Alien species and pollution threaten soil microbiota biodiversity

We have already had the opportunity to discuss the effects that some factors, such as climate change and human activities (including agricultural practices, such as tillage, trampling, etc.) have on the composition of soil microbial communities. But that is not all. The introduction of alien species, as well as pollution in various forms, also represent fearsome threats to soil biodiversity.

In natural ecosystems, different species evolve together, generating a situation in which none of them, individually, can dominate the others and creating a dynamic balance, which is the basis of the longevity and resilience of the ecosystems themselves. This is true until external factors intervene to break these balances. Among these, we can mention invasive species coming from environments geographically and genetically distant from the one the settle in. They notoriously represent a risk to the biodiversity of the ecosystems in which they land, including that of the soil. Several more or less recent studies have focused on the negative effects that some alien plant species, invading new habitats, can have on the soil microbiota, highlighting a crucial aspect: once established, alien species modify the microbial communities by emitting specific root exudates and favouring the multiplication of microorganisms capable of sustaining their nutrition. In this, alien species are sometimes favoured by a more remarkable ability to develop root biomass compared than native species. The disturbance generated by this new situation can lead to a drastic decrease in the presence of microorganisms potentially beneficial for the health of the soil and for the development of nutrient cycles.

To give an example, Prosopis juliflora, a tree species of Latin American origin, tolerant to high temperatures and salinity stress, has invaded semi-arid areas of Africa, producing organic substances that are toxic to native plants (the phenomena of allelopathy) and profoundly modifying the pattern of nitrogen-fixing bacterial species present in the soil.

Even organisms whose presence in soils we are used to considering as absolutely positive can actually become a threat to biodiversity if introduced into environments in which they had never been found. This is the case of earthworms, on whose ability to reduce the quantity and variety of native soil invertebrates, as well as alter the microbial communities in the soils they settle in, several studies have been conducted.

Various pollutants can have direct and indirect effects on microbial balances. Remaining in the agricultural sector, pollutants can be represented both by improperly used agrochemicals and fertilizers, which can also give rise to salinization phenomena, and by other substances that reach cultivated soils for reasons other than agricultural activity, such as wastewater from industrial activities and acid rain.

Heavy metals such as copper, iron and zinc are essential for the physiological functions of soil microorganisms. Still, when their concentrations exceed certain limits, they become toxic, causing a decrease in microbial biomass, numerically speaking, and favouring microbial populations resistant to heavy metals, qualitatively speaking.

Acid rain is a mixture of wet and dry deposits of material from the atmosphere containing high amounts of sulfuric and nitric acid. Once they reach the soil, they lower its pH, drastically decreasing it and thus altering the balance of the microbial communities present.

The attention paid by the world of research to soil microbiota has gradually grown over the last 25 years, generating new fundamental knowledge and practical applications for agriculture, including inoculated fertilizers.

This knowledge is progressively enriched and allows us to reach a more profound understanding of the links between the performance of cultivated plants and the microorganisms of the soil and subsoil. And, if certain disturbances of the microbial balance of our soils are difficult to avoid, the choice of how to practise agriculture is in our hands.

Aiming for agricultural practices that respect the vitality of the soil and, where possible,  its restoration means having a far-sighted vision.