[Effects of Soil Texture on Autotrophic CO₂ Fixation Bacterial Communities and Their CO₂ Assimilation Contents]

Autotrophic bacteria can assimilate atmospheric carbon dioxide (CO₂) and convert CO₂ into organic carbon. The CO₂ fixation by autotrophic bacteria is important for the improvement of carbon sequestration in agricultural soils. However, the effect of soil texture on autotrophic CO₂ fixation bacteria and their CO₂ fixation capacity is still unknown. Here, two paddy soils with different textures (loamy clay soil and sand clay loam soil) were incubated with continuous ¹⁴C-CO₂ in a glass chamber. The two soils were developed from the same parent. At the end of 110 days incubation, the ¹⁴C-CO₂ incorporated in soil organic carbon (¹⁴C-SOC), microbial biomass carbon (¹⁴C-MBC) and dissolved organic carbon (¹⁴C-DOC) were measured to explore the effects of soil texture on the autotrophic bacterial CO₂ fixation rates. The effect of soil texture on the composition and diversity of autotrophic CO₂ fixation bacterial community was investigated using cloning and sequencing of the cbbL gene, which encodes ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO) in the Calvin cycle. The results showed that the average contents of ¹⁴C-SOC, ¹⁴C-MBC and ¹⁴C-DOC were 133.81, 40.16 and 8.10 mg·kg^-1 in loamy clay soil, respectively, which were significantly higher than their corresponding contents in sand clay loam soil (P<0.05). This suggested that soil texture not only affected the amounts of autotrophic bacteria CO₂ fixation but also had an effect on the transformation of microbial assimilated ¹⁴C in soil. The cbbL gene libraries of two soils were significantly different as revealed by libshuff analyses (P<0.05). Phylogenetic analysis showed that cbbL sequences from the loamy clay soil were closely affiliated with known cultures such as Rhodoblastus acidophilus, Blastochloris viridis, Thauera humireducens, Mehylibium sp.and Variovorax sp., whereas these sequences belonging to the sand clay loam soil were related to branching lineages originating from Rhizobiales and Actinomycetales.Rarefaction curve, clone library coverage and diversity index analysis based on bacterial cbbL clone libraries indicated that the loamy clay soil had higher cbbL gene diversity compared to the sand clay loam soil. These results suggested that soil texture had a pronounced effect on the composition and diversity of autotrophic CO₂ fixation bacterial communities. The higher clay content, nutrient availability and cation exchange capacity may stimulate the growth and activity of autotrophic bacteria, and result in the higher amounts of ¹⁴C in loamy clay soil. These data broaden the understanding and knowledge of mechanisms of microbial carbon fixation and their influencing factors in agricultural soils.