Effect of growing season upon microbial status of peppermint (Mentha x piperita L.) rhizosphere

In this study we assessed the number of microorganisms in rhizosphere of peppermint (Mentha × piperita L.) as one of the important aromatic species in Serbia. The trial was set up on chernozem soil at experimental field of Backi Petrovac, Institute of Field and Vegetable Crops, Novi Sad (northern Serbia) in 2012, 2013 and 2014. Rhizosphere soil samples were collected from both conventional and organic grown peppermint. In each year of investigation two samplings (June and July) were performed for microbiological analysis. Soil samples were analyzed by the standard methods in four replications and average number of microorganisms was calculated at 1.0 g absolutely dry soil. The results were analyzed in accordance with three-way model of analysis of variance. The obtained results showed that a significantly higher number of ammonifiers, azotobacters and free nitrogen-fixing bacteria was recorded in 2012 compared to 2013 and 2014. Also, a significantly higher number of azotobacters and free nitrogen-fixing bacteria was obtained in organic than in conventional growing. In addition, significant differences in number of fungi and actinomycetes were recorded between the sampling terms, i.e. significantly higher numbers of these groups of microorganisms were found in the second term.