『Abstract
Denitrifying aquifers are sources of the greenhouse gas N2O. Isotopic signatures reflect processes of production
and reduction of N2O, but it is not clear
to which extent these can be used to quantify those processes.
We investigated the spatial distribution of isotopologue values
of N2O (δ18O, average δ15N,
and 15N site preference, SP) in two denitrifying sandy
aquifers to study N2O production and reduction
and associated isotope effects in groundwater. For the first time,
we combined this approach with direct estimation of N2O
reduction from excess-N2 analysis. Groundwater
samples were collected from 15 monitoring wells and four multilevel
sampling wells and analysed for NO3-,
decreased N2O, dissolved O2,
excess N2 from denitrification and isotopic
signatures of NO3- and N2O. Both aquifers exhibited high NO3-
concentrations with average concentrations of 22 and 15 mg N L-1,
respectively. Evidence of intense denitrification with associated
N2O formation was obtained from mean excess-N2 of 3.5 and 4.3 mg N L-1, respectively.
Isotopic signatures of N2O were highly variable
with ranges of 17.6-113.2‰ (δ18O), -55.4 to 89.4‰ (δ15Nbulk)
and 1.8-97.9‰ (SP). δ15N and δ18O of
NO3- ranged from -2.1‰ to 65.5‰
and from -5‰ to 33.5‰, respectively.
The relationships between δ15N of NO3-,
δ15Nbulk and SP were not in good agreement
with the distribution predicted by a Rayleigh-model of isotope
fractionation. The large ranges of δ18O) and SP of
N2O as well as the close correlation between
these values could be explained by the fact that N2O
reduction to N2 was strongly progressed but
variable.
We confirm and explain that a large range in SP and δ18O
is typical for N2O from denitrifying aquifers,
showing that this source signature can be distinguished from the
isotopic fingerprint of N2O emitted from
soils without water-logging. We conclude that isotopologie values
of N2O in our sites were not suitable to
quantify production or reduction of N2O or
the contribution of different processes to the total N2O
flux, apparently because these values were not only governed by
individual pathways but eventually also by the spatial distribution
of substrates and activity within the aquifers. These observations
could be explained by the dynamics of N2O
production, reduction and transport in water-saturated systems
with heterogenic distribution of microbial activity and by a combination
of diffusive and enzymatic isotope effects.』
1. Introduction
2. Materials and methods
2.1. Sites
2.2. Sampling and laboratory analyses
2.3. Statistics
2.4. Estimating isotope effects of N2O turnover
3. Results
3.1. Concentrations of NO3-,
N2O and N2
3.2. Isotopic values
4. Discussion
4.1. Evidence for N2O production and
consumption from N concentration patterns
4.2. Isotopic values
4.2.1. Identifying the contribution of nitrification and denitrification
to groundwater N2O
4.2.1.1. Evidence from δ15N-NO3-
and δ15Nbulk
4.2.1.2. Combining δ15N-NO3-,
δ15Nbulk and 15N site preference
4.2.1.3. Impact of microbial community structure
4.2.2. Characterising N2O reduction dynamics
using δ18O and SP of N2O
4.2.3. Deviations from proposed models
4.2.3.1. Validity of the Rayleigh model
4.2.3.2. Control of the SP/δ18O slope
4.2.4. Range of isotopic values
4.2.5. Implications for the global isotopic budget of N2O
5. Conclusions
Acknowledgements
References