Impact of Seasonal Variations on the Colonial Populations of Bacteria and Fungi in Soil and on Buried Plant Stems

Abstract

Forest soils and stems buried in them usually have varying degrees of colonization and abundance of bacteria and fungi. This study was undertaken to determine the effects of seasonal variations on the population of bacteria and fungi isolated from forest soil and on plant stems buried in the soil. Soil sampling and stem burial studies were conducted over a 12-month period in 2019. Serially diluted soil samples were plated on suitable media for bacterial and fungal growth and thereafter counted after incubation. Buried stems were removed from the soil, rinsed and placed in flasks containing suitable media for fungal and bacterial cultivation. Colonial growth was counted after incubation. Soil moisture was highest during the wet season months of July (27.7%), August (23.5 %), September 26.1 %) and October (29 %) whereas the average soil moisture content was lowest in the dry season. Seasonal pH did not significantly affect microbial population levels in the various months. Colony counts for Pseudomonas spp. during the dry season months (January, February, March and April) were very low. Growth of the bacterium showed peaks in the May through October during which counts reached 109 cells per gram of soil except in August with counts of 108 cells per gram of soil. Micrococcus spp. and Bacillus spp. also showed similar trends in colony counts with little variations. Fungi were generally fewer in number than bacteria and only one peak which reached 107 cells/g soil was obtained for Fusarium spp. and Rhizopus spp. in September and October respectively. The density of Trichoderma spp. per gram of soil peaked at 106 cells in June, July, September and October. Counts for Aspergillus spp. was negligible in January, February, March and April but reached 106 cells per gram of soil in June, July and August. The colonization of Pseudomonas spp. on buried plant stem varied between 62% in June to 76% in October while Micrococcus spp. had levels which varied from 65% in May to 84% in June and 72% in October. Fusarium species were found most frequently on the stem every month except in February, March and April. Low colonization of Aspergillus spp. on stems occurred in January, February, March, November and December. Highest numbers of this organism was found in August, September and October. Rhizopus spp. was observed in 85 and 80% of the stem in September and October respectively but lower percentages of colonization occurred in January, February, March and April. In all the dry season months (January-April), all bacterial and fungal populations had low densities but their counts increased in the rainy season. Fungi were generally fewer in number than bacteria in both soil and stem burial experiments.