Forest trees and microbes choreograph their hunt for a ‘balanced diet’ under elevated CO2

Oak trees change their fine roots and ‘energise’ soil microbes by supplying them with a cocktail of small organic compounds to supplement the trees’ supply of essential nutrients when exposed to higher levels of carbon dioxide.

In a study published in the Proceeding of the National Academy of Sciences (PNAS), researchers including from the University of Exeter discovered that trees growing in a CO₂-rich atmosphere tactically choreograph in-soil trading of carbon for nutrients through “do it yourself” and “outsourcing” strategies.

The study, carried out at the University of Birmingham’s BIFoR-FACE institute, a large outdoor forest research facility, involved burying Perspex-sided root boxes in the forest, which gave scientists access to the soil and roots below giant 180-year-old English oak trees that are growing in what is expected to be a mid-21^st-century atmosphere – ie an atmosphere that contains over a third more CO₂.

The trees demonstrated their ability to respond to the change of atmosphere by adjusting multiple carbon investment strategies for sourcing soil nutrients.

The measurements, made after elevating CO₂ in the forest for five years, showed that the oak trees increased their fine root branching systems by 73%, helping the trees explore more of the soil at all times of the year in a “do it yourself strategy” for finding and taking up nutrients. “Outsourcing” strategies, which involve trade partnerships with the soil microbial community, showed distinct seasonal patterns.

There was a 63% increase in the release (‘exudation’) of a cocktail of small organic molecules early in spring and autumn, which ‘prime’ soil microbes to release nutrients locked up in soils, so meeting tree nutrient demands.

There was also a 17% increase in the amount of symbiotic root-associated fungi in autumn.

More remarkable still, trees under elevated CO₂ not only invested more carbon in their in-soil trading, but they also altered the cocktail of chemicals released as root exudate, providing yet one more ‘outsourcing’ strategy to gain nutrients and sustain growth.

Dr Michaela Reay, lead author from the University of Bristol, who carried out the research while at the University of Birmingham, said: “Roots do not simply take up nutrients and water from soils but rather exhibit smart and dynamic choreography, which involves highly specialized tradeoffs with soil microbes via varied nutrient exploration strategies throughout the year.  

“These findings indicate that trees are more agile in optimizing growth than previously thought and will continue to be agile under higher CO₂ atmospheres as long as soil nutrient supplies are sustained.”

Senior author Professor Sami Ullah from the University of Birmingham said: “These mechanistic insights into how trees growing in future atmospheres acquire soil nutrients will have significant policy implications with direct relevance for climate mitigation initiatives such as the Paris Climate Agreement, the EU Green Deal, and the UK and EU net zero ambitions by 2050.”

“There is growing importance of forests to act as carbon sinks to contribute to offset essential CO₂ emissions. Forest growth not only relies on CO₂, but also requires an ample supply of soil nutrients to keep capturing and sequestering atmospheric CO₂.”

Professor Iain Hartley from Exeter University said: “Trees in this forest increasing their growth under elevated CO2 was slightly unexpected, and suggests mature temperate forests could play a key role in the fight against climate change. It remains important to investigate whether the strategies the oak trees are using will continue to allow for greater access to soil nutrients.”

Since the nutrients available in soil are not unlimited, the question now remains whether the extra carbon investment of trees in nutrient acquisition may eventually exhaust soil nutrient stocks.

Forest responses to enforced changes, such as higher atmospheric CO₂, can take many years to reach a new balance point; the ongoing research at BIFoR-FACE will assess whether nutrient supplies can continue to meet tree nutrient demands.

“Elevated CO2 alters relative belowground carbon investment for nutrient acquisition in a mature temperate forest,” is published in PNAS.