MICROBIAL ABUNDANCE IN RHIZOSPHERE OF SUGARBEET IN DEPENDANCE OF FERTILIZATION AND INOCULATION WITH AZOTOBACTER CHROOCOCCUM PUBLISHEDNastasija Mrkovački, Jelena Marinković, Nikola Čačić, Dragana Bjelić
The nonsymbiotic N fixing azotobacter is an obligate aerobe living in the soil, the rhizosphere and the plant root. The ability of various strains to fix atmospheric N positively affect plant growth and yields. Microbial interactions with roots may involve either endophytic or free living microorganisms and can be symbiotic, assosiative or casual in nature. Associative and free living microorganisms may also contribute to the nutrition of plants through a variety of mechanisms including direct effects on nutrient awailability (N2-fixation), enhancement of root growth (PGPR – plant growth promoting rhizobacteria) as antagonists of root pathogenes or as saprophytes that decompose soil detritus and subsequently increase nutrient availability through mineralization and microbial turnover. Therefore, the abundance of azotobacter, fungi, actinomycetes and the total number of microorganisms in inoculated and non-inoculated sugar beet rhizosphere were determine in this study. Simultaneously we studied the effects of application of nitrogen fertilizer, manure and harvest residues. Samples of rhizosphere soil were taken three times in the course of growing season (May, July, September). The experiment included two variants (inoculated with Azotobacter chroococcum and non-inoculated) at four fertilization levels (non-fertilized control, 50, 100, 150 and 200 kg N/ha), in five replication. Total number of microorganisms was determined in soil agar (dilution of 106). Fungi were determined on Chapek agar (dilution of 104), actinomycetes on a synthetic agar (dilution of 104) and azotobacters on Fiodor substrate (dilution of 102). The results of the study for all four fertilization types and all four levels of added N showed that azotobacter abundance was higher in inoculated treatments than in the non-inoculated ones. The highest increases of total microbial abundance and number of free N-fixing bacteria were recorded in the inoculated treatments with NPK fertilizer alone and in the inoculated treatments with 100 kg N/ha. The highest percentage increases of azotobacter were obtained in the inoculated treatments with manure and manure plus harvest residues, respectively and in the variant without N.
microorganisms; azotobacter; rhizosphere; sugarbeet