Salinization altered variations in organic carbon fractions by changing specific carbon-fixing microorganisms and pathways in soda saline-alkali soil

Salinization is a major global cause of soil degradation and has severe ecological consequences on soil organic carbon (SOC) and microorganisms. However, research on how carbon fixation microorganisms (CFMs) impacted by salinization affect variation in carbon fractions remains limited. We collected soil samples under three salinization levels from soda saline-alkali land, to assess the variations of environmental factors and carbon fractions with salinization gradient. We also analyzed microbial functional potential based on metagenomics absolute quantitative data obtained from different saline-alkali soils. We found that except for dissolved organic carbon, SOC and its fractions declined from low-salinized (LS) to high-salinized (HS). Among them, the SOC content of HS was 52.24% lower than that of LS, and dominant microbes also declined. The majority of genes accountable for the 6 carbon fixation pathways in CFMs have been picked out. Among these, the absolute abundance of Dicarboxylate/4-Hydroxybutyrate cycle (DC/4-HB cycle) and 3-Hydroxypropionate bicycle (3-HP bi-cycle) was highest and most significant across different saline-alkali soils. The negative impact of CO32- on CFMs was greater than the positive effect of Ca2+. CO32- affected the synthesis of microbial biomass carbon by altering the absolute abundance of prominent carbon fixation pathways (e.g., abfD, smtB involved in DC/4-HB and 3-HP bi-cycles) contributed by microbes (mainly Pseudomonadota and Acidobacteriota), thereby decreasing carbon fractions, and ultimately reducing carbon sequestration in saline-alkali soil. Overall, these findings provided comprehensive insights into the effect of carbon fixation microorganisms on variation of carbon fractions influenced by salinization in saline-alkali regions, and have biological guidance for the development and utilization of saline-alkali land and the enhancement of organic carbon.