Soil Microbes Transform Inorganic Carbon into Organic Carbon by Dark
Fixation Pathways in Desert Soil
Abstract
Soil inorganic carbon (SIC) represents the main soil carbon pool in
drylands with a high geologic residence time for carbon sequestration.
Recent studies have shown that SIC is not stable as previously supposed,
and can be employed by certain microbes and transformed into organics in
soils; however, this transformation remains largely unexplored. We
performed in situ 13C tracing in desert bulk
soil and employed metagenomics to predict the microbial metabolic
processes associated with carbon transformation. The tracing data showed
that the 13C signature profile in soil organic carbon
(SOC) originated from SIC with a 13C-SOC content of
6.881 mg m-2 during the feeding periods. Metagenomic
analysis identified genes encoding enzymes related to microbial
CO2 and HCO3-
fixation, accounting for 0.448% (based on Kyoto Encyclopedia of Genes
and Genomes database) and 0.668% (based on Evolutionary genealogy of
genes: Non-supervised Orthologous Groups database) of all ascertained
genes. Our results confirmed that a considerable portion of the
determined genes and taxa were responsible for heterotrophic fixation.
The microbes involved in dark microbial fixation, particularly
chemoautotrophic and heterotrophic pathways, were from a broad taxonomic
range. Although the amount of SOC derived from the dark microbial
fixation process was not assessed, the present study highlights a
neglected carbon transformation process mediated by soil microbes in
drylands and provides insights into carbon transformation of SIC to SOC
in dryland soil.