Now that the outside temperature is quite low, most of us are enjoying the coziness of our homes while we wait for warmer weather. Some of you may be thinking about this year’s crops already and planning ahead for seeding time. Many of you have already applied some nitrogen (N) fertilizer, but others may have waited due to the high prices. As you may know, the N does not stay in one form forever; rather it participates in a cycle within the ecosystem. But what is this cycle? In this week’s edition of Growing Possibilities, we will be taking a brief look at the N cycle and how microbes play their part.
So what is the N cycle? The N cycle is the pathway N takes through the ecosystem, going between the soil, various organisms, and the atmosphere. The N goes through a variety of changes throughout this process, going from an inaccessible form to bioavailable form and back again. So let’s break down this cycle, starting with atmospheric N. Atmospheric N is the largest proportion of our atmosphere and is inaccessible to most organisms. In order to become accessible to most organisms, the atmospheric N must go through N fixation. Nitrogen fixation occurs when particular microbes convert the atmospheric N into ammonia (1).
Next the N goes through nitrification. Nitrification consists of two parts: first converting the ammonia into nitrite, and then converting the nitrite into nitrate. Again, like the N-fixation, nitrification can only be done by a select few microbes, including bacteria and archaea. Now when the N is in its nitrate form it can go in one of two directions: assimilation or denitrification. For assimilation, the plants will uptake the nitrate and use it for various things, including but not limited to photosynthesis, protein synthesis, and DNA synthesis. Upon death, the N in the tissues then goes through ammonification. Ammonification is turning the organic N in tissues back into ammonia via decomposing microbes like fungi and bacteria, where it can then go through the nitrification process again. But what if the N had taken the denitrification route? Denitrification is converting nitrate back into atmospheric N, making it unavailable to most organisms again and finishing the cycle. Denitrification is carried out by a variety of microbes in both the water and soil (1).
So that is the natural N cycle, but growers have been adding N into their fields via fertilizers for generations. Nitrogen fertilizers are made using the Haber-Bosch process, which takes the atmospheric N and converts it into ammonia. This process has revolutionized the farming industry since it helped us ensure the crops had enough N to produce greater yields. But globally we have been over applying N fertilizers and more than 50% of it is lost to the environment (2). This over use is causing eutrophication, which is the proliferation of microbes like algae in water systems. Eutrophication causes decreases in oxygen in the water, which ultimately kills aquatic life (3).
So what can we do? In general, farmers can reduce their N fertilizer application to reduce these effects or they can do more efficient use of N fertilizer. One way to do that is to start planting more legumes to help input N into their soil more naturally. Legumes have a symbiotic relationship with certain N-fixing bacteria. These bacteria fix large portions of the N required for the plant from the air, meaning less N fertilizer is required to produce a good crop. To ensure their legumes have enough N-fixing bacteria to fix N, farmers apply inoculants like XiteBio® SoyRhizo® (for soybeans) and XiteBio® PulseRhizo® (for peas, lentils, and faba beans) to their legumes. Also, after a legume crop has been harvested, there is more residual N left in the soil from the N-fixing bacteria than there would have been if a different crop had been planted, meaning less N fertilizer is required for the next year’s crop (3). Therefore, adding legumes into your crop rotation is an excellent way to reduce over application of N fertilizers and to naturally participate in the N cycle.
References:
- https://www.nature.com/scitable/knowledge/library/the-nitrogen-cycle-processes-players-and-human-15644632/
- https://www.frontiersin.org/articles/10.3389/fmicb.2018.02424/full
- https://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419/