The nitrogen cycle and trees
Trees, like all living organisms, need various nutrients to grow and survive. A tree will obtain these elements from the soil, water, or the atmosphere. Of all of the elements necessary for tree growth, the one nutrient that is most often in short supply is nitrogen (N2). This is because nitrogen in its elemental form (N2) is a gas, which makes it unavailable to most plants unless the plant can obtain it from the atmosphere. The two forms of nitrogen available to most plants in the soil and water are nitrate (NO3) and ammonium (NH4). The nitrate form of nitrogen is highly soluble in water and will often leach out of the soil before plants have a chance to fully utilize all of it. This form is also somewhat unstable so nitrogen reverts back to a gaseous state in the atmosphere in a process known as denitrification. Ammonium (NH4), on the other hand, is more readily available to plants because it is more stable; a plant has to decompose organic matter before it can extract nitrogen through ammonium.
Plants called nitrogen fixers can take nitrogen right out of the air. Many of them are from the legume family, and they are all characterized by producing seed ponds. Some examples of legumes include black locust, Kentucky coffee tree, soybeans, and clover. Legumes enrich the soil by taking elemental nitrogen from the air, then depositing some in the soil in a form that is more available for plants.
Most plants, however, have to obtain the nitrogen they use from the soil or water utilizing either the NO3 nitrate or NH4 ammonium form. The nitrate form of nitrogen, because it is highly soluble in water and somewhat unstable, often leaches away or reverts back to the gaseous state.
Article by FCFCDB
Nature Note for 10/26/14