Dryland farm soil may fix atmospheric carbon through autotrophic microbial pathways

The absorption of CO2 by dryland farm soil has been reported, but understanding of its mechanisms is still limited. Recent investigations suggest that the absorption of atmospheric carbon by dryland farm soil is probably related to autotrophic carbon fixation in the soil. However, little evidence for this has been presented. Therefore, we hypothesized that autotrophic microbes employing carbon fixation pathways exist in dryland farm soil. To verify this hypothesis, metagenomic sequencing and analysis methods were used to identify autotrophic microbes harboring genes of carbon fixation pathways in the soil of six typical farmlands in a dryland area. The results showed that autotrophic microbes harboring genes of six carbon fixation pathways existed at each of the six sites, and their average relative abundance at our study sites was 1.68%. The dominant phyla of autotrophic microbes in dryland farm soil were Actinobacteria and Proteobacteria. Among the determined carbon fixation pathways, the reductive citrate cycle was the most representative pathway with the highest relative abundance. Moreover, the composition of these autotrophic microbes was correlated with total phosphorus and soil pH, while the composition of carbon fixation genes was not correlated with soil properties. Overall, the present study highlights that dryland farm soil may fix atmospheric carbon through autotrophic microbial pathways, which provides a new reference for exploring the absorption of atmospheric carbon in farmland soils of drylands.