Teaching the nitrogen cycle is one of my favourite lessons. There's so much to connect to: history and everyday life. But, students also need a clear model to understand what's going on. Without both, the nitrogen cycle can seem quite abstract and meaningless to many students.
Below I give you two stock and flow models. One for 16-18 and one for 14-18. I describe how I built the model in a lesson with my 16-18 year olds. See this post if you want to learn more about how I build them step by step.
Age of students I have used this model with: 16–18
What the model shows
I designed this model to be spatially intuitive for students: where nitrogen can be physically found at any one time. Notice the line labelled “surface”. This splits the nitrogen cycle into three main domains:
the atmosphere
the biomass of food webs, and,
the soil
There's another distinction I wanted to make clear when designing the model.
The inflows and outflows of atmospheric nitrogen, entering and exiting the ecosystem.
The flows that cycle between the soil and the food web, within the ecosystem.
How I’ve used this model (with 16–18 year-olds).
I began by drawing the stocks without any flows.
And I made explicit the distinction between nitrogen in the atmosphere, in biomass, and in the soil. I named the stocks, and then began telling students the problem ecosystems face. The most important flow to focus on to begin with was the outflow of nitrogen from the atmosphere and into the biosphere.
Nitrogen is an important element of organic molecules, amino acids and nucleotides, for example. Organisms need to accumulate it, but most is stuck in the atmosphere. The trouble is that diatomic nitrogen is incredibly stable and it's difficult to force it to react and form part of another molecule. The other issue I told them, is that only certain species can fix nitrogen, and those are bacterial.
This is a bottleneck for ecosystems, as the rate of flow of nitrogen fixation determines the input of nitrogen to the whole ecosystem.
I then added the flow from atmospheric nitrogen to the stock of ammonia. And, I drew the flow of plants “assimilating” the ammonia.
I told students how the price of food correlates with nitrogen content. We discussed some examples, donuts begin cheap (fats and carbs), meat being expensive. In fact, Raubenheimer and Simpson (2020) suggest that animals will continue to eat until they accumulate their daily nitrogen intake, as an essential variable of homeostasis. Which could go some way to explaining why humans can overeat easily on foods of mainly carbs and fats.
I asked the students if they knew of any plant food that is high in protein. Some of them mentioned legumes and we discussed examples, such as lentils, chickpeas, peas, peanuts, soy beans, broad beans, etc. I showed them an image of root nodules of legumes and told the students of the symbiotic relationship between the plants and Rhizobium bacteria.
I then drew the other flow of nitrogen fixation and added the variables that affect it. This completed the two flows of fixing nitrogen.
It was time to put these flows into historical context. Increasing human population sizes required increased flows of nitrogen. Every ancient agricultural civilisations had at least one legume in its repertoire.
For example, the Aztecs had climbing beans, the Middle East had lentils and chickpeas, the East had mung beans and soybeans, and Africa had the black-eyed pea. Legumes, of course, form an essential part of vegetarian and vegan diets. I also told the students about cover crops, such as legumes, which fix nitrogen while a crop field is not in use.
Next I discussed the processes of nitrification that ultimately convert ammonia into nitrates. I explained to students that plants would assimilate ammonia, but in high concentrations it is toxic and that makes nitrate the preferred form of nitrogen. This adds to the narrative that rates of nitrogen flow into the food web are limited by the action of bacteria.
I added the flows of nitrifying and assimilating (of nitrate), and then discussed the rates.
I contrasted fixation with nitrification as fixation requires low oxygen concentrations, as exemplified by the production of leghaemoglobin in legumes to protect their symbionts. Nitrification, however, requires oxygen and can be limited by low concentrations, which can happen, for example, in waterlogged fields. A well-aerated soil, therefore, can increase nitrification rates.
Here I paused to return to the historical context. Industrial nations that wanted to feed and increase their population, therefore needed to think about the problem of nitrate concentrations in soils. Gorman (2013) tells this story well. Hundreds of years ago, in England it was the law that the manure found in stables was property of the monarch, and there was a team of people in charge of collecting it.
I then added the ammonifying flow that returns nitrogen from the food web back to the ammonia stock. This represents the completion of the cycling of nitrogen within ecosystems.
I discussed with students that these flows are typically much faster than the inputs and outputs to ecosystems. Therefore, ecosystems act as accumulators of nitrogen. However, collecting manure wasn’t enough. As populations increased further, inputs to the system had to increase.
Islands in the pacific off Chile and Peru, which had accumulated nitrogen-rich bird poo for milenia, suddenly became hot property—enhancing tensions between those countries. Europe and America imported huge quantities.
The Atacama desert became a lucrative resource. It’s the driest desert in the world. Over millions of years ocean water sprayed across the desert to never be washed away by rain. There was an input but no output, so huge deposits of minerals accumulated. These deposits were exported to feed populations abroad.
But, as demand outstripped supply, the Haber process was invented to industrially fix nitrogen and produce artificial fertiliser (and munitions). Humans have now duplicated the nitrogen flows across the globe.
To finish the model, I added the denitrification flow and the variables that affect it.
Not only can waterlogging reduce nitrate production (decreased inflow), an absence of oxygen increases the rate of denitrifying nitrates (increased outflow). I gave the example of swamps, which retain hardly any nitrogen. This is the reason carnivorous plants evolved to obtain nitrogen from insects (while still acting as autotrophs).
It was now time to have students grapple with potential variation in the system. To do this I asked them all the things they could think of doing if they were a farmer. The answers I expected and discussed were avoiding the waterlogging of fields to decrease rates of denitrification, spreading manure over fields to increase the rate of decomposition, growing legume cover crops (like clover), and applying nitrates directly to the soil.
I finished the lesson by showing and discussing with the students a graph of the human population and an estimate of what it would be had the Haber process not been invented.
Below I'll leave you the simplified version I've used with my 14-16 year olds.
My books: Difference Maker | Biology Made Real, or my other posts.
Download the first chapters of each book for free here.
Age of students I have used this model with: 14–16
References
Gorman, H. 2013. The Story of N: A Social History of the Nitrogen Cycle and the Challenge of Sustainability. USA: Rutgers University Press.
Raubenheimer, D., & Simpson, S. 2020. Eat Like the Animals: What Nature Teaches Us About the Science of Healthy Eating. USA: Houghton Mifflin Harcourt.
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