Abstract
The control of soil nitrogen (N) availability under elevated atmospheric CO 2 is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO 2 on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO 2 (ambient + 400 µmol mol −1 ) in open top chambers under Scots pine ( Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N 2 O emissions were significantly higher in soils from the elevated CO 2 treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO 3 – production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO 2 treatment in the field when incubated in the laboratory under elevated CO 2 (increase of 100%), but there was no effect when incubated under ambient CO 2 . Net nitrate production of the soil originating from the ambient CO 2 treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO 2 on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO 2 through higher soil CO 2 concentrations. Alternatively, lower microbial NH 4 assimilation under elevated CO 2 might explain the higher net nitrification. We conclude that elevated atmospheric CO 2 has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO 2 concentrations compared to atmospheric concentrations.