microbial composition of such a crust. The inoculated cyanobacteria proliferated by 5 logarithmic units in the heated soils which were colonized very quickly and, after 2 months of incubation, the cyanobacterial filaments and associated fungal hyphae made up a matrix in which surface soil particles were gathered into crusts of up to 1.0 cm in thickness. These crusts were composed, on average, of 2.5×10¹⁰ cyanobacteria, 2.8×10⁶ algae, 6.1×10¹⁰ heterotrophic bacteria (of which 1.2×10⁸ were acidophilic, 1.3×10⁶ were Bacillus spp. and 1.5×10⁸ were actinomycetes) and 77.8 m fungal mycelium (1.4×10⁶ were fungal propagules) g^–1 crust. Counts of most microbial groups were positively correlated to cyanobacterial numbers. The efficacy of treatment depended on both the class of inoculum and the type of soil. The best inoculum was the mixture of the four strains and, whatever the inoculum used, the soil over lime showed the most developed crust followed by the soils over schist, granite and sandstone; however, the latter was comparatively the most favoured by the amendment. In the medium term there were no significant differences between the two inocula rates used. Biofertilization increased counts of cyanobacteria by 8 logarithmic units while heterotrophic bacteria, actinomycetes, algae and fungal propagules rose by >4 logarithmic units, acidophilic bacteria and Bacillus spp. by around 3 logarithmic units and fungal mycelia showed an 80-fold increase. The results showed that inoculation of burned soils with particle-binding diazotrophic cyanobacteria may be a means of both improving crust formation and restoring microbial populations.