The connection between microbes and carbon cycle is more complicated than expected

In some circumstances, a new study explains, microbial respiration, with the resulting release of carbon, can increase even when biomass production remains constant. A factor that must be included in climate projection models

by Matteo Cavallito

Soil microbes are known to play a key role in regulating the carbon cycle. However, the dynamics that influence the respiration of microorganisms on the one hand and the soil’s ability to store carbon on the other are not yet fully understood. As a result, some of the most established hypotheses explaining these mechanisms may now be called into question by a new interpretation.

This has been proposed in recent weeks by researchers at the Institute of Earth Environmental Sciences of the Chinese Academy of Sciences who, in a study published in Science Advances, offer a new perspective on climate change projection models. This overturns previous hypotheses on the relationship between respiration and carbon sequestration.

A link between efficiency, storage, and respiration

Influence of microorganisms on carbon cycle in soil can be understood by considering two essential values: the heterotrophic respiration rate (Rh) and the microbial carbon use efficiency (CUE). The first value measures the amount of CO2 released into the atmosphere when microbes degrade plant material. The second measures the efficiency with which microorganisms convert absorbed organic carbon into their own biomass instead of releasing it in the form of carbon dioxide. These aspects, of course, are closely related.

To put it simply, when use efficiency is high, more carbon is fixed in biomass and, subsequently, in the soil. When it’s low, in contrast, most part of the element is released through microbes respiration.

Scientists, a statement points out, have traditionally assumed that CUE decreases linearly with increasing Rh in all ecosystems. The latest study, however, has shown that the relationship between the two variables is not actually uniform. Instead, it varies non-linearly with ecosystem productivity.

A decoupling between efficiency and respiration

“When the total microbial carbon uptake remains constant, higher assimilation for growth combined with lower heterotrophic respiration indicates more efficient biomass production, which enhances soil organic carbon retention” the study explains. Under changing environmental conditions, however, microbial growth efficiency is more stable than respiration levels. Furthermore, additional assessments suggest that warming accelerates respiration itself but does not have a clear or consistent effect on increasing microorganism biomass.

“These differential responses,” the research continues, “imply a decoupling between microbial carbon assimilation efficiency, commonly described as use efficiency and heterotrophic respiration under specific environmental constraints.”

This decoupling “challenges the long-held assumption that CUE uniformly declines with increasing Rh.” Moreover “It may also represent a key source of uncertainty in linking CUE to soil C storage and dynamics.” The study made it possible for the first time to empirically test this phenomenon by highlighting the mechanisms that regulate it.

Initial empirical confirmation from the study

The research team used a total of 1,094 paired observations from a range of global natural ecosystems, which revealed distinct patterns in different zones. In low-productivity regions, such as arid and cold areas, for example, CUE decreased as Rh increased, in line with the traditional hypothesis.

In high-productivity areas, such as tropical and temperate ecosystems, on the contrary, carbon use efficiency decoupled from respiration rate once the latter exceeded a critical threshold.

According to the authors, in particular, under conditions of limited availability of nutrients such as nitrogen and phosphorus, microorganisms can sustain growth by investing in the production of energy-intensive enzymes and efficient nutrient recycling. These dynamics affect the carbon cycle.

Stabilization at 27%

Specifically, when the heterotrophic respiration rate exceeds approximately 340 grams of carbon per square meter per year, the research explains, the efficiency of microbial carbon use stops decreasing according to the traditional pattern and stabilizes around an average value of approximately 0.27. In other words, this means that further increases in respiration intensity are not associated with a significant reduction in the proportion of assimilated organic carbon converted into new microbial biomass, which remains on average around 27% of the total, while the remaining 73% is mainly returned to the atmosphere in the form of CO₂.

The mechanism also explains why vegetation greening can accelerate carbon loss in the soil, while nutrient input can improve carbon sequestration.

The study’s conclusions have important implications, suggesting the need to update predictive models to improve the accuracy of climate projections. The research’s findings, in fact, “imply a limited potential for natural ecosystems to serve as effective soil carbon sinks under global change (e.g., vegetation greening).” Therefore, they also emphasize “the importance of incorporating microbial metabolic adaptability into future mechanistic assessments of soil carbon dynamics.”