The undercover benefits of inoculants.
The role of biological nitrogen fixation in an agricultural system
Written by: Allison Friesen
Nitrogen. A vital element.
As a grower, when you think of nitrogen the words fertilizer, availability and essential come to mind. But why is nitrogen so important and why do your crops need it to survive? Nitrogen is a fundamental element for plant growth, development and reproduction. It’s so vital because it’s a major component of chlorophyll, the pigment plants use to convert sunlight energy to produce sugars from carbon dioxide and water. It’s also a major component of amino acids, which are the building blocks of proteins. But despite being one of the most abundant elements on earth, it’s not directly available to plants and can only be taken up in certain forms. This is why nitrogen deficiency is one of the most common nutritional problems in plants. Plants can receive the usable forms of nitrogen (typically ammonium and nitrate) from several sources (Figure 1):
- The addition of ammonia and/or nitrate fertilizer (The Haber-Bosch process)
- Organic matter decomposition
- The conversion of atmospheric nitrogen by natural processes (e.g. lightning)
- Biological nitrogen fixation
Figure 1. The nitrogen cycle. Source: http://www.cropnutrition.com/efu-nitrogen
Biological nitrogen fixation is carried out by a specialized group, collectively referred to as plant-growth promoting microorganisms (PGPM). Some examples include Rhizobium, Bradyrhizobium, Azotobacter and Azospirillum. The conversion of nitrogen from the atmosphere (N2) requires a lot of energy because of its extremely strong bond. Symbiotic nitrogen-fixing PGPMs like Rhizobium and Bradyrhizobium obtain this energy from the rhizosphere (or the area surrounding the roots) of legume crops. They then use the enzyme nitrogenase to break the strong bond and drive the conversion of atmospheric nitrogen into ammonium (NH3/NH4+), a form of nitrogen that can be readily taken up by the plant.
Why we need rhizobia.
It’s estimated that approximately 80% of fixed nitrogen on the planet is due to the activity of rhizobia bacteria. It’s also estimated that the rhizobium-legume association is responsible for producing 35 million tons of nitrogen annually (Sassitsch et al. 2002). Inoculants take advantage of the unique and mutually beneficial relationship between legumes and rhizobia to make nitrogen available for use by the plant. These rhizobia are located in nodules on the plant’s roots (Figure 2) and convert atmospheric nitrogen into ammonium and in return, the plant provides the rhizobia with energy, water and nutrients. The overall result is enhanced plant growth and nutrient uptake, resulting in increased yield potential.
Figure 2. Nodule formation on peas.
The “other” inoculant benefits.
The fact that inoculants enhance crop growth is the most attractive feature which drives their incorporation into pulse production. But inoculants actually provide several other environmental benefits that often go unnoticed. As more inoculants are being made commercially available, there has been a substantial reduction in fertilizer use. Overuse of fertilizers has led to an upset in the nitrogen cycle causing surface water and groundwater pollution. Environmental pollution from fertilizer nitrogen escaping root zones is high because fertilizers are not used efficiently by crops (Sessitsch et al. 2002). In general, 60 to 90% of total applied fertilizer is lost and only the remaining 10 to 40% is taken up by plants (Bhardwaj et al. 2014). Biological nitrogen fixation offers more flexible management compared to fertilizers because the pool of nitrogen becomes more slowly available. This is ideal for the current legume crop but it’s also convenient for future rotations of non-legumes. Research shows that when cereals are grown on field-pea stubble, they produce more yield per acre than cereals grown on canola stubble (Canola Council of Canada). Because of these yield benefits, legumes are often not grown as continuous crops but are rotated with non-legumes like cereals or oilseeds.
But the benefits don’t end there. Not only do inoculants fix nitrogen and enhance plant growth, they also promote other beneficial nitrogen-fixing bacteria in the soil, increase the supply of other essential nutrients and they can provide control of bacterial and fungal diseases. On top of that, inoculants can also increase the stability and fertility of the soil, particularly in poor environments. Below is a summary of the benefits inoculants provide to both the crop and the surrounding environment.
- Improve fixation of nutrients in the soil
- Provide food supply for growth of microorganisms, including earthworms
- Increase soil stability, structure and root growth
- Promote decomposition and increase organic matter levels in the soil
- Promote the development of beneficial fungi like mycorrhiza
- Produce growth stimulants for the plant
- Enhance other beneficial bacteria and fungi
- Encourage processes to remove toxins from contaminated soils
Inoculants and the plant-growth promoting microorganisms they utilize play an important role in agriculture. Especially since increased inoculant use can reduce reliance on fertilizers and other chemicals. Overall, inoculants can improve productivity in a relatively short amount of time, increase soil fertility, mitigate soil contamination and promote biological control of pathogenic organisms. With an increasing global demand for protein-rich crops, inoculants will continue to play an important role in sustainable agriculture.
- Bhardwaj, Deepak, Wahid Ansari, Mohammed, Kumar, Ranjan, and Tuteja, Narendra. 2014. Biofertilizers function as a key player in sustainable agriculture by improving soil fertility, plant tolerance and crop productivity. Microbial Cell Factories. 13:66.
- Sessitsch, A., Howieson, J.G., Perret, X., Antoun, H., and Martinez-Romero, E. 2002. Advances in Rhizobium Research. Critical Reviews in Plant Sciences. 21(4): 323-378.
- Canola Council of Canada, 2013. Crop rotation.