Tillage Systems Influence the Abundance and Composition of Autotrophic CO2-fixing Bacteria in Wheat Soils in North China

Autotrophic bacteria are widely distributed in agricultural soils and can significantly incorporate atmospheric CO2 into soil microbial biomass. However, knowledge about field autotrophic bacteria under different tillage systems is limited. In this field study, the effects of 6 years of tillage on autotrophic bacteria were evaluated by high-throughput sequencing of the ribulose 1,5–bisphosphate carboxylase/oxygenase (RubisCO) gene (cbbL). The results showed that tillage systems had strong influence on the abundance, composition, and activity of cbbL-carrying bacteria. Conventional tillage had more abundant cbbL and 16S rRNA genes as well as higher RubisCO activity, when compared with no tillage. The cbbL sequences were mainly affiliated with Proteobacteria and Actinobacteria, which accounted for >96% of the identified community. The predominant cbbL-carrying bacteria in conventional tillage with straw returning were Proteobacteria (94.3%) at the phylum level and Variovorax paradoxus (42.9%) at the species level; in contrast, those in no tillage with straw removal included Actinobacteria (31.5%) at the phylum level and Thermomonospora curvata (29.1%) at the species level. The copy numbers of cbbL correlated positively with dissolved organic carbon and negatively with soil bulk density. Mantel tests confirmed that cbbL-carrying bacterial composition was significantly affected by soil bulk density and also by microbial biomass carbon. Straw returning enhanced the abundance of cbbL-carrying bacteria and had unique species such as Acidiphilium multivorum, which was not detected in treatments with straw removal. These results suggest that the potential for CO2 mitigation by tillage depends on soil conditions and is much more variable and complex than previously considered.