Hidden soil chemistry helps rainforests hold their nutrients despite storms

Biological and chemical processes in rainforests are essential for the way these ecosystems hold onto nutrients, yet many of the details are still unclear. One important question is what happens to nitrite, formed through the transformation of nitrate, a very reactive form of nitrogen that usually does not last long in soils. It has often been assumed that only microbes control what happens to nitrite. However, new results show that chemical reactions on their own—without the help of living organisms—can also lock nitrite into the soil by attaching it to organic materials, which is the natural matter formed from decayed plants and animals. This hidden process may be one of the keys to why rainforest soils remain fertile despite heavy rainfall.

Professor Francisco Matus from Universidad de La Frontera led a team of scientists to examined this question closely. Their findings have been published in the journals Forests and Journal of Environmental Management, scientific publications dedicated to studies of forest ecosystems.

Using a safe chemical label called isotopic labelling, which allows scientists to follow how elements move, the team studied soils from both volcanic areas and non-volcanic landscapes in rainforests. When they removed oxygen from the environment, creating an oxygen-free or anoxic condition, they observed that nitrate, and especially nitrite added quickly begin to disappear. Within a very short time, nearly half of it was gone, and over several days, much of it had been turned into a form of organic nitrogen, first dissolved in the soil water and then incorporated into solid organic matter becoming part of a stable pool that is less prone to immediate loss but crucial for long-term fertility. In fact, close to three-quarters of the nitrite was held in organic matter, regardless of the origin of the soil.

What stood out most was that the amount of nitrate and nitrite added to the soil mattered more than the kind of rock the soil originally came from. Put simply, how much nitrite entered the system determined how much was kept in organic form. As Professor Matus explained: “These results show that a large share of the added nitrate transformed to nitrite can be abiotically incorporated into the DON and SON of unpolluted old-growth temperate rainforest, whether developed on volcanic or non-volcanic soils”. Here DON and SON refers to dissolved and solid organic nitrogen, meaning nitrogen that is mixed into the water and attached to components  within the soil organic form.

Another key insight is that rainforests usually lose nitrogen not in its free mineral form, but bound together with organic materials. This new evidence suggests that soils can trap nitrite converted from nitrate through natural chemical reactions, reducing the loss of nutrients from the ecosystem. According to Professor Matus, this resilience shows that the amount of organic matter available in the soil is more important for stabilizing nitrogen than the type of soil derived from the parent rock. 

Beyond the rainforests themselves, these discoveries add important knowledge to how we understand the global nitrogen cycle, which is the movement of nitrogen between the air, soil, plants, and water. In wet forests, nutrients are often washed away faster than in drier regions, but this chemical pathway helps explain why so much nitrogen is not lost in its mineral form. The researchers also highlight the “Ferrous Wheel Hypothesis,” an idea that iron in the soil helps recycle nitrogen into organic forms, giving rainforests another way to retain their nutrients even under heavy rainfall.

Overall, Professor Matus’s studies highlights that chemical reactions, not just microbes, are critical in keeping rainforests healthy and fertile. As he summarized: “This reveals the natural resilience of unpolluted temperate rainforests to nitrogen  loss, with implications for long-term ecosystem stability and nutrient cycling”.

Journal Reference

Matus F., Dyckmans J., Stock S.C., Merino C., Dippold M.A., Kuzyakov Y. “Abiotic Nitrite Incorporation into Organic Matter in Volcanic and Non-Volcanic Soil Within Rainforest Ecosystems.” Forests, 2025; 16(930). DOI: https://doi.org/10.3390/f16060930

Matus, F., Álvarez, E., Godoy, R., Iturriaga-Vásquez, P., Farías-Cea, A., Parada, J., Merino, C., Nájera, F., Mendoza, D., Jofré, I., Knicker, H., Dippold, M. A., Kuzyakov, Y., Schluesselburg, L., Boy, J. “Ferrous wheel hypothesis II: Abiotic incorporation of mineral nitrogen into organic pools in volcanic soils of temperate forest ecosystems.” Journal of Environmental Management, 2025 391 (126311). DOI: https://doi.org/10.1016/j.jenvman.2025.126311

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