Nitrogen fixation adds nitrogen to the cycle. Nitrogen-fixing bacteria include Rhizobium and Azotobacter . Rhizobium is found in a symbiotic relationship in root nodules of legumes. Denitrification results in reversal of nitrification and occurs in anoxic soils and sediments as well as anaerobic deep ocean waters. This process can be accomplished by physical reactions and the bacterium Pseudomonas denitrificans .
Nitrogen fixation balances denitrification in most ecosystems.
However, anthropogenic influences have thrown the nitrogen cycle out of balance. It is increasing in concentration every year and the source of most of it remains unknown. Nitric acid contributes to acid rain. Figure 11-15 illustrates the phosphorus cycle. Note the absence of an atmospheric compartment.