The nitrogen cycle involves various microorganisms and chemical transformations. Which of the following processes is carried out by bacteria specifically under anaerobic conditions, such as in waterlogged soils?
conversion of ammonium ions to nitrite ions (nitrification)
conversion of nitrate ions to nitrogen gas (denitrification)
conversion of nitrogen gas to ammonium ions (nitrogen fixation by Azotobacter)
decomposition of urea to ammonium ions (ammonification)
