The role of soil and forests in greenhouse gas sequestration has been studied for a long time. However, forests are also home to invisible organisms that may affect the climate.
“Soil, water and peatlands have been studied in the Biogeochemistry Research Group at the University of Eastern Finland since the mid‑1980s, led by Professor Emeritus Pertti Martikainen. When Martikainen retired in 2016, Professor of Microbial Biogeochemistry Jukka Pumpanen took over as the group’s leader,” says Academy Research Fellow Henri Siljanen from the Department of Environmental and Biological Sciences.
Earlier studies have found that mosses act as filters in soil, and bacteria have been observed to produce greenhouse gases.
“Now, a new generation of researchers is turning their gaze up from the ground, to the tree canopy,” Siljanen notes.
Researchers are currently investigating the ability of lichens in the tree canopy to sequester a potent atmospheric greenhouse gas, nitrous oxide, also known as laughing gas. The role of lichens in greenhouse gas sequestration has not been studied before, as previous research has focused on soil and plants.
Our research also focuses on microbes living in the trees and soil, and their symbiotic relationship with plants.
Henri Siljanen
Academy Research Fellow
Microbes living in spruce branches can also consume nitrous oxide. A study on this topic, authored by Postdoctoral Researcher Dhiraj Paul, was recently accepted for publication in ISME Communication.
Microbes can support tree growth by producing growth hormones while the tree, in turn, provides sugars for the microbes and captures methane from the atmosphere. The rate of interaction between plants and microbes varies depending on whether the forest is young or old.
“The role of microbes living in lichens in greenhouse gas sequestration has not been studied at all before.”
“Processes taking place in the tree canopy can be compared with those in the soil, making this line of research significant for the development of climate models. Climate models, in turn, support decision‑making when, for example, assessing forest biodiversity.”
Atmospheric gases can be collected and analysed in real time with portable measuring devices
A forest more than 160 years old, located on the border of the nature reserve at the top of Puijo hill, was chosen as the study site for the ongoing project.
“This site is well suited for research because it has many different lichen species. It is also close to the university,” Siljanen explains.
“Puijo is home to the SMEAR research station, which studies atmospheric particles and gases. Through SMEAR, we are involved in the Atmosphere and Climate Competence Centre, ACCC, funded by the Research Council of Finland, but the station has not been able to study this specific area directly.”
Portable measuring devices can be used to determine the concentrations of atmospheric water vapour, methane and nitrous oxide in real time. Instead of just releasing emissions, the tree canopy may also capture atmospheric methane and nitrous oxide.
Lichen and microbe samples are also collected from the area. They are preserved in liquid nitrogen on site, which brings microbial and enzymatic activity to a halt. The samples are taken to the university laboratory, where their water content is measured.
The organisms’ DNA and RNA are isolated from the samples using targeted metagenomic sequencing, and the results are compared against a comprehensive database. This enables rapid and accurate identification of microbial species and diversity.
Vincenzo Abagnale from Italy is one of three doctoral researchers involved in the project. He is currently working in the laboratory with lichen and tree samples collected from the Puijo forest and from a site in the Sodankylä region, analysing how atmospheric gas measurements can be linked to microbial activity in these ecosystems.
A preprint of his research has been published, and the study is currently under review in Environmental Microbiology.
I became interested in this topic when I realised how many different organisms, fungi and bacteria live in the Puijo forest.
Vincenzo Abagnale
Doctoral Researcher
“It was easy to obtain a sampling permit for this site near the nature reserve from the local Centre for Economic Development, Transport and the Environment,” Abagnale notes.
“My research focuses on understanding whether microscopic organisms living in lichens can actively influence the regulation of greenhouse gases and how these small‑scale biological processes might affect the climate system more broadly.”
The release of nitrous oxide is a result of human activity – but also of natural processes
Samples for the project have previously been collected from Estonia, the Czech Republic and northern Lapland in Finland.
The highest nitrous oxide concentrations were measured on birch stems in the village of Agali in Tartu, Estonia, where the behaviour of young birch trees was studied. A report related to this research is under review in Microbial Ecology.
The study site houses a drained peatland where the water level fluctuates significantly, partly due to the activity of beavers. Last year, extremely heavy rainfall caused a rapid rise in the water level.
“In the future, it would be interesting to continue research in the Puijo nature reserve. For instance, we could study how soil recovers from human activity by closing off a small area, approximately 100 square metres in size, from visitors. This would allow us to examine the recovery of microbial processes and the ability of the soil to capture atmospheric greenhouse gases,” Siljanen says.
Next year, the ability of soil, spruce needles and lichens in the tree canopy in Puijo to produce and consume greenhouse gases will be measured. Greenhouse gas fluxes will be measured at soil level throughout the year as well as higher up in the canopy using a crane. The Sodankylä measurement site will serve as a point of comparison.
“This will give us a year‑round comparison data of greenhouse gas metabolism. With these data we can improve the sensitivity of climate models regarding plants and lichens. We are also studying the activity and diversity of microbial genes related to the consumption and production of greenhouse gases.”
Research articles:
V Abagnale, C Palacin-Lizarbe, D Paul, J Kerttula, HMP Siljanen. 2023. Activity and abundance of nitrous oxide consuming bacteria in Platismatia glauca cryptogamic lichen in boreal Finnish spruce forest. bioRxiv, 2023.05. 09.539975
Dhiraj Paul, Inga Paasisalo, Anuliina Putkinen, Christopher M Jones, Lukas Kohl, Sara Hallin, Mari Pihlatie, Henri MP Siljanen. (2025) Microorganisms in the phyllosphere of Norway spruce controlling nitrous oxide dynamics. ISME Comm. DOI: 10.1093/ismeco/ycaf196.
Krishnapriya Thiyagarasaiyar, Dhiraj Paul, Johanna Kerttula et al. Genetic potential for N₂O metabolism in tree tissues: Insights into nitrogen cycling gene abundance and nosZ diversity across trees, 23 December 2025, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-8339723/v1]
