RUDN University Soil scientist found out the difference between the contribution of bacteria and fungi in the carbon cycle in the soil

Mineralization of soil organic matter is a necessary stage of the carbon cycle. Organic residues completely decompose to oxides and salts — that is, to a form available for plant nutrition. At the same time, carbon dioxide is released into the atmosphere — the main greenhouse gas that leads to global warming. The intensity of mineralization is affected by the proportion of so-called dissolved organic carbon - water-soluble organic compounds that enter the soil, for example, from plant roots. These substances are divided into hydrophilic (sugars, amino acids, carboxylic acids, etc.) and hydrophobic (lignin and lipids). It is assumed that hydrophilic compounds accelerate the mineralization of soil organic matter to a greater extent — they are more accessible to microorganisms that decompose it. However, experimental evidence for this has not yet existed.

"We sought to quantify the differences in the mineralization of hydrophilic and hydrophobic fractions of the soil, as well as to clarify the underlying mechanisms controlled by bacteria and fungi," said Doctor of Biological Sciences Yakov Kuzyakov, Head of the Center for Mathematical Modeling and Design of Sustainable Ecosystems, RUDN University.

It is possible to find out how hydrophilic and hydrophobic substances affect the rate of decomposition of soil organic matter by the volume of carbon dioxide released by the soil. Soil samples for the experiment were taken at two points in the Chinese province of Hunan - moist soil of a rice field and dry soil from a hill. The points were chosen in such a way that the microbial composition differed in them — this will allow us to find out the contribution of individual groups of microorganisms. Soil scientists added hydrophilic and hydrophobic organic residues to the samples, after marking them with a rare isotope of carbon —13C. For 40 days, the samples were stored in a dark container at a temperature of 25℃, carbon dioxide emissions were measured every few days.

On the first day, mineralization was 3.6-70 times faster for the hydrophilic fraction. Moreover, the "embedding" of the hydrophilic fraction in bacteria was 11.4-16.4 times greater than in fungi. For the hydrophobic fraction, the ratio is the opposite — their embedding in fungi is 1.5 times greater than in bacteria. This indicates that fast-growing bacteria have a greater influence on the assimilation of the hydrophilic fraction, and slow—growing fungi - hydrophobic. Decomposition of both hydrophobic and hydrophilic compounds took place faster in dry soil from high ground — there was released about 90 mg/kg-1 more carbon dioxide than from moist rice soil (49.4-54.5 mg/kg-1).

"Understanding the dependence of the carbon cycle in the soil on its composition will help in the future to develop methods for maintaining soil fertility based on biotechnology and regulation of the soil microbiome," said Doctor of Biological Sciences Yakov Kuzyakov, Head of the Center for Mathematical Modeling and Design of Sustainable Ecosystems, RUDN University.

The results are published in the journal Biology and Fertility of Soils.