Significant Role for Microbial Autotrophy in the Sequestration of Soil Carbon

Soils were incubated for 80 days in a continuously labeled (CO2)-C-14 atmosphere to measure the amount of labeled C incorporated into the microbial biomass. Microbial assimilation of C-14 differed between soils and accounted for 0.12% to 0.59% of soil organic carbon (SOC). Assuming a terrestrial area of 1.4 x 10(8) km(2), this represents a potential global sequestration of 0.6 to 4.9 Pg C year(-1). Estimated global C sequestration rates suggest a missing sink for carbon of between 2 and 3 Pg C year(-1). To determine whether (CO2)-C-14 incorporation was mediated by autotrophic microorganisms, the diversity and abundance of CO2-fixing bacteria and algae were investigated using clone library sequencing, terminal restriction fragment length polymorphism (T-RFLP), and quantitative PCR (qPCR) of the ribulose 1,5-bisphosphate carboxylase/ oxygenase (RubisCO) gene (cbbL). Phylogenetic analysis showed that the dominant cbbL-containing bacteria were Azospirillum lipoferum, Rhodopseudomonas palustris, Bradyrhizobium japonicum, Ralstonia eutropha, and cbbL-containing chromophytic algae of the genera Xanthophyta and Bacillariophyta. Multivariate analyses of T-RFLP profiles revealed significant differences in cbbL-containing microbial communities between soils. Differences in cbbL gene diversity were shown to be correlated with differences in SOC content. Bacterial and algal cbbL gene abundances were between 10(6) and 10(8) and 10(3) to 10(5) copies g(-1) soil, respectively. Bacterial cbbL abundance was shown to be positively correlated with RubisCO activity (r = 0.853; P < 0.05), and both cbbL abundance and RubisCO activity were significantly related to the synthesis rates of [C-14]SOC (r = 0.967 and 0.946, respectively; P < 0.01). These data offer new insights into the importance of microbial autotrophy in terrestrial C cycling.