Microbial nitrogen transformations tracked by natural abundance isotope studies and microbiological methods: A review

Nitrogen is an essential nutrient in the environment that exists in multiple oxidation states in nature. Numerous microbial processes are involved in its transformation. Knowledge about very complex N cycling has been growing rapidly in recent years, with new information about associated isotope effects and about the microbes involved in particular processes. Furthermore, molecular methods that are able to detect and quantify particular processes are being developed, applied and combined with other analytical approaches, which opens up new opportunities to enhance understanding of nitrogen transformation pathways. This review presents a summary of the microbial nitrogen transformation, including the respective isotope effects of nitrogen and oxygen on different nitrogen-bearing compounds (including nitrates, nitrites, ammonia and nitrous oxide), and the microbiological characteristics of these processes. It is supplemented by an overview of molecular methods applied for detecting and quantifying the activity of particular enzymes involved in N transformation pathways. This summary should help in the planning and interpretation of complex research studies applying isotope analyses of different N compounds and combining microbiological and isotopic methods in tracking complex N cycling, and in the integration of these results in modelling approaches.