Soil formation is a very slow and complex process involving the interaction of abiotic factors and biotic factors, among which the latter is the most active agents.

However, traditional soil classification systems are mainly based on the data of soil physiological and chemical properties along soil profiles, lacking of the data of soil microbiology. 

To reveal the correlations between microbial communities and soil classification, a research group led by WANG Guanghua from the Northeast Institute of Geography and Agroecology of the Chinese Academy of Sciences collected soil samples based on the diagnostic horizons from Ustic and Udic Isohumosols in agricultural soils across Heilongjiang Province of China. They investigated the abundance and communities of bacteria, archaea, and fungi.

Their findings were published in CATENA on June 10.

Isohumosols is a soil order nominated based on Chinese Soils Taxonomic Classification. It is roughly equal to Mollisols, and also commonly called black soils.

According to the researchers, the abundance of bacteria, archaea, and fungi consistently decreased by more than 90% in C horizon (parent material horizons) compared with those in Ah horizons (humus horizons).

In addition, all soil microbial community structures were obviously divided into Ustic and Udic groups, and a distinct succession of microbial communities was detected from Ah horizon to C horizon at individual sites. "The soil microbial communities in C horizons could better characterize the formation of the two suborders of Isohumosols," said WANG.

The researchers used a machine learning approach to establish random forest modeling. Results indicated that soil microbial communities of diagnostic horizons could serve as quantitative indices well, reflecting the two suborders of Isohumosols with more than 95% accuracy and more than 92% accuracy in different soil diagnostic horizons.