Trees are not only excellent at absorbing carbon dioxide from the air, but they play a “surprising” role in retaining and transforming other pollutants such as nitrogenous compounds, according to the results of a new international study that demonstrates the crucial role that trees play. Microorganisms present on tree leaves remove nitrogenous compounds from the air.
The authors of the study have discovered the presence of nitrifying microbes in the leaves of trees that interact with nitrogenous compounds present in the atmosphere (ammonia and ammonium, highly polluting) and transform them into nitrates. This transformation process is called “nitrification”. The scientific team behind the study is led by Rossella Guerrieri, associate researcher at Creaf (Marie Sklodowska Curie grant) and associate professor at the Department of Sciences and Technologies at the University of Bologna.
The research has been carried out in European forests in slightly polluted areas (in the Scandinavian countries), moderately polluted areas (in the Mediterranean area) and highly polluted areas (in Central Europe). To find these microbes, the research group has used the innovative Generation Sequence Analysis (NGS) technique, which consists of massive sequencing of environmental DNA to detect the presence of new organisms. These analyzes have been carried out in the laboratories of the IBB-Parc de Recerca UAB. To evaluate the nitrifying properties, they have analyzed rainwater samples inside and outside the forest and have even analyzed leaves from trees of different heights, which have been collected by professional climbers in all countries.
Rossella Guerrieri recalls that “Previous research had already shown that the tree crowns, thanks to the cuticle of the leaves and their morphological structure (for example, the presence of hairs), served as a passive filter.” However, “they do not I was convinced that it was a simple and passive mechanism. Aware of the great biodiversity found in forests, and particularly in the treetops, I began to wonder if, on the other hand, this process did not depend on the intervention of microorganisms that live in the leaves,” indicates the researcher from the Creaf.
The nitrification process is a key part of the nitrogen cycle that, until now, was thought to only occur underground. It is an essential process because it increases the availability of nitrates in the soil, a form of nitrogen that trees need for growth and are able to absorb. Thus, when the air is slightly contaminated with nitrogenous compounds, thanks to nitrification in the leaves, the concentration of nitrates in the soil increases, fertilizing the soil and providing more nutrients to the forest.
Now, in areas where the air is highly polluted, especially where a lot of ammonia reaches (due to the aerial dispersion of fertilizers from agriculture or other human activities), the activity of the microbes on the leaves skyrockets. This causes nitrates to be produced in large quantities, which leak into the aquifers, compromising the quality of groundwater, or even disperse back into the atmosphere. This “over-fertilization” harms forests and the quality of groundwater.
This discovery, published in Nature GeoScience, reveals the great importance of such small organisms. Without them, the tree crowns would not be able to transform these atmospheric pollutants and all the nitrogen would be returned to the atmosphere, increasing the greenhouse gas balance.
The study is signed by 24 scientists from six countries, including researchers from centers in Catalonia Maurizio Mencuccini, Stefania Mattana, Josep Peñuelas, Joan Cáliz, Anna Barceló and Emilio O. Casamayor.