Local Nitrogen Cycle Overview

Here is the fourth entry in a series exploring new and orphaned knowledge of nitrogen cycling to update the view of the forest gardener – there are many articles to be written so stay tuned. 

In this article, we’ll take a step back and look at the bigger picture of the local nitrogen cycle, and see broadly how the various inputs and outputs interact. This article will also be written a bit less in-depth than the previous ones, because we’re not covering new ground (where we require extraordinary evidence for extraordinary claims).

Why do we care about nitrogen at all? Nitrogen is one of the main limiting nutrients for plant growth in a wide variety of ecosystems around the world. It can be very soluble (when present as nitrate) and can easily leach too deeply to be captured by plant roots. It can’t be mined from the subsoil by deep-rooted plants (unlike phosphorus). Any major ecosystem disturbance (deforestation, excessive fertilization) can see lots of soil nitrogen escape via runoff, leaching or denitrification processes.

Luckily, there is a network of processes that brings nitrogen both into, and out of the ecosystem – this is called the nitrogen cycle. This cycle operates on many different scales. At the global scale, nitrogen is present in the atmosphere (in practically inexhaustible amounts – 79%), which is fixed by nitrogen-fixing bacteria into a solid-form, taken up by plants, and then as the plants die, the nitrogen is decomposed from organic compounds, and eventually denitrified and re-enters the atmosphere.
The nitrogen cycle also operates at a much smaller scale, cycling within an ecosystem whereby nitrogen travels from the plants, back to the soil, and then back to the plants – we can think of this as the local nitrogen cycle.

The Local Nitrogen Cycle

At the level of scale of a forest garden, the amount of available nitrogen is dominated by the rates of nitrogen transfer between plants and soil. This is really a special case of what’s called a nutrient cycle [Nutrient cycle].

To gain a deeper understanding of the local nitrogen cycle, we need to know what are the pieces that make it up, and what are the connections between these pieces. These can be thought of as ‘pools’ and ‘flows’ (flows can be further broken down into external inputs, external outputs and internal flows). A nitrogen pool is a place where nitrogen has gathered, and a flow is a channel that moves nitrogen from one pool to another. Click on the graphic below to read more about nitrogen pools and flows that are important within the scale of a forest garden.

We’re going to keep returning to the local nitrogen cycle many times as it helps explains so many other concepts, so it’s worth becoming familiar with the different pieces and to start to view it as a system made up of parts that connect together. It’s also important to note that all of these pools and flows vary depending on practically every variable (e.g. temperature, water availability, day length, soil profile, pH, micronutrient balances, C:N ratio, existing plant communities, etc.) imaginable. The cycle shown above also doesn’t include all flows – some of which may be quite important (e.g. some plants have the ability to uptake organic nitrogen directly, small amounts of ammonium can be directly volatilized into the atmosphere, etc.).

Hopefully the above diagram gives you a clearer understanding of how the various flows and pools of nitrogen connect together to make up the local nitrogen cycle. What we want to understand next is – how do these flows and pools behave as a whole system? What happens if we reduce, of increase the flow of nitrogen from one pool to another? What happens if we’re manually intervening in the nitrogen flows (i.e. heavy pruning, chopping and dropping)? Where are we losing our nitrogen to (and hence less yields)?

We can’t really answer these questions exactly. But we can get a pretty accurate big picture idea of how things work in order to make better decisions. We’ve put together a simplified model simulation of the local nitrogen cycle of a forest garden. It only contains the largest input (nitrogen fixation), the largest output (denitrification – which is true under many conditions – but if it’s leaching it doesn’t really change that it’s a loss of nitrate), and the most important feedback loop (ammonification -> nitrification -> assimilation -> decomposition). The model makes basic assumptions about what is the limiting factor for the flow rate (e.g. that sunlight/warmth is the limiting factor for nitrogen fixation, the amount of plants is the limiting factor for how much nitrates they can absorb). This is enough complexity to draw some new conclusions from without getting stuck into details.

Have a play with some of the basic presets and see what happens to the nitrogen. You can speed up the simulation to make it faster, and pause it to get a closer look. Once you have a feel for how things work, open up the Variables panel and run a simulation of Young forest with a low denitrification rate, versus high denitrification rate. Some of the questions you might like to explore are:

• What happens to bare subsoil when the nitrogen fixing rate is high, versus low?
• What happens in a young forest when the denitrification rate is high, versus low?
• What happens to a young forest if you keep mulching all the plants?

Hopefully there are a few things you start to see emerging as patterns from this rough model. Some of the things I see are:

• Once the amount of growing plants exceeds a certain amount, excess nitrate barely leaves the system – before that point, a lot is getting lost. It really is important for us as gardeners to gain sufficient plant cover, as soon as possible and to avoid bare soil.
• The different pools work somewhat separately – e.g. nitrate can be produced when plants don’t need it.
• Isn’t there some way we can slow down denitrification – it’s taking away all the nitrogen. Well… this will be an important future topic. 🙂

Well that’s about it for this article. I hope you learned something, I definitely did writing it! If you found anything interesting, or have any questions about nitrogen cycling or the model simulation – please leave in the comments

References

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https://www.hill-laboratories.com/assets/Documents/Technical-Notes/Agriculture/UNDERSTANDING-SOIL-NITROGEN-TESTS.pdf