Abstract
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.
Highlights
Nitrous oxide (N2O) is produced in soils during the processes of nitrification and denitrification (Firestone et al, 1980)
Some N2O may be assimilatorily reduced to NH3
Reduction of N2O to NH3 is thermodynamically advantageous compared to the reduction of N2
Summary
Nitrous oxide (N2O) is produced in soils during the processes of nitrification and denitrification (Firestone et al, 1980). It is implicitly assumed that complete denitrification (reduction of N2O to N2) is the only process responsible for observed sink activity. Once produced by a soil organism, a molecule of N2O is presumed to take one of the three known routes (Ostrom et al, 2007) (Fig. 1): (1) complete denitrification to N2 within the cell prior to its escape into the gas phase (reviewed in Zumft, 1997); (2) escape from the cell into the gas phase of soil and potentially to the atmosphere; or (3) complete denitrification to N2 upon re-entering a cell capable to reduce N2O We hypothesise a fourth pathway of assimilatory reduction to NH3 may be responsible for some of observed N2O consumption in soil (Fig. 1)
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