The Nitrogen Cycle
How the cycle works in Iowa’s cropping system
WATCH: How the nitrogen cycle impacts Iowa’s cropping system
What is nitrogen?
Nitrogen is an element that is essential to human life. It comes in many forms including dinitrogen, nitrate, ammonium, and organic nitrogen. It’s an essential nutrient for all living organisms and is a critical component of proteins, DNA, and chlorophyll.
If you don’t remember grade school biology, that’s okay! Clorophyll is what plants use to convert sunlight into their own energy (photosynthesis). In plants, nitrogen is also in amino acids, which are the building blocks of proteins.
What is the nitrogen cycle?
The nitrogen cycle is a process that moves nitrogen into different forms between the air, soil, water, and living organisms.
Since it’s called “the nitrogen cycle”, you might think it moves in a perfect circle, but it’s much more complicated than that! Click through to see all the steps individually.
FIXATION
Some plants like legumes can “fix” nitrogen from the air. They pull atmospheric nitrogen (N2) from the air and turn it into ammonia (NH3). Soybeans do this using nodules on their roots. Fixation is how Iowa ended up wtih a bank of 10,000 lbs of naturally occuring nitrogen in the soil per acre! Not all of it is plant available.
ammonification
This happens when plants or animals die or release waste. Living organisms in the soil like bacteria decompose the matter into ammonium. This happens at the end of every crop growing season with the “trash” that’s left in the field. It also happens when manure is applied as fertilizer.
NITRIFICATION
Soil bacteria converts ammonia (NH3) or ammonium (NH4+) into nitrites, then into nitrates. Nitrate is the most plant available form of nitrogen. It’s water soluble, meaning it is also vulnerable to moving with water. In Iowa, this happens in early spring, which means two problems: no plants on bare fields to absorb it, and heavy rain which can make it move.
assimilation
Plants absorb nitrates from the soil to form plant proteins. Animals then consume the plants and take in nitrogen, which is also essential for human life. This is critical to the growing system. It’s what makes Iowa’s crops grow (and all plants grow).
denitrification
Bacteria break down nitrates, converting them to nitrogen gas. A good example of this is the use of wetlands. In a wetland, microbes don’t have oxygen to breath, so instead, they breath nitrate and release N2 back into the atmosphere.
How the nitrogen cycle fits into the Iowa cropping system
There are two key concepts to understand the process:
- Plants can get nitrogen from both natural sources in the soil and applied nitrogen.
- The most plant-available form of nitrogen is nitrate – it’s water soluble making it easy for the plant to use it, and for it to move with water.
Now let’s dig in!
Nitrogen Fixing: Nitrogen fixing plants, like soybeans, pull nitrogen from the air. Via the steps of the nitrogen cycle, much of it is then stored underground.
- Iowa stores about 10,000 lbs of naturally occurring nitrogen in the soil per acre.
- The bank was built up over millenia.
- This stored nitrogen is not usable until it’s turned into nitrate.
Nitrification: In the spring, microbes get more active because of the warm and damp weather. They turn some of the naturally stored nitrogen into nitrate.
Assimilation: In late spring and summer, Iowa crops use nitrogen – both from the natural bank and from applied sources. But it has to be in a plant available form: nitrate.
- Corn uses about 205 lbs of nitrogen per acre.
- Soybeans take up nitrate from soil organic matter or natural sources.
In the fall, the crop is harvested and what’s left in the field decomposes. Via the nitrogen cycle, some of this returns to the ground’s natural bank.
The graphic below explains these steps. We’ve added numbers to help you read it, but remember, the nitrogen cycle doesn’t always move in a perfect circle!
Why do farmers apply fertilizer if it’s naturally found in the soil?
There are a few reasons.
- Most importantly, farmers have to replenish what they remove in harvest. If they do not, they are unsustainably mining Iowa’s soils, one of our most important natural resources.
- Because much of the naturally occurring nitrate is produced at the wrong time. Microbes are active when it starts warming up. They end up producing nitrate about a month before the growing season.
- The amount of naturally produced nitrate is different every year; it’s highly dependent on temperature, precipitation, soil health, and more.
- Some nitrate is lost to water, unfortunately. What makes nitrate plant available, also makes it vulnerable to water.
The graphic below visualizes the timing of nitrate production and corn/soybean production.
How we can use the cycle to our benefit
All on-farm conservation practices are a way to better utilize the nitrogen cycle. We can delay the natural production of nitrate, divert nitrate from moving, and use the denitrification part of the cycle to remove it from nitrate form.
Cover crops
Cover crops hold nitrogen until a farmer terminates it so it can be released for the main crop. Legume cover crops like clover or hairy vetch are also nitrogen fixing, so they can pull nitrogen from the air. Learn more.
Soil Health
Building organic matter through reduced tillage and cover crops makes the biology of the soil stronger, meaning more efficient conversion of nitrogen to plant available forms. With better soil biology, farmers can apply less fertilizer. Learn more.
drainage
Saturated buffers and bioreactors direct tile water into carbon rich soils and woodchips where microbes can denitrify the water. Learn more.
wetlands
Wetlands are natural nitrogen sponges, using plants and saturated soils to denitrify water after it leaves the field. Learn more.
prairie
Dense root systems can stop nitrogen from moving through the soil profile and build diverse microbial communities. Learn more.
