How the invisible will save what’s visible

Use of microbial inoculants in agriculture to tackle climate change

Challenges of Climate Change

We often hear about climate change and global warming, but they refer to non-overlapping concepts. Climate change includes global and local phenomena including the alteration of the water cycle, ice melting, increase in sea levels and their acidification, increase in the frequency of extreme phenomena such as heat waves, strong rainfall, cyclones and droughts. Instead, when we mention global warming we are referring to the increase in average annual temperatures around the globe.

Both climate change and global warming are consequences of human activity. The reason is the steep increase in greenhouse gas emissions mainly due to industrial activities and energy production, even if a significant share is imputable to agriculture (13.5%) (IPCC, 2007).

Food is an essential element of our lives and its production must be adapted to the ongoing climate change. In fact, the agricultural sector is faced with numerous challenges in order to be able to increase or even just maintain productivity, above all is the need to reduce its environmental impact.

Fig. 1 Greenhouse gas emissions in CO equivalents due to different sectors in 2004. (IPCC, 2007, modified)

The world population is in fact increasing: the UN has estimated that we will be 9.8 billion in 2050 and the necessity will be to increase agricultural production accordingly. The problem is that environmental phenomena such as floods, droughts and temperature increases limit the growth and development of plants.

A stressed plant will have limited growth with a negative impact on productivity. To support global productivity, therefore, there are two possibilities: increase the land destined for crops, removing it from native ecosystems (in a nutshell, deforestation), or increase the productivity of the crops themselves while keeping the dedicated surface unchanged. How are we going to increase the food we produce without deforesting further?

A Miniature Ecosystem: the Rhizosphere

“No man is an island.” And neither is any plant.

The poet John Donne would have liked to know about the microbiota, the set of microorganisms that live on or in a host organism and that constitute a real community, with different roles and more or less close interactions.

The microbiota in question is the one found in contact with the roots and in the soil around them: the rhizosphere. Here we find pathogens, commensals, saprophytes and organisms beneficial to the plant.

The latter help the plant to grow, to perform many of its physiological functions such as the acquisition of nitrogen and stress resistance and are called PGPR (Plant Growth Promoting Rhizobacteria). The importance of these organisms is such that they are increasingly used as biofertilizers as an alternative to traditional practices that contribute to depleting the soil, often involving the use of polluting and/or unsustainable chemical compounds.

The usefulness of soil microorganisms, however, does not stop at improving productivity: these inoculants, if managed correctly, can help us deal with the factors that put the planet at risk.

Fig. 2 General schematization of the components and processes occurring in the rhizosphere. (White R.A. Ill et al., 2017)

Addressing climate Change

Microbial inoculants can help us in several ways depending on the specific function they perform in the community. We can distinguish their contribution into three groups.

First of all, it is possible to recognize the organisms that contribute to the reduction of greenhouse gas emissions such as N2O-reducing bacteria which reduce the emission of nitrous oxide, but also methanotrophs which reduce the emission of methane. It is also possible to identify the bacteria that increase the efficiency of CO2 sequestration, both by the plant as in the case of PGPRs – which help the plant to grow in biomass – and by the soil – thanks to microorganisms CO2-fixing microorganisms.

Finally, it is important to underline the importance of microorganisms that help the plant in the management of drought and salinity stress. Here we recognize EPS-like compounds producers that improve the soil’s water-holding capacity; PGPRs which stimulate root development so that it can reach water deeper in soil and PGPRs which stimulate antioxidant activity, improving the plant’s stress resistance.

But is a cocktail of “good” microorganisms enough? Actually it is not that simple. The rhizosphere presents a well-oiled community, where each organism performs a more or less important function and modifying it through microbial inoculants can have effects that we do not expect (even counterproductive). For this reason it is necessary to study secondary succession, in the specific climate and soil context of each environment in order to find effective and targeted solutions.

Protect the Biodiversity of the Terrestrial Microbiome

To mitigate climate change, efforts are required in all sectors, not just agriculture.

It should be emphasized that the best remedy to tackle climate change is to eliminate net emissions from the industrial, energy and transport sectors through targeted international regulations and reforestation.

The soil microbiota can also help us in the latter case. The reforestation process is slow and delicate and more and more attention is paid to the biodiversity of the microbiome, responsible for more than half of plant growth in the various ecosystems (Averill et al., 2022).

As in the use of microbial inoculums in cultivated soils, great attention must be paid to the biodiversity of the microbial community also in the case of reforestation, in order not to make the same mistake of the past by cultivating monocultures that are not resilient to pathogens and constantly changing climatic conditions. A biodiverse microbiome represents an important resource for our planet.

BIBLIOGRAPHY

Arias P. A. et al., 2021: Technical Summary. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte V., et al.]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 33−144.

Averill, C., Anthony, M. A., Baldrian, P., Finkbeiner, F., Van den Hoogen, J., Kiers, T., … & Crowther, T. W. (2022). Defending Earth’s terrestrial microbiome. Nature Microbiology, 7(11), 1717-1725.

Backer R, Rokem JS, Ilangumaran G, Lamont J, Praslickova D, Ricci E, Subramanian S and Smith DL (2018) Plant Growth-Promoting Rhizobacteria: Context, Mechanisms of Action, and Roadmap to Commercialization of Biostimulants for Sustainable Agriculture. Front. Plant Sci. 9:1473.

IPCC, 2007: Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, Pachauri, R.K and Reisinger, A. (eds.)]. IPCC, Geneva, Switzerland, 104 pp.

Liu, X., Le Roux, X., & Salles, J. F. (2022). The legacy of microbial inoculants in agroecosystems and potential for tackling climate change challenges. Iscience, 103821.

White III, R. A., Rivas-Ubach, A., Borkum, M. I., Köberl, M., Bilbao, A., Colby, S. M., … & Jansson, C. (2017). The state of rhizospheric science in the era of multi-omics: A practical guide to omics technologies. Rhizosphere, 3, 212-221.