Metagenomics Study of Soil Microorganisms Involved in the Carbon Cycle in a Saline-Alkaline Meadow Steppe in the Songnen Plain in Northeast China

Soil microorganisms play an important role in regulating and contributing to carbon cycling processes in grassland ecosystems. Soil salinization is one of the major problems causing soil degradation, which negatively affects carbon cycling in grassland ecosystems. However, its effects on carbon cycle immobilization-related functional genes in soil microorganisms remain unknown. Therefore, we conducted a metagenomic study to examine how different levels of salinity in the Songnen grassland affect the diversity of soil microorganisms and functional genes related to carbon cycling. The relative abundance of Ascomycetes increased with higher salinity, while the relative abundance of Proteus and Sclerobacillus decreased. Moreover, it decreased the relative abundance of carbon cycle immobilization-related functional genes. With an increase in soil salinisation, the relative abundance of the glycoside hydrolase (GH)130 family significantly increased, while the relative abundance of soil carbohydrate enzymes belonging to GH3 and GH55 families significantly decreased. Soil pH and electrical conductivity (EC) had a significant impact on soil microbial diversity and carbon cycle immobilization-related functional genes, as assessed using a structural equation model (SEM). An increase in soil pH directly reduced the soil microbial diversity and Shannon diversity of carbon cycle immobilization-related functional genes. It was found that the increase of grassland salinization reduced the diversity of bacteria and fungi, and affected the diversity of functional genes related to carbon cycle immobilization by reducing the total bacterial diversity. These results suggest that the increase of salinity has a negative feedback on soil carbon cycling in grassland, and the microbial diversity and functional gene diversity of moderate saline-alkali grassland (MDS) are the highest. The study findings elucidate the mechanism of soil salinization in grasslands, affecting microbial and gene diversity related to carbon cycle immobilization. They provide a theoretical framework for soil carbon sequestration and degradation restoration in grasslands.