[Carbon dioxide assimilation potential, functional gene amount and RubisCO activity of autotrophic microorganisms in...

Carbon dioxide (CO2) assimilation by autotrophic microorganisms plays a significant role in carbon sequestration in terrestrial ecosystems. Here, experiments were carried out to determine the contribution of autotrophic microorganisms to atmospheric CO2 fixation in 6 representative agricultural soils. Soils were incubated continuously in an atmosphere of 14CO2 and the distribution of labeled C into soil organic carbon (14C-SOC) was determined after 110 d. Meanwhile, the amounts of the cbbL genes were determined by Quantitative PCR and the RubisCO activity was measured in different soils. The results showed that substantial amounts of 14CO2 were fixed into 14C-SOC (ranged 10.63-133.81 mg x kg(-1) after 110 d of continuous labeling, with an annual, global rate of about 0.57-7.3 Pg. The microbially fixed C was also incorporated into the active carbon pool [the dissolved organic C (14C-DOC) and in the microbial biomass C (14C-MBC)], and ranged from 0.96 to 8.10 mg x kg(-1) and 1.70 to 49.16 mg x kg(-1), respectively. The proportion of 14C-SOC in SOC was 0.09%-0.64%. The 14C-DOC /DOC and 14C-MBC /MBC were 5.07%- 4.3% and 2.51%-13.12%, respectively. Thus, the distribution and transformation of microbially fixed C had a larger influence on the dynamics of DOC and MBC than on the total SOC dynamics. Moreover, the abundance of soil bacteria cbbL gene and RubisCO activity were in the range of 2.40 x 10(7) - 1.9 x 10(8) copies x g(-1) and 34.06-71.86 nmol x (g x min)(-1), respectively. The 14C-SOC content was significantly correlated with both the 14C-MBC content (P < 0.01) and the RubisCO activity (P < 0.01) in all tested soils. We concluded that autotrophic CO2 assimilation by soil microbes is significant to the global C cycle.