Vinay Kumar
In a woodland in Staffordshire, rings of pipes have been pumping extra carbon dioxide over 180-year-old oak trees every growing season since 2017, giving scientists a rare, decade-long window into how mature forests might behave in a warmer, more carbon-rich future. The facility, known as BIFoR FACE and run by the University of Birmingham, raises CO2 levels inside its treatment rings to around 573 parts per million, close to what the whole atmosphere is projected to reach by the 2050s, compared to roughly 424 parts per million in untreated control rings nearby. After six years of this treatment, researchers found the fumigated oaks had grown noticeably more wood than their neighbours, but that raised an old and stubborn question in forest science: where was the extra nitrogen needed to build all that wood actually coming from?
Why nitrogen has long been the sticking point for this idea
Trees cannot manufacture nitrogen themselves. They pull it from the soil, where most of it stays locked away inside dead leaves, roots and older organic matter until microbes slowly break it down and release it in a usable form. This has been the central objection to the idea that a more carbon-rich atmosphere will automatically make forests grow faster and store more carbon, since previous experiments have shown that growth boosts from extra CO2 can fade once the soil simply runs short of nitrogen to support it. A well-known 2010 study at an Oak Ridge sweetgum plantation documented exactly this kind of fade-out, and the same researcher, Richard Norby, is a co-author on this newer oak study.
What the Birmingham team actually measured in the soil
According to thestudy published in Science Advances titled Faster-and-tighter nitrogen cycle supports mature forest productivity under elevated CO2, led by Manon Rumeau while she was at the University of Birmingham, researchers sampled soil beneath the CO2-treated oaks every month from February to November 2022, sealing samples in bags, burying them under the leaf litter and digging them up 28 days later to measure how much usable nitrogen had been released. Over the full year, soil under the high CO2 oaks released about 29 per cent more usable nitrogen than soil in the control rings, and the trees took up correspondingly more of it, an increase that closely matched what the extra soil nitrogen supply could provide.According to the University of Birmingham, this matters because in the untreated control rings, tree nitrogen uptake outpaced what the soil was releasing, meaning those trees were gradually drawing down a nitrogen reserve the forest cannot refill indefinitely. In the high CO2 rings, by contrast, soil supply and tree demand for nitrogen were almost perfectly matched.
How trees appear to be unlocking extra nitrogen from the soil
The mechanism behind this shift centres on tree roots. Rumeau explained that trees secure additional nitrogen from soil by releasing an easily broken-down mixture of organic carbon compounds through their roots, sometimes described as a kind of natural energy drink for soil microbes, which stimulates those microbes to break down organic matter and release nitrogen that would otherwise stay locked away.Consistent with this explanation, soil cores from the high CO2 rings contained 39 per cent more fine roots and released 26 percent more carbon dioxide themselves, a sign of significantly more root and microbial activity underground. Microbes in these rings also freed nitrogen from organic matter roughly 30 per cent faster, though the researchers noted this particular figure carries more uncertainty, since the experiment only used three replicate rings per treatment.
Why researchers expected a leakier system and found the opposite
One open question going into the study was whether unlocking more nitrogen from the soil would also mean losing more of it. Once nitrogen becomes available, some of it typically converts into nitrate, which can wash away with rainfall or be converted by microbes into gas and lost to the atmosphere. A forest breaking down more organic matter might reasonably be expected to lose more nitrogen this way. Instead, Rumeau said the team found what she described as a faster but tighter nitrogen cycle rather than a faster but leakier one, largely because trees appeared to be taking up the extra nitrogen before it had much chance to be lost.Nitrate levels in the high CO2 soil never actually rose, despite the faster overall nitrogen cycling, and instruments running at the site between 2020 and 2022 recorded 74 per cent less nitrous oxide, a potent greenhouse gas, being released under elevated CO2, though that particular finding has so far only been presented at a scientific meeting and has not yet been published in a peer-reviewed journal.
Why this finding comes with real limits
The soil beneath this particular forest holds a finite nitrogen reserve, and while researchers calculate the forest would take decades to exhaust it at the current pace, the portion microbes can easily access could run low considerably sooner. Other potential nitrogen sources have not stepped in to fill any gap either, with microbes capable of pulling nitrogen directly from the air contributing less than half a pound per acre regardless of CO2 levels, while nitrogen arriving through air pollution, once a meaningful extra input, is itself declining across the UK and other regions as air quality improves.Researchers were also careful to note that this pattern will not necessarily hold everywhere. A similar CO2 enrichment experiment run on eucalypt woodland in Australia showed no comparable growth speed up under elevated CO2, since phosphorus rather than nitrogen was the nutrient limiting growth at that particular site, underscoring that different forests may respond quite differently depending on which nutrient constrains them most.
What researchers still need to figure out
One major uncertainty remains unresolved. Since the same soil microbes that release nitrogen also release carbon dioxide in the process, researchers still cannot say for certain whether the additional carbon entering the soil through tree roots ultimately outweighs the carbon microbes are releasing back into the atmosphere as they work. Rumeau has identified accurately measuring all carbon inputs and outputs in the soil as the next essential step, since only that fuller accounting can determine whether soil beneath these fumigated oaks is ultimately gaining carbon or losing it, a distinction that matters enormously for how much confidence forests deserve as a long-term, nature-based climate solution.
