New review publication: Do soil microbes ultimately contribute to climate change, or mitigate it?

The multitudes of microbes living within soil are massive players in global carbon levels, as collectively they are responsible for the cycling of huge amounts of organic carbon. The soil microbiome can both produce and consume greenhouses gases, which makes it an extremely important factor in the context of our changing climate and climate feedback. However, as Microbiome Insights Scientific Advisory Board member Janet Jansson explains in a new Nature Reviews article, we still don’t know whether soil microbes will ultimately act as a sink or a source of atmospheric carbon. 

Integrating soil microbial ecology into landscape-scale climate models remains a challenge due to a lack of knowledge surrounding how soil carbon and nitrogen pools will change in response to various climate scenarios. There is also the matter of grappling with the various microbial responses to climate changes in different ecosystems around the world. In order to begin a discussion on the topic, Jansson and her co-author presented an overview of the current research describing responses of soil microbiomes to several anticipated climate-related changes, including: elevated atmospheric CO2 levels, elevated temperature, increased drought, increased precipitation and flooding, and increased frequency of fires. Each of these factors influence microbe growth, plant growth, and decomposition as well as microbial activity—all of which play a role in determining whether soil is increasing or decreasing greenhouse gas levels. 

Several unique climate-sensitive soil ecosystems are also identified in the review, each with the potential to react differently to the anticipated changes listed above. The Arctic is the most sensitive and is also changing the most quickly. Recent estimates of permafrost thawing suggest that anywhere from 5-10% of the carbon stored there is susceptible to microbial decomposition as the ground warms and the microbes activate. This represents a carbon source that could contribute to further warming and positive climate feedback.

Forests and grasslands represent approximately 30% and 26% of global land surface, respectively. Both are important carbon sinks at the moment, but under scenarios predicting increased drought, fire and in the grasslands periodic flooding, this could change. In scenarios or regions with increased precipitation, wetlands stand to change from sinks to sources too as increased moisture and anaerobic conditions favour CH4 production. Dryland areas on the other hand are predicted to increase by 11-23% by the end of the century. This change in water availability could have profound impacts on soil microbiomes but it is currently difficult to generalize microbial responses to drought across soils and regions. 

While there is uncertainty in how each of these regions and soil types might react to climate change, there are proposals for manipulating soil microbiomes to mitigate climate changes. Tinkering with soil’s ability to sequester carbon, via the numerous biochemical reactions that naturally take carbon via plants and other autotrophs and deposit it into the soil ecosystem, is one proposition. This would require careful collaboration between plant breeders and soil microbiologists in order to identify the best pairings of beneficial microorganisms with plant genotypes. Another idea is treating microorganisms as beneficial plant inoculants. Plant growth promoting bacteria and fungi might be used in this scenario to counteract the negative consequences of drought. 

According to the authors, while these ideas may help mitigate some negative effects of climate change, “we do not by any means propose that this will be sufficient to counterbalance the loss of soil and the generation of greenhouse gases that is already occurring.” Rather, they advocate for an integrated approach of best practices for soil management that supports plant production, the storing and supply of clean water, maintenance of biodiversity, carbon sequestering, and resilience against climate change. To do so, they argue that fine-scale data from microbiome studies be combined with landscape-scale resolution of ecosystem services and climate models. The authors add, “most importantly, we need political will and a global dedicated effort to curb the emissions of greenhouse gases that are at the root of climate change.” The role of soil microbiomes in providing plant nutrients and their importance in ensuring healthy soil for future generations cannot be taken for granted. 

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