Asymbiotic or non-symbiotic N-Fixing organisms, usually species of bacteria, have the ability to ‘fix’ nitrogen (N) from the atmosphere where it is abundant as N2 gas, and produce NH3 without being associated with plant or fungi.
The process used to fix N by asymbiotic organisms is highly energy intensive. The enzyme that serves as the fixation facilitator in bacteria is nitrogenase. To convert one molecule of N2 to one molecule of NH3, requires 16 ATP (adenosine triphosphate) molecules (the chemical fuel for many biological reactions) and 8 electrons. This compares to only 5 ATP to produce one peptide bond in the production of amino acids, which is classified itself as a ‘high energy requirement’ (Smercina et al., 2019).
The energy required by asymbiotic organisms to fix N may come from a variety of sources depending on their spatial presence in the surface soil biome. The sources may consist of soil organic matter, and intermediary compounds during the process of plant/animal decay, or the decay of other microorganisms. A rich source of energy also comes from root exudates of growing plants. If N-fixing organisms are active inside living plant tissue, the energy for N-fixation has to come from the plant itself.
Lately, biological-based products have been marketed to enhance crop production. One class of products essentially consists of seed inoculants whose marketers claim they will either infect the seed, which enables them to perform N-Fixation functions similar to Rhizobia in legumes, or to inhabit the immediate soil outside the root (the rhizosphere) and use plant-root exudates as food-stuffs, enabling the N-fixing organisms to fix N from the air. There is much more marketing information available to corn growers on these products than there is availability of published unbiased data from the North Central Region (NCR) of the U.S.
Trials to determine the yield benefit from one or more of these products added to different rates of N fertilizer were conducted by researchers from most of the land-grant universities in the NCR through the 2022 growing season to determine their possible value to farmers.
Overall summary – 61 site years of N rate trials with and without the use of biological N fixing products were conducted in corn, spring wheat, sugar beet and canola in 10 states including Kansas within the NCR. Of the 61 site-years, 59 site-years had no yield increase with use of the product over N rate alone. Two site-years in corn had yield increases due to product use over the N rates alone. Given the low rate of positive benefits to the use of these products, growers should be skeptical of products that claim to provide asymbiotic/nonsymbiotic N-fixation for the purpose of allowing a farmer to decrease fertilizer N rate. It is good for farmers to be curious; however, the wise grower needs to test products of interest on their own farm in a replicated manner and search for unbiased data on product performance before using them on whole fields. There is a recently published primer on how one might conduct on-farm research titled “Promoting adoption of precision nitrogen management technologies through on-farm research” by Thompson et al., 2022.
This information came from NDSU Extension publication SF2080 (April 2023).
Stacy Campbell is an Agriculture and Natural Resources agent for Cottonwood Extension District. Email him at scampbel@ksu.edu or call the Hays office, 785-628-9430.
