Microbial secretions emerge as major contributors to soil carbon

Research reported by nature.com has identified a second major microbial route by which carbon enters and remains in soil. Microbes do not contribute only through the remains of cells that have died; substances they release while alive may supply comparable amounts of carbon and may be incorporated into mineral-associated organic carbon at similar or higher rates.

The finding comes from a study of cropland soils across China, where researchers developed an isotope-tracing technique to measure the production of extracellular polymeric substances. These materials are released outside microbial cells and form mixtures that can include sugars, proteins and other biological polymers. Their consistency is often described as mucus-like, but their significance in long-term soil carbon storage has been less clearly measured than that of dead-cell residues.

A missing part of the microbial carbon cycle

Soil organic carbon is built partly from plant material, but microbes help determine what happens to that material after it enters the ground. They process plant-derived carbon and convert some of it into microbial carbon. That carbon can remain in the soil, becoming part of the wider stock of organic matter.

Scientists have established that cellular residues—the remnants left when microbial cells die—make an important contribution to this storage. The new work addresses a less developed question: how much carbon is supplied by microbial products released beyond the cell before death?

Extracellular polymeric substances provide one answer. Microbes produce and release them into their surroundings, where they can interact with soil particles and minerals. The study’s results indicate that this process is not a minor supplement to carbon storage through dead cells. Across the surveyed soils, production of the extracellular substances was comparable to the production of microbial cellular residues.

The researchers also found a statistical relationship between the abundance or production of these substances and mineral-associated organic carbon in different soils. That association does not by itself establish every step of the process, but it points to a connection between microbial secretions and one of the soil carbon pools considered important for persistence.

Tracking carbon into mineral-bound stores

To quantify the extracellular pathway, the researchers developed a method based on water containing oxygen-18, a form of oxygen that can be traced through biological processes. They used the approach in a national-scale survey of agricultural soils in China, allowing the production of extracellular polymeric substances to be assessed across many sites rather than in a single controlled sample.

The study also used laboratory microcosm experiments in which carbon was labelled with carbon-13. This made it possible to follow carbon originating from extracellular polymeric substances and determine whether it entered the mineral-associated organic carbon pool.

Those experiments showed that carbon from the microbial substances was transferred into the mineral-associated pool at rates comparable to, and in some cases greater than, the rates measured for microbial cellular residues. The result strengthens the case that microbial secretions are not merely temporary material in the soil environment. They can become part of a carbon fraction associated with minerals, where organic matter is held in a form relevant to soil carbon accumulation.

Why the finding matters

Mineral-associated organic carbon is one of the major destinations for carbon in soil. Understanding how carbon reaches this pool is important because the route determines how researchers interpret changes in soil carbon and how they assess the influence of microbial communities.

The work expands the picture of microbial carbon processing. A focus on dead cells alone would overlook carbon released by living microbes, even though the study indicates that this extracellular input can be equally important. The findings therefore provide a mechanism for explaining how microbial communities influence the movement and retention of carbon in soils.

The results may also help refine research into how soil carbon responds to global change. Conditions that alter microbial activity could affect not only the formation of cellular residues, but also the release and fate of extracellular polymeric substances. The study does not present the extracellular pathway as a replacement for cellular residues. Instead, it identifies both as substantial routes through which microbial processing can feed mineral-associated soil carbon.