LAUSR

Abstract In soils both δ13C and δ15N values of soil organic matter (SOM) tend to increase with soil depth. This change in soil C and N isotopic composition is at least partially linked to soil microbial activity that has been suggested to drive fractionation during decomposition, preferential microbial decomposition of components of soil organic matter and mixing of diverse sources of C. Soil microbes are capable of fixing N2 and CO2 from the soil atmosphere. Apart from surface photosynthetic CO2 fixation and chemoautotrophic fixation, dark anaplerotic (i.e. non-photosynthetic) fixation of CO2 is especially important for provision of C-skeletons for amino acid synthesis. We hypothesized that these N2 and CO2 fixing processes may contribute to determining SOM δ13C and δ15N values. Soils from 10 sites across South Africa differing in soil properties were incubated in the dark for 3 d under continuous exposure to 13CO2- and 15N2-enriched atmospheres with varying soil moisture (10, 50 and 100% of field capacity) and temperature (4, 25, 40 °C). We did not detect significant N2 fixation in any treatment. Significant soil microbial anaplerotic CO2 fixation, however, occurred in all soils. Highest rates of anaplerotic CO2 fixation occurred in soils at 50% field capacity and 25 °C, suggesting a link with microbial biotic activity. Soils with low C and N concentrations and low C:N ratios exhibited the highest rates of CO2 fixation, indicating a possible link between anaplerotic CO2 fixation rates and soil nutrient status. The higher rates of CO2 fixation in soils with low nutrients may indicate that soil microbes rely increasingly on anaplerotic fixation as SOM-N declines, forcing greater reliance on de novo amino acid synthesis, and thus anaplerotic CO2 fixation. Diffusion of bulk atmospheric CO2 (δ13C ca. −10‰) into the soil atmosphere (δ13C ≪ −10‰) drives soil atmospheric CO2 δ13C values up towards those of the bulk atmosphere. Anaplerotic CO2 fixation in this CO2 may contribute to determining soil δ13C values.