Denitrification and Nitrous Oxide Production in Successional and Old‐Growth Michigan Forests

Soils from 12 successional and old‐growth forests in Michigan were examined for N 2 O production under each of three incubation conditions: without amendment, in a 20% (vol/vol) acetylene atmosphere, and in an acetylene‐amended atmosphere after a simulated rainfall. Intact cores were taken during the growing season and incubated in the laboratory for 90 to 180 min in a porespace‐recirculation system that sampled for N 2 O and CO 2 at 10‐min intervals. Measurable rates of N 2 O production occurred in cores from all sites and under all incubation conditions; in general but not always, rates were higher in the presence of C 2 H 2 than in its absence, and increased upon simulated rainfall. Under acetylene‐ and water‐amended conditions, mean production was highest in an old‐growth and a late‐successional hardwood site and in a recent clearcut (30 to 80 mg N 2 O‐N m −2 d −1 ); intermediate rates (2 to 3 mg N 2 O‐N m −2 d −1 ) were observed in soils from a midsuccessional hardwood stand and from two old‐field communities, and low rates (<0.6 mg N 2 O‐N m −2 d −1 ) occurred in soils from a young sand dune community, from a midsuccessional and a late‐successional hardwood stand, and from a managed, a midsuccessional, and an old‐growth conifer community. Nitrate production, CO 2 production and water content could explain 65% of the variation in rates of N 2 O production among sites under acetylene‐amended conditions. Nitrate pool sizes and pH differed substantially among sites but were not well correlated with N 2 O production. The presence of a class of cores that produced N 2 O only in the absence of acetylene suggests that sources of N 2 O other than denitrifiers may be important in some sites.