Significant Role for Microbial Autotrophy in the Sequestration of Soil Carbon

Abstract

Soils were incubated for 80 days in a continuously labeled 14 CO 2 atmosphere to measure the amount of labeled C incorporated into the microbial biomass. Microbial assimilation of 14 C differed between soils and accounted for 0.12% to 0.59% of soil organic carbon (SOC). Assuming a terrestrial area of 1.4 × 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 14 CO 2 incorporation was mediated by autotrophic microorganisms, the diversity and abundance of CO 2 -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 [ 14 C]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.