ABSTRACTSoils were incubated for 80 days in a continuously labeled14CO2atmosphere to measure the amount of labeled C incorporated into the microbial biomass. Microbial assimilation of14C differed between soils and accounted for 0.12% to 0.59% of soil organic carbon (SOC). Assuming a terrestrial area of 1.4 × 108km2, 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 whether14CO2incorporation 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 dominantcbbL-containing bacteria wereAzospirillum lipoferum,Rhodopseudomonas palustris,Bradyrhizobium japonicum,Ralstonia eutropha, andcbbL-containing chromophytic algae of the generaXanthophytaandBacillariophyta. Multivariate analyses of T-RFLP profiles revealed significant differences incbbL-containing microbial communities between soils. Differences incbbLgene diversity were shown to be correlated with differences in SOC content. Bacterial and algalcbbLgene abundances were between 106and 108and 103to 105copies g−1soil, respectively. BacterialcbbLabundance was shown to be positively correlated with RubisCO activity (r= 0.853;P< 0.05), and bothcbbLabundance and RubisCO activity were significantly related to the synthesis rates of [14C]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.

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