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. Very little phosphorus is readily available to organisms, most is tied up in rocks, sediments, and soils. Hence, it can be a limiting nutrient in ecosystems. Soluble phosphate is made available to organisms by the weathering of the mineral apatite.