Biochar Can Curb or Boost Greenhouse Gas Depending on Soil

Two adjacent fields in Jiangxi Province, China, grow different crops — one peanuts, one rice. The soils beneath them look similar enough: both acidic, both fertilised at similar rates, both collected from the same shallow depth. Mix biochar into either of them and you’d expect a broadly similar result. You’d be wrong.

A new study has found that biochar, the carbon-rich material made by heating crop waste in low-oxygen conditions and increasingly promoted as a climate-friendly soil amendment, produces sharply opposite effects depending on whether the ground beneath you is waterlogged or not. In dry upland soil, it suppressed nitrous oxide emissions more effectively than lime — the standard treatment for acidic farmland. In flooded paddy soil, it sent them soaring.

The results, published in Nitrogen Cycling, come at an awkward moment for biochar’s growing reputation. Advocates have long pointed to evidence that mixing charred biomass into agricultural soils can sequester carbon, improve soil structure, and cut emissions of nitrous oxide — a greenhouse gas roughly 265 times more potent than carbon dioxide over a century, and the most significant ozone-depleting substance currently being released into the stratosphere. That story, it turns out, is only half the picture.

The team, led by Jinbo Zhang at Hainan University and Christoph Müller at Justus Liebig University in Giessen, Germany, used an elegant approach to figure out exactly what was happening. Rather than simply measuring how much nitrous oxide the soils released, they used isotope analysis to fingerprint where the gas was coming from — distinguishing between bacterial denitrification, fungal denitrification, nitrifier denitrification, nitrification, and a fifth pathway called heterotrophic nitrification, which other methods typically can’t separate out.

In the dry upland soil, biochar did something rather specific to the resident microbial community. It reduced populations of Chaetomium — a fungal genus that produces nitrous oxide at prodigious rates, somewhere between 100 and 207 nanomoles per millilitre per day in pure culture — while simultaneously boosting expression of the nosZII gene, which encodes an enzyme that converts nitrous oxide into harmless nitrogen gas. The effect was to squeeze the gas from both ends: less production, more destruction. “Our findings suggest that biochar can help redirect microbial processes so that more nitrogen ends up as stable nitrogen gas rather than as a climate damaging emission,” the authors wrote.

The paddy soil told a different story. There, all five pathways lit up simultaneously after biochar addition. The thin layer of water sitting above the soil creates a slightly anaerobic environment that allows nitrification to proceed, generating nitrate substrate while preventing the complete reduction of that nitrate to nitrogen gas. Biochar amplified every step in this already-active system. At the highest application rate tested (5 per cent by weight), cumulative nitrous oxide emissions increased more than 14-fold compared to untreated soil.

What makes this particularly tricky is that the two fields sit side by side. Same geology, same climate, same fertilisation regime. The decisive variable is water.

The distinction matters because the global push to promote biochar as a climate solution has often glossed over land-use type. Large-scale meta-analyses tend to average across soil conditions, potentially masking cases where the amendment causes harm. “This tells us that biochar is not a one size fits all solution,” the team noted. “Its climate benefits depend strongly on where and how it is applied.”

The isotope approach the researchers used — running three separate signatures through a Bayesian statistical model called FRAME — also revealed something about the standard comparison for managing soil acidity. Lime, in the form of quicklime, is routinely spread on acid soils to raise pH. In the upland soil, it helped somewhat. But biochar outperformed it on nitrous oxide reduction by a considerable margin, probably because it does more than just change pH: it adds dissolved organic carbon, which in turn favours the microbes responsible for converting nitrous oxide to nitrogen.

Whether any of this translates to real fields is still an open question. The study was conducted in laboratory incubation conditions over just four days. Fields are messier — variable moisture, temperature fluctuations, different crop roots interacting with the soil microbial community in ways that are hard to replicate in a flask. The researchers are calling for longer-term field studies before anything resembling large-scale deployment is contemplated.

Still, the mechanistic detail is genuinely useful. Knowing that Chaetomium abundance and nosZII gene expression are the key levers in upland soils gives future researchers specific targets to probe. “By identifying the microbial pathways behind these emissions, we can begin to design smarter soil management practices,” the team concluded. The prospect, eventually, is biochar prescribed not just by soil type but by the microbial community lurking within it.

Study link: https://www.maxapress.com/article/doi/10.48130/nc-0025-0021