Forest trees growing in air containing higher levels of carbon dioxide (CO2) radically change the way they 'cycle' nitrogen, a key plant nutrient, with implications for forests as a nature-based climate solution.
In a new study in Science Advances, researchers at the University of Birmingham's world leading Free Air CO₂ Enrichment (BIFoR FACE) facility in Staffordshire, UK examine how a ~180-year-old oak woodland responded after six years of exposure to the CO₂ levels expected by the 2050s and beyond.
They found that key processes managing nitrogen in the forest increased significantly under the increased CO2 conditions. Nitrogen is a critical nutrient for living things, including trees, and while elevated CO2 levels lead to a 'carbon fertilization' effect, previous studies have not unpacked the way trees and their microbial partners manage nitrogen in such future scenarios.
In the study, the oak woodland's soil nitrogen cycling processes, responsible for meeting tree nutritional needs, increased by about 30%. Rather than remaining 'hungry' for more nitrogen, the trees maintain healthy nutrition by working with microbes to 'mine' it from organic matter in the soil.
Dr Manon Rumeau, who completed the study at the University of Birmingham and is now Post-Doctoral Researcher at the Université de Pau et des Pays de l'Adour is the study's lead author. Dr Rumeau said: "Trees secure nitrogen from soils by releasing an easily decomposable cocktail of organic carbon through their roots. This natural 'energy drink', known as root exudates, stimulates soil microbes to break down organic matter and release nitrogen that would otherwise remain locked away.
"Releasing the nitrogen in soil from soil organic matter can make it vulnerable to loss, seeping away as a gas or liquid solution. Surprisingly, the study finds that the high-CO2 ecosystem holds on to its nitrogen more effectively and conservatively, creating a 'faster but tighter' nitrogen cycle."
Senior author Professor Sami Ullah from the BIFoR team at the University of Birmingham said: "The 'faster but tighter' nitrogen cycle under elevated CO₂ comes from shifts in how soil microbes behave. These tiny organisms produce more nitrogen that tree roots can easily use, while other microbial processes, that turn soil nitrogen into forms prone to be lost from the ecosystems, are greatly reduced."
Increased tree growth with sustained nutrition seen in the study may play a beneficial effect given the role trees are playing in combating climate change, but researchers warn that this boost in nitrogen supply may not last forever. The cycling process observed in the study relies on nitrogen stored in soil organic matter, which is not an unlimited store, and the team identify that the processes identified could eventually deplete these reserves.
Professor Ullah added: "Declining pollutant nitrogen deposition in the UK and elsewhere—once an additional source of nutrients—may further increase the risk of future nitrogen limitation."
The study is the latest of more than 40 BIFoR FACE studies focused on understanding the various ways in which elevated CO2 levels affect forests ecology and the potential role of forests in climate change mitigation.
The Birmingham Institute of Forest Research (BIFoR)'s FACE experiment celebrates its 10th anniversary in 2026. The BIFoR FACE facility is supported by the JABBS foundation, the University of Birmingham, the John Horseman Trust, the Ecological Continuity Trust, the UK Natural Environmental Research Council, and private donations.
