The fate of N2O consumed in soils

Abstract

Soils are capable to consume N2O. It is generally assumed that consumption occurs exclusively via respiratory reduction to N2 by denitrifying organisms (i.e. complete denitrification). Yet, we are not aware of any verification of this assumption. Some N2O may be assimilatorily reduced to NH3. Reduction of N2O to NH3 is thermodynamically advantageous compared to the reduction of N2. Is this an ecologically relevant process? To find out, we treated four contrasting soil samples in a flow-through incubation experiment with a mixture of labelled (98%) 15N2O (0.5-4 ppm) and O2 (0.2-0.4%) in He. We measured N2O consumption by GC-ECD continuously and δ15N of soil organic matter before and after an 11 to 29 day incubation period. Any 15N2O assimilatorily reduced would have resulted in the enrichment of soil organic matter with 15N, whereas dissimilatorily reduced 15N2O would not have left a trace. None of the soils showed a change in δ15N that was statistically different from zero. A maximum of 0.27% (s.e. ±0.19%) of consumed 15N2O may have been retained as 15N in soil organic matter in one sample. On average, 15N enrichment of soil organic matter during the incubation may have corresponded to a retention of 0.019% (s.e. ±0.14%; n=4) of the 15N2O consumed by the soils. We conclude that assimilatory reduction of N2O plays, if at all, only a negligible role in the consumption of N2O in soils.