Key Takeaways
- A Counter-Intuitive Finding: Adding biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More (with fertilizer) to this boreal soil increased emissions of the potent greenhouse gas nitrous oxide (N2O) by 1.56 times compared to fertilizer alone.
- Hydrochar Performed Better: A different type of char, hydrochar, restricted N2O emissions. Without fertilizer, it even showed the potential to absorb N2O from the air, turning the soil into a gas “sink.”
- A Hit to Crop Yield: Biochar also decreased the yield of timothy grass, a valuable forage crop, likely by locking away nitrogen fertilizer.
- Bad for Long-Term Carbon: Biochar reduced the amount of stable, long-lasting carbon in the soil, while hydrochar helped stabilize it.
- Not All “Char” Is Equal: This study shows that the “char” type and soil type matter immensely. What works in one place can have negative effects in another.
Amending soil with “char”— is widely studied as a way to fight climate change. The general idea is that it locks carbon in the soil for centuries and can help reduce greenhouse gas emissions from agriculture. But a new study shows this is not a one-size-fits-all solution, and in some cases, it can even make things worse. The research, led by Hem Raj Bhattarai and colleagues in the journal Biochar, reveals the complex and sometimes contradictory effects of two different char types, biochar and hydrochar, on a boreal grassland soil. The scientists hypothesized that both chars would reduce emissions and boost plant growth, but their findings tell a much more complicated story.
The most significant and unexpected result was for nitrous oxide (N2O), a greenhouse gas 300 times more potent than carbon dioxide. When biochar was added with nitrogen fertilizer—a common farming practice—it did not decrease emissions. Instead, it significantly increased N2O emissions by 1.56-fold compared to the fertilized control soil. The study suggests this happened because the biochar, which has a very high surface area, created a perfect home for N2O-producing microbes. It boosted a process called nitrification, leading directly to more N2O being released.
In stark contrast, the hydrochar told a different story. When added with fertilizer, hydrochar restricted N2O emissions relative to the biochar. It appeared to support a different set of microbes, ones that can “breathe” N2O and turn it into harmless N2 gas. Even more striking, the hydrochar added without fertilizer (H) had the lowest N2O emissions of all. It even showed several “uptake events,” where the soil actually pulled N2O out of the atmosphere, hinting at a potential to turn the soil into an N2O sink. For the other major greenhouse gases, methane (CH4) and carbon dioxide (CO2), the chars had no significant effect on cumulative emissions.
Beyond emissions, the chars had a surprising impact on the plants. While the total biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More yield did not change, the biochar amendment negatively affected a specific crop. It significantly reduced the yield of timothy grass compared to the control. The researchers believe the biochar’s high surface area may have “sponged up” the added nitrogen fertilizer, immobilizing it and making it unavailable for the timothy grass to use. This finding challenges the idea that biochar is a universal booster for crop yields.
Finally, the study examined the very reason for using char: storing carbon. While both chars increased the “particulate” carbon in the soil (which was mostly just the char itself), the real test is whether they help create stable carbon. This stable form, called Mineral-Associated Organic Carbon (MAOC), is created by microbes and can last for centuries. Here again, biochar failed. The biochar amendments significantly reduced the amount of stable MAOC compared to the soil’s starting point. Hydrochar, however, supported a significantly larger microbial community. These microbes, in turn, helped to form and stabilize the long-lasting MAOC.
This research is a critical reminder that “biochar” is not one thing. The biochar used here (made from birch at 600°C) performed poorly in this specific soil, increasing a potent greenhouse gas, harming a key crop, and hindering long-term carbon storage. The hydrochar (made from birch bark at 220°C) showed promise on all three fronts. The study concludes that the specific properties of the char, like its surface area, and its interaction with the local soil and plants are what determine if it will be a climate solution or, unexpectedly, part of the problem.
Source: Bhattarai, H. R., Honkanen, E., Ruhanen, H., Soinnie, H., Gil, J., Saghir, S., Lappalainen, R., & Shurpali, N. J. (2025). Effects of biochar, hydrochar and nitrogen fertilization on greenhouse gas fluxes, soil organic carbon pools, and biomass yield of a boreal legume grassland. Biochar, 7(114).
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