The bacterium Nitrobacter transforms nitrite to nitrate. This genus of bacteria is found in the soil. The bacteria fix atmospheric CO 2 in the dark from energy derived from the oxidation of nitrite. The bacteria are slow growing. The acquisition of nitrogen by plants involves an expenditure of energy.
Nitrogen fixation can only occur under certain conditions.
In root nodules of legumes, nitrogen fixation occurs only in the presence of leghaemoglobin. Some nitrogen-fixing bacteria live in the guts of animals. A pool of labile phosphorus contains 100 units of phosphorus. Phosphate is precipitated as insoluble iron and manganese phosphates. Iron and manganese phosphates are soluble under anoxic conditions.