22 September 2023

Earthworms are changing Canadian forests


Invasive species of earthworm affect microbial composition by altering overall soil conditions, Canadian research explains. Influencing nutrient and carbon cycling

by Matteo Cavallito


The increasingly invasive presence of earthworms is changing soil composition in Canada’s boreal forests. This is suggested by a study of the University of Alberta. The survey, one of the first to highlight the link between the species and the microbial community paints a particularly interesting picture. In fact, the ongoing change could affect ecosystem services carried out by the soil.

Fungi and soil bacteria, in particular, are the main factors affecting plants’ management of the carbon and nutrient cycle. “As earthworm invasion progresses in these boreal systems, these changes in the soil microbial communities may have an impact on forests at a large scale,” said Justine Lejoly, researcher and author of the research, in a statement released by the Canadian university.

The study

“While most research on the topic has been carried out in northern temperate forests” the study states, “little is known about the impact of invasive earthworms on soil microbial communities in hemiboreal and boreal forests, characterized by a slower decay of organic matter (OM).” The influence exerted on the population composition of microorganisms stems from the ability of earthworms to influence the structure, pH and availability of nutrients and organic matter in the soil.

The authors analyzed soil samples from different forest sites in Alberta and Quebec. They compared areas characterized by the presence of invasive earthworms and those that were devoid of them.

In summary, “In mineral soil horizons, earthworm invasion was associated with higher fungal biomass and greater relative abundance of ectomycorrhizal fungi.” At the same time, “Oligotrophic bacteria were less abundant in invaded mineral soils (…) while the opposite was observed for the copiotrophic Bacteroidota.” Finally, “earthworm-invaded mineral soils harboured higher fungal and bacterial species diversity and richness.”

A boost for diversity

The study, then, showed how earthworms were able to influence the presence of microbes in the first layer of mineral soil, that is, below the organic-rich forest soil belt. Which, according to the researcher, would appear surprising. Previous studies, the research notes, had shown by contrast how earthworms contributed to reduced plant diversity.

The increased presence of fungi capable of living symbiotically with trees highlighted by the study could have positive effects. “In theory, having higher diversity means there are more types of bacteria and fungi present, which potentially makes them more resilient to disturbance and better equipped to maintain soil functions in different scenarios,” Lejoly indeed points out. But there is more.

Beware of carbon

In fact, the survey also highlighted a potentially problematic aspect: the presence of earthworms, the research still points out, would promote the spread of bacteria characterized by a strong growth rate. This issue would result in a possible acceleration of nutrient cycling with a consequent release of carbon into the atmosphere.

This mechanism seems to confirm the results of a previous study that earthworm activity could reduce the amount of carbon stored in forest soil by as much as 94 percent. In addition, the potential increase in nutrients caused by earthworm activity could change the composition of plant species in boreal forests. Thus facilitating the growth of invasive plants and increased grass cover.