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Trees not only absorb CO2, they also clean the air of nitrogenous compounds

The study was carried out in beech and pine forests in Europe. Image: Galdric Mossoll.
The study was carried out in beech and pine forests in Europe. Image: Galdric Mossoll.

A new international study demonstrates the crucial role of micro-organisms on tree leaves in removing nitrogenous compounds from the air. The study shows that trees are not only excellent at absorbing carbon dioxide, but also play a surprising role in retaining and transforming other pollutants such as nitrogenous compounds. 

The scientific team, led by Rossella Guerrieri, a Marie Sklodowska Curie researcher at CREAF while this research was being carried out, has discovered the presence of nitrifying microbes in tree leaves that interact with nitrogenous compounds present in the atmosphere (ammonia and ammonium, which are highly polluting) and transform them into nitrates. This transformation process is called nitrification. 

The research has been carried out in European forests in lightly polluted (Scandinavian countries), moderately polluted (Mediterranean area) and highly polluted (Central Europe) areas. To find these microbes, the research group used the innovative Generation Sequence Analysis (NGS) technique, which consists of massive sequencing of environmental DNA to detect the presence of new organisms. These analyses were carried out in the laboratories of IBB-Parc de Recerca UAB and the computational analysis of these data was performed at CEAB  with the participation of Joan Cáliz and Emilo O. Casamayor. To evaluate the nitrifying properties they have analysed rainwater samples inside and outside the forest and even analysed leaves from trees of different heights, collected by professional climbers in all countries.

“Previous research had already shown that tree canopies, thanks to the cuticle of the leaves and their morphological structure (e.g. the presence of hairs), had a passive filtering function. But she was not convinced that this was a simple, passive mechanism. Aware of the great biodiversity found in forests, and in particular in tree canopies, I began to wonder whether, on the other hand, this process did not depend on the intervention of micro-organisms living on the leaves”.

ROSSELLA GUERRIERI, lead author of the article and professor at the University of Bologna. 

Nitrification, a delicate balance 

The process of nitrification is a key part of the nitrogen cycle which, until now, was thought to occur only underground. It is an essential process because it increases the availability of nitrates in the soil, a form of nitrogen that trees need for their growth and are able to absorb. Thus, when the air is low in nitrogenous compounds, nitrification in the leaves increases the concentration of nitrates in the soil, fertilising the soil and providing more nutrients to the forest. 

However, in areas where the air is highly polluted, especially where there is a lot of ammonia (from the aerial dispersion of fertilisers from agriculture or other human activities), the activity of leaf microbes is triggered. This causes nitrates to be produced in large quantities, which leach into aquifers, compromising groundwater quality, or even re-disperse back into the atmosphere. This over-fertilisation damages forests and groundwater quality. 

This discovery, published in Nature GeoScience, reveals the great importance of such tiny organisms. Without them, tree canopies would not be able to transform these atmospheric pollutants and all the nitrogen would be returned to the atmosphere, increasing the greenhouse gas balance. 

This study was led by Associate Professor R. Guerrieri from the Department of Science and Technology of the University of Bologna, with the collaboration of an international team. Researchers from CREAF such as Maurizio Mencuccini, Stefania Mattana and Josep Peñuelas and from CEAB have also participated. 

Reference article: 

Guerrieri, R., Cáliz, J., Mattana, S. et al. Substantial contribution of tree canopy nitrifiers to nitrogen fluxes in European forests. Nat. Geosci. 17, 130–136 (2024). https://doi.org/10.1038/s41561-023-01364-3 

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