In a corner of Staffordshire, England, a woodland of oak trees that are nearly 180 years old has been breathing air loaded with extra carbon dioxide for six years straight. Scientists at the University of Birmingham wanted to know what would happen to these ancient oaks when they lived in the kind of air we might see in the 2050s and beyond. Their findings, published in the journal Science Advances, show something unexpected: the trees got better at pulling nitrogen from the soil.
Nitrogen is a vital nutrient for trees, just like vitamins are for people. Scientists already knew that extra CO₂ can help trees grow faster, a phenomenon called the "carbon fertilization effect." But researchers had not fully understood how trees would find enough nitrogen to support that growth in a warmer, more CO₂-rich world.
The team discovered that the oaks did not go hungry. Instead, they teamed up with tiny soil microbes to "mine" nitrogen from organic matter in the ground. The trees release a sugary substance through their roots called root exudates, which acts like an energy drink for soil bacteria and fungi. These microbes then break down organic matter and release nitrogen that would otherwise stay locked away in the soil.
The study found that key nitrogen-cycling processes in the forest soil increased by about 30 percent over the six-year period. The researchers called this a "faster but tighter" nitrogen cycle. More nitrogen becomes available for tree roots, while processes that normally let nitrogen escape from the soil as gas are greatly reduced.
"Trees secure nitrogen from soils by releasing an easily decomposable cocktail of organic carbon through their roots," said Dr. Manon Rumeau, the study's lead author who now works at Université de Pau et des Pays de l'Adour in France. "Surprisingly, the high-CO₂ ecosystem holds on to its nitrogen more effectively and conservatively."
This news is hopeful for forests and the climate. Trees help combat climate change by absorbing CO₂ from the air, so if they can keep growing healthily in a high-CO₂ world, that is good news. But the researchers urge caution. The nitrogen supply that helps trees thrive is not infinite. The soil reserves that microbes tap into can eventually run low. What is more, pollution that once added extra nitrogen to soils from rain is declining in the UK and elsewhere, which could create future nitrogen shortages.
The study comes from the BIFoR FACE facility, one of the world's leading research sites for understanding how forests respond to higher CO₂ levels. It is the 41st study from this team, which continues to explore the complex relationship between forests and a changing climate.