Unlocking hidden value of soil nitrogen through organic carbon
Crop growers can rely on pre-existing soil nitrogen to provide a portion of their crop nutrient needs. With enough nitrogen in the soil, they can ideally save money on expensive nitrogen inputs.
Soil nitrogen availability is not a straightforward thing, though. In “Re-thinking soil nitrogen availability to crops in the context of soil organic carbon” — the most widely read paper in the Canadian Journal of Soil Science in 2025 — retired Agriculture and Agri-Food Canada soil scientist Keith Reid argues a more cyclical approach is required to get the most benefit from soil nitrogen and to prevent nitrogen loss.
WHY IT MATTERS: Understanding the natural timing of nitrogen mineralization allows you to cut fertilizer costs by accurately accounting for the nutrients already in your soil. By managing the cycle between carbon and nitrogen, you protect your long-term soil health while preventing expensive inputs from washing away or gassing off.
Mineralization
Nitrogen exists in soil as either organic nitrogen or mineral nitrogen (nitrate or ammonium). Only mineralized nitrogen is accessible by the plant. Mineralization occurs as soil organisms use the carbon compounds in organic matter as food and release ammonium as a byproduct.
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Because carbon and nitrogen are linked in soil organic matter (carbon-to-nitrogen ratios typically range from 10:1 to 12:1), the release of nitrogen will be proportional to the amount of carbon consumed. This process is fastest when the soil biology faces warm, moist and well-aerated conditions, but it is also limited by the amount of readily degraded organic matter.
Understanding the timing of nitrogen release
In most temperate climates, including Ontario, the greatest net mineralization is in the early part of the growing season. Crops then absorb the mineralized nitrogen during the summer, combine it with the carbon fixed during photosynthesis, and return it to the ground as root exudates and crop residues.
Nitrogen mineralization occurring in the early growing season averages 60 to 130 kilograms of nitrogen per hectare — about half of the total nitrogen uptake — for most growing regions in Canada. The actual rate, however, is influenced by soil type, climate, soil management and cropping system. The additional nitrogen supporting harvested yield must come from biological nitrogen fixation, manure or fertilizer.
The hidden cost of degrading organic matter
“If you’ve got a balanced system — where nothing is happening to change the amount of organic matter available for decomposition — you have return of nitrogen in the residues that roughly matches the mineralization. As a long-term average, it’s going to balance out. What you take out, you put back,” says Reid.
“If you have conditions where you built up soil organic matter, or changed a farming system so organic matter is declining, that means there’s an increase, or push, of nitrogen from soil organic matter into plant available forms.
“If we’re in a position of degrading organic matter, we can grow crops with less fertilizer at the cost of degrading organic matter. But, if that organic matter isn’t releasing nitrogen at the time crop needs it, or if we don’t account for the extra nitrogen from the soil, it’s lost to environment. The point is not to give the final answers, but to nudge the direction of research. Because if we don’t account for the timing of nitrogen mineralization, it will be lost at end of season.”
Nitrogen fixation as a cycle
The overall purpose of authoring the paper, says Reid, was to argue for a move away from a linear, extractive way of thinking about available soil nitrogen, and “towards circularity by explicitly including the annual cycles of carbon and nitrogen from soil to vegetation, and back again.”
“Ultimately, this should improve the descriptions of nitrogen flows through agricultural ecosystems and allow better management of nitrogen, and minimization of nitrogen losses to air and water,” he says.
“Over my career, the holy grail was predicting how much to feed the crop to get everything it needs without over-applying. We have to really think about that connection with soil organic matter, balanced in relation to how much nitrogen is available to crops every year. It’s been there in the background. It’s never been explicitly considered.”
Key Takeaways
- Financial Savings: Growers can lower their fertilizer bills by accurately accounting for the nitrogen already present in their soil.
- The Carbon Link: Nitrogen release is driven by soil organisms consuming carbon, meaning soil organic matter is the engine behind nutrient availability.
- Predictable Timing: In temperate climates, the highest rate of nitrogen mineralization typically occurs early in the growing season.
- Balance is Key: Maintaining a circular system ensures that the nitrogen returned through crop residues matches what is taken out by the harvest.
- Risk of Loss: Failing to account for the timing of soil nitrogen release leads to wasted nutrients that are lost to the environment rather than used by the crop.
Source: Farmtario.com