Roots and fungi important for long-term forest carbon storage

New research published today in the international scientific journal Science indicates that plant roots and root-associated fungi play a much more important role for long-term carbon storage in forest soils than previously believed.

– The knowledge on carbon dioxide uptake and carbon storage in forest soils is important if we are to prevent forests from becoming a source of carbon dioxide in the future, says researcher Alf Ekblad of Örebro University and one of the members of the Swedish research team behind the study.

– The aim is to facilitate long-term measures and to manage the forest ecosystems in a way that guarantees their ability to take up atmospheric carbon dioxide. It is therefore important to understand how, when and why the forest ecosystems take up and store carbon, says Alf Ekblad, professor of biology at Örebro University.

The research project, led by researchers at the Swedish University of Agricultural Sciences, has overturned the prevailing idea that the principal part of the carbon stored in forest soils comes from aboveground plant litter, or the top layer of the forest soils, consisting of for instance needles and wood.

Roots and fungi the source

Our findings show that the carbon enters the soil from beneath rather than falling from above. The carbon stored in forest soils originates, to a great extent, from roots and fungi, known as mycorrhizal fungi, living in symbiosis with the plant roots. Trees gain carbon dioxide via photosynthesis, upon which the carbon is accumulated in high-energy carbohydrates, says Alf Ekblad.

Some of the carbohydrates are transported down to the roots and to the associated fungi, where the carbohydrates provide energy and building blocks for new roots and filaments, known as hyphae. Previously, ecologists have believed that there is a fast-paced and almost complete breakdown of hyphae and roots when they die and that the carbon contained in them is released into the atmosphere in the form of carbon dioxide.

– What we found in our study is however that the breakdown of roots and fungal hyphae is significantly slower than previously believed and that breakdown residues of roots and hyphae constitute a considerable proportion of the large carbon volumes stored in the soil, says Alf Ekblad.

The researchers estimate that 50-70 per cent of the stored carbon in forest soils have been accumulated via plant roots and that the older the forest is, the greater the significance of the root’s carbon supply. The discovery is based on a study of the carbon exchange on 30 forested islands in the Lakes Uddjaure and Hornavan in northern Sweden.

Slowing or increasing the greenhouse effect

– On the larger islands, there have been forest fires over the last century, while the forests on the smaller islands have grown without being disturbed by fires for almost 5000 years. We were able to see that it took a long time for the carbon storage to recover after a fire and that the humus layers in soils where carbon was stored was a lot thinner in forests affected by fires, says Alf Ekblad.

It is therefore the interplay of several different processes that affects the carbon cycle in forests. The balance between carbon storage in forest soils and its release into the atmosphere via breakdown processes is decisive of whether forests will help slow or increase the greenhouse effect.

– There are large volumes of carbon deposited in forest soils – the carbon stored in the ground exceeds by three times the volume of carbon dioxide in the atmosphere. The soil respires approximately ten times more carbon dioxide than what is released through society’s combustion of fossil fuels. In other words, it is incredibly important to design new models that make it possible to predict how various environmental changes and forestry measures affect carbon storage in forest soils, says Alf Ekblad.

Text: Linda Harradine