Heterotrophic fixation of CO(2) in soil.

The occurrence of heterotrophic CO(2) fixation by soil microorganisms was tested in several mineral soils differing in pH and two artificial soils (a mixture of silica sand, alfalfa powder, and nutrient medium inoculated with a soil suspension). Soils were incubated at ambient ( approximately 0.05 vol%) and elevated ( approximately 5 vol%) CO(2) concentrations under aerobic conditions for up to 21 days. CO(2) fixation was detected using either a technique for determining the natural abundance of (13)C or by measuring the distribution of labeled (14)C-CO(2) in soil and bacteria. The effects of elevated CO(2) on microbial biomass (direct counts, chloroform fumigation extraction method), composition of microbial community (phospholipid fatty acids), microbial activity (respiration, dehydrogenase activity), and turnover rate were also measured. Heterotrophic CO(2) fixation was proven in all soils under study, being higher in neutral soils. The main portion of the fixed CO(2) (98-99%) was found in extracellular metabolites while only approximately 1% CO(2) was incorporated into microbial cells. High CO(2) concentration always induced an increase in microbial activity, changes in the composition of the microbial community, and a decrease in microbial turnover. The results suggest that heterotrophic CO(2) fixation could be a widespread process in soils.