Nitrogen Fixation and Agriculture
Nitrogen is one of the most abundant elements on earth. It accounts for 78% of the earth’s atmosphere in the form of N2 . Plants require nitrogen for their metabolic processes as well as growth. It is a key component of amino acids, the building blocks of proteins, as well as chlorophyll. However, plants are unable to fulfill their needs with the di-nitrogen available in the earth’s atmosphere. They require nitrogen in the form of nitrate or ammonium, both of which are more complex forms (NH3, NH4, NO2 or NO3) that are found in the soil. There are three ways that the atmospheric nitrogen is converted into nitrate/ammonium that is found in the soil. The first is through atmospheric fixation events such as lightning strikes, rain, and snow, relying upon climatic events. Another alternative is through man-made fertilizers that utilize the Haber process to produce ammonium. Finally, some plants have a symbiotic relationship with diazotrophic bacteria that fix atmospheric nitrogen into ammonium .
Diazotrophs are bacteria that contain nitrogenase, the enzyme responsible for biological nitrogen fixation (BNF). Diazotrophs include cyanobacteria, green sulfur bacteria, azotobacteraceae, rhizobia, and frankia.
The reaction for BNF is as follows: N2 + 8 H+ + 8 e− → 2 NH3 + H2
The majority of plants that have a symbiotic relationship with diazotrophs come from the legume family, known formally as Fabacae . These include plants such as clovers, soybeans, alfalfa, lupines, peanuts, and rooibos. Legumes contain root nodules that harbor the diazotrophs, providing them with the anaerobic conditions necessary to fix nitrogen . While the plant lives, most of the fixed nitrogen goes to the plant itself. However, after the death of the plant, the fixed nitrogen is released, acting as a natural fertilizer for the soil and providing usable nitrogen to other plants (non-legumes).
The Haber process used in the production of fertilizer, while chemically efficient, requires boiling, cooling, and very high pressure throughout the process. This requires the use of fossil fuels, and results in high costs to farmers. About 2% of the world’s energy goes into the production of fertilizer alone. It must be replaced frequently and represents a strong cost to the environment, both in terms of the amount of energy used up and the negative impacts of fertilizer runoff into rivers and other sensitive ecosystems . Although fertilizer is by far the most common way of providing ammonium to crops, many farmers are already using alternative methods involving nitrogen-fixing bacteria. Some farmers use crop-rotation techniques in which they plant leguminous crops some seasons in order to fertilize the soil for future harvest of other crops. Another method is to plant leguminous plants alongside the crop in question, as long as it doesn’t block out sunlight or hurt the growth of the other crop. Current research is going into finding a diazotroph that is able to fix nitrogen for all types of crops- scientists are looking for an alternative ways to trigger the mechanism on plants that produces the root nodule in order to attract rhizobia or other diazotrophs .
Reasons for move to BNF
There are a few reasons why there needs to be a move towards more plants being able to fix their own nitrogen. The first reason is the need for increased yields in crops over the next few decades combined with the expensive nature of nitrogen fertilizers. Grain crop yields have increased substantially over the last thirty years while the area of land harvested has stayed relatively flat. For example, in 1975 the area harvest for grains was just over 700 million hectares for a total production of roughly 1,250 metric tonnes. This comes out to a yield of roughly 1.75 tonnes per hectare. In 2010, the total harvested area remained flat around 700 million hectares while the global grain production had risen to roughly 2,500 million metric tonnes. This comes to a yield of roughly 3.5 metric tonnes per hectare, a very significant increase. A lot of the increased in yields can be attributed to increased fertilizer use, in some cases as much as 75%. With the population doubling in the last 30 years, the increase in yield has been essential to our survival. Over the next few decades, the population is expected to rise even further, which will require yields to rise even further.
Secondly, fertilizer costs are a large part of farmers’ budgets. In 2011, fertilizers for corn and wheat were roughly 20% of a farmer’s budget, compared to less than 10% for soybeans. This is due to the fact that soybeans do not need as much fertilizer because of their symbiotic relationship with nitrogen fixing bacteria. Fertilizer costs have increased nearly 50% in the previous years, putting pressure on farmers to find alternative methods of providing nitrogen to their crops.
Finally, there are environmental costs associated with the excessive use of fertilizer. The nitrogen that is not taken up by plants can “accumulate in soil, water, the atmosphere and coastal oceanic waters [and] contribute to the greenhouse effect, smog, haze, acid rain, coastal ‘dead zones’ and stratospheric ozone depletion.” Crops are only able to harness somewhere between 30% and 50% of the fertilizer that is applied. Fossil fuels are another factor in the environmental costs of fertilizers. Roughly 2% of the world’s fossil fuel consumption is used in producing fertilizers. If major crops can be enhanced to become nitrogen fixers, then the world would use substantially less fossil fuels. We would be able to decrease fertilizer production and limit the cost of transportation to the farmland. We would cut down the costs of fertilizer because the plants would be able to biologically fix nitrogen. The increase in yield that is needed would not be sacrificed, and there would be less excess nitrogen in water and soil because plants that fix nitrogen themselves make the necessary amount.
What is being done
Within the past decade or so, scientists have been actively trying to make our crops capable of being nitrogen fixers, and therefore less reliant on inorganic fertilizers. The most significant crop for us to achieve nitrogen fixation with would be corn. The US produces more corn than any other crop. It has been shown that corn can establish a relationship with nitrogen fixing bacteria. Rhizospheric or endophytic relationships can be established “with various nitrogen-fixing bacteria such as azospirillum, Klebsiella, Pantoea, Herbaspirillum, Bacillus, Rhizobium etli, and Burkholderia." Establishing relationships means less nitrogen fertilizer and lower costs for farmers. This can be extremely beneficial to poor farmers in developing countries. This, from a study in 2010, may have been a little optimistic.
An article in February of 2012 states that “for years scientists have been working hard to discover the key to nitrogen fixation in corn”, meaning that it has not quite been established yet. They believe they have found the bacterium though in Gluconacetobacter diazotrophicus, which was originally found in sugarcane in the 1980’s. The study states they have been successful at introducing the bacterium in the lab, but will need to do further tests in the field. There is hope that the bacterium will be able to live in the corn throughout the growing cycle, but it is unclear if it will be passed on to the next generation. The G. diazotrophicus bacterium is somewhat special in that it is able to be effective in soil that is already highly concentrated with nitrogen, and it can enter the plant at multiple sites. This is unlike the Rhizobium bacterium found in soybeans because it shuts down in soil with a high concentration of soil and must enter the plant through the root nodule.
There are some websites that claim to sell nitrogen fixing bacteria that work for all crops. One of these is called TwinN, a “free living Nitrogen fixing bacteria that will infect all crops, and make available large quantities of nitrogen while improving root growth and solid health.” The bacteria “produce nitrogen ‘in tune’ with the plant, so the plant is never under nor over supplied with nitrogen.” The costs of using these bacteria versus normal inorganic fertilizer are cut in half. This does not conclude a solution has been found though because of the crop's inability to pass on the bacteria to the next generation. More research and study will be needed to make nitrogen fixation a widespread reality and to decrease our need for inorganic fertilizers.
(2) Ingraham, John L. March of the Microbes: Sighting the Unseen (Ch. 5)
Daven Karp and Andrew Sprague, students of Rachel Larsen in Bio 083 at Bowdoin College