There are many symbiotic relationships in nature. Plants and herbs can be grown together to enhance growth, helpful predator insects are attracted by a specific flower to combat a specific crop pest, and even bacteria interact with special leguminous plants to transform nitrogen into a form that plants and animals can use. In this blog, I would like to focus on nitrogen fixation, a very special relationship between bacteria, plants, the soil, and the atmosphere.

In brief, nitrogen fixation is a process where inert N2 (nitrogen gas) is converted into usable ammonia (NH3). This form of nitrogen is important to plants and animals as it helps to manufacture amino acids, proteins, nucleic acids and other nitrogen-containing components necessary for life.

Nitrogen fixation by legumes is a partnership between a bacterium and a plant. The plant supplies all the necessary nutrients and energy for the bacteria. Examples of legume plants include Alfalfa, Fava Beans, Vetch, Peanuts, Soy Beans, and Clover. Other plants benefit from nitrogen-fixing bacteria when the bacteria die and release nitrogen to the environment or when the bacteria live in close association with the plant.

A common soil bacterium, Rhizobium, invades the root of a legume and multiplies within the root cells forming bump-like masses called nodules. Within these nodules, nitrogen fixation is done by the bacteria, and the NH3 (ammonia) produced is absorbed by the plant. A way to determine whether or not nitrogen fixation is occurring in a plant is to investigate the roots. When fixation occurs the nodules turn from white or gray to pink.

It is a common misconception that nitrogen fixing plants deliver nitrogen directly to the soil via their root systems. The following is from the W.C. Lindemann, a Soil Microbiologist from New Mexico State University:

The amount of nitrogen returned to the soil during or after a legume crop can be misleading. Almost all of the nitrogen fixed goes directly into the plant. Little leaks into the soil for a neighboring non-legume plant. However, nitrogen eventually returns to the soil for a neighboring plant when vegetation (roots, leaves, fruits) of the legume dies and decomposes. When the grain from a grain legume crop is harvested, little nitrogen is returned for the following crop. Most of the nitrogen fixed during the season is removed from the field. The stalks, leaves and roots of grain legumes, such as soybeans and beans contain about the same concentration of nitrogen as found in non-legume crop residue. In fact, the residue from a corn crop contains more nitrogen than the residue from a bean crop, simply because the corn crop has more residues. A perennial or forage legume crop only adds significant nitrogen for the following crop if the entire biomass (stems, leaves, roots) is incorporated into the soil. If forage is cut and removed from the field, most of the nitrogen fixed by the forage is removed. Roots and crowns add little soil nitrogen compared with the aboveground biomass.

Taking the implications of the above paragraph seriously, it is important to till in a legume cover crops in order to utilize the nitrogen fixed from the atmosphere. This process is similar to carbon sequestration process mentioned in the previous blog. When we incorporate plant matter back into the soil we feed the microbial life of the soil foodweb. These microbes mineralize nutrients in the soil, aid aggregation of soil particles, and help to form humus that improve overall health and vitality of the soil.

To read W.C. Lindemann’s paper and to learn more about nitrogen fixation check out the following links:

http://www.cahe.nmsu.edu/pubs/_a/a-129.pdf

http://overton.tamu.edu/clover/cool/nfix.htm