Soil is a net source of CO2,
because roots and microorganisms are mineralizing plant residues and soil
organic matter. However, also the reverse process, namely CO2
fixation, occurs and may have significant implications for the assessment of
bound residue formation as determined by the use of radiolabelled pollutants as
well as for the isotopic composition of microbial biomass - if exposed to
atmospheric CO2 - and soil organic matter. We incubated an arable
soil under an atmosphere enriched with 13CO2 in the dark
for up to 81 days to study the processes involved in CO2 fixation by
soil microorganisms and the fate of the CO2-derived carbon in soil. CO2
was fixed along with heterotrophic microbial activity in the soil. The amount
of CO2 fixed corresponded to about 7% of the microbial biomass and
was linearly correlated with respiration. The process was almost completely
suppressed by fumigation with CHCl3. We found 13C-enrichment
in the fatty acids and the amino acids, but not in the amino sugars. This points to maintenance metabolism of the soil microbial
cells without significant growth during the incubation. For each individual
fatty acid, more label was found in the total fraction
(microbial biomass + non-living organic matter) than in the polar lipid fatty
acid fraction (microbial biomass only). Part of the label thus already has been
transferred from the biomass to non-living soil organic matter. The amino acids
carrying the highest label were asparagine and aspartate, indicating oxaloacetate
as a direct product of CO2 fixation. The reaction therefore is
related to anaplerotic reactions in heterotrophic organisms. Our results show
that CO2-fixation by heterotrophic microorganisms is an essential
process of the carbon cycle in soil and thus also contributes to the formation
of soil organic matter.