How do plants gain access to vital nitrogen, and what are the consequences of agricultural fertilizer overuse?

Nitrogen is usually limited in terrestrial ecosystems, since it is not released into the ground from rocks (as opposed to phosphorus). Instead, the bacteria and plants have to extract it from the nitrogen molecules of the air in order to build it into their cells. For this reason, fertilizers are used in agriculture to supply the optimal amount of nitrogen (as well as phosphorus and potassium) to plants. Nitrogen reserves in the soil are easily dissolved in water and therefore, they are quickly washed from the fields into rivers, streams and lakes.

Therefore the intensive use of fertilizers can pollute groundwater - this has rendered many water sources and wells unsuitable for drinking. The easily soluble nitrates in surface waters, so they are usually not a limitation for algae growth. On the other hand, compounds of phosphorus are less mobile and are often absorbed on soil particles or in various ions. Therefore they are slower to move and less frequent in natural waters, so the availability of phosphorus is typically a limiting factor of algae growth. A bigger problem is that phosphates bind to other particles and become difficult to move and absorb - making it difficult to get into the feeding network.