Enumeration of Microbes and Gas Production During Denitrification and Nitrogen Fixation Processes in Soil

Abstract

Dry plant material contains 2-4% nitrogen, making it an essential nutrient for all plants. The nitrogen cycle regulates the pathways which transform nitrogen from a relatively inert dinitrogen gas to forms of organic nitrogen such as proteins and nucleic acids. Denitrification and nitrogen fixation are the two most important processes that remove and add nitrogen to the soil, respectively. The aim of the study was to gain information on the denitrification and nitrogen fixing activities in soil and sediment employing the acetylene technique and assuring the gas chromatography analysis by total plate count and most probably number. The results indicated that acetylene (0.1 atm) inhibited N₂O reduction and caused stoichiometric accumulation of N₂O during the conversion of NO₃^- to N₂. N₂O was an obligatory intermediate in the sequence of steps between N₂O^- and N₂. The appearance of CO₂ and accumulation of N₂O would be suitable criteria for the presence of denitrifiers in appropriately enriched media and the acetylene reduction test is a suitable assay for nitrogen fixing activity. There was an obligatory requirement for organic carbon as a carbon and energy source for denitrification and nitrogen fixation to take place. The results showed that acetylglucosamine can be used as a carbon and energy source for denitrification but not as a nitrogen source (C:N ratio of 5:1). NH₄⁺ has no effect on denitrification activity but it inhibited the nitrogenase activity. The presence of air in the gas phase affects both the denitrification and nitrogen fixing activity while adding H₂O encouraged anaerobic conditions.