21 August 2023

Soil microbes will release 40 percent more CO2 by 2100


Climate change is increasing the respiration of soil microbes and related CO2 emissions. A phenomenon, says a Swiss study, set to accelerate between now and the end of the century

by Matteo Cavallito


CO2 emissions from soil microbes could rise 40 percent by the end of the century. This increase is driven by climate change, which, in turn, is also influenced by the effects of this phenomenon – so-called heterotrophic respiration – in an obvious vicious circle. This is suggested by Swiss research published in the journal Nature Communications.

“The projected rise in microbial CO2 emissions will further contribute to the aggravation of global warming, emphasising the urgent need to get more accurate estimates of the heterotrophic respiration rates,” said Alon Nissan, a researcher at the ETH Zurich Institute of Environmental Engineering and lead author of the study, in an article published by the same Swiss institute.

Moisture and temperature affect the respiration of microbes

“Within the terrestrial carbon cycle, soil respiration, the emission of CO2 through root (autotrophic) and microbial (heterotrophic) respiration is the largest carbon efflux into the atmosphere,” the research explains. “Therefore, reliable quantification of how soil respiration may be affected by climate change is critical for predicting future atmospheric CO2 concentrations.”

Heterotrophic respiration, the researchers continue, is primarily linked to the interaction of two elements: soil temperature and moisture.

While increasing temperature increases respiration, moisture affects the phenomenon in a complex way. When soil water content is low, microbial activity decreases. However, the same thing happens when moisture is excessive and thus limits the supply of oxygen from the atmosphere to the soil. This depends on the lower conductivity of water in comparison with air. Consequently, the researchers explain, optimal conditions for the phenomenon occur when the soil water concentration reaches an intermediate level.

The new model

The two variables are obviously at the basis of the model developed by the researchers. “We first quantified soil heterotrophic respiration starting from the pore (micro) scale, integrating parameters that are biological, chemical and physical,” the authors explain. Next, the scientists made larger-scale predictions of these respiration fluxes.

Next, “considering soil temperature and moisture variation in space and time, we show that the model yields estimates of recent trends in soil heterotrophic respiration rates at the global scale that are in line with observations.” Finally, they explain, “we use this mechanistic model to simulate how soil heterotrophic respiration might change under the worst-case future climate scenario.”

a) Proiezioni future della respirazione eterotrofa del suolo per diverse regioni geografiche. I punti rappresentano le stime basate sulla media di dieci modelli climatici. b) Mappa superficiale della respirazione eterotrofa del suolo in funzione dell'umidità del terreno e della temperatura superficiale (T). Il punto più grande per ogni zona geografica corrisponde al suo stato attuale nel 2021, con la dimensione del simbolo che rappresenta il contributo relativo alla respirazione eterotrofa del suolo globale (Tropicale 67%, Subtropicale 23%, Temperata 10% e Artica 0,1%). Fonte: Nissan, A., Alcolombri, U., Peleg, N. et al. Global warming accelerates soil heterotrophic respiration. Nat Commun 14, 3452 (2023). https://doi.org/10.1038/s41467-023-38981-w https://www.nature.com/articles/s41467-023-38981-w#MOESM3 Attribution 4.0 International (CC BY 4.0)

a Future projections of soil heterotrophic respiration for different geographical regions. Points represent estimates based on the ensemble average of ten climate models. b Surface map of soil heterotrophic respiration as a function of the soil moisture and surface temperature (T). The large point for each geographical zone corresponds to its current state in 2021, with symbol size representing the relative contribution to global soil HR (Tropical 67%, Subtropical 23%, Temperate 10% and Arctic 0.1%). Source: Nissan, A., Alcolombri, U., Peleg, N. et al., “Global warming accelerates soil heterotrophic respiration”. Nat Commun 14, 3452 (2023). Attribution 4.0 International (CC BY 4.0)

Respiration will increase by 40 percent

The study showed how CO2 emissions from microbes are expected to accelerate on a global scale by the end of the century. “Estimates from the model show that heterotrophic respiration has been increasing since the 1980s at a rate of about 2% per decade globally,” the study states. “Using future projections of surface temperature and soil moisture, the model predicts a global increase of about 40% in heterotrophic respiration by the end of the century under the worst-case emission scenario.”

The increase in emissions varies across climate zones. Under the worst-case scenario, polar regions – which are characterized by ideal moisture conditions for heterotrophic respiration – will experience a 119 percent increase by 2100.

This figure far exceeds the increases that will be experienced by the tropics (38 percent), subtropics (40 percent) and temperate zones (48 percent). The weight of different zones on the global microbial respiration phenomenon, however, is definitely variable. Of the total, 67% of microbe respiration comes from the tropics, 23% from the subtropics, 10% from the temperate zones and only 0.1% from the Arctic or polar regions.