Abstract. Nitrous oxide (N2O) is the primary atmospheric constituent involved in
stratospheric ozone depletion and contributes strongly to changes in the
climate system through a positive radiative forcing mechanism. The
atmospheric abundance of N2O has increased from 270 ppb (parts per billion, 10−9 mole mole−1) during the
pre-industrial era to approx. 330 ppb in 2018. Even though it is well known
that microbial processes in agricultural and natural soils are the major
N2O source, the contribution of specific soil processes is still
uncertain. The relative abundance of N2O isotopocules
(14N14N16N, 14N15N16O,
15N14N16O, and 14N14N18O) carries
process-specific information and thus can be used to trace production and
consumption pathways. While isotope ratio mass spectroscopy (IRMS) was
traditionally used for high-precision measurement of the isotopic
composition of N2O, quantum cascade laser absorption spectroscopy
(QCLAS) has been put forward as a complementary technique with the potential
for on-site analysis. In recent years, pre-concentration combined with QCLAS
has been presented as a technique to resolve subtle changes in ambient
N2O isotopic composition. From the end of May until the beginning of August 2016, we investigated
N2O emissions from an intensively managed grassland at the study site
Fendt in southern Germany. In total, 612 measurements of ambient
N2O were taken by combining pre-concentration with QCLAS analyses,
yielding δ15Nα, δ15Nβ,
δ18O, and N2O concentration with a temporal resolution of
approximately 1 h and precisions of 0.46 ‰, 0.36 ‰, 0.59 ‰, and 1.24 ppb,
respectively. Soil δ15N-NO3- values and
concentrations of NO3- and NH4+ were measured to further
constrain possible N2O-emitting source processes. Furthermore, the
concentration footprint area of measured N2O was determined with a
Lagrangian particle dispersion model (FLEXPART-COSMO) using local wind and
turbulence observations. These simulations indicated that night-time
concentration observations were largely sensitive to local fluxes. While
bacterial denitrification and nitrifier denitrification were identified as
the primary N2O-emitting processes, N2O reduction to N2
largely dictated the isotopic composition of measured N2O. Fungal
denitrification and nitrification-derived N2O accounted for 34 %–42 % of total N2O emissions and had a clear effect on the measured
isotopic source signatures. This study presents the suitability of on-site
N2O isotopocule analysis for disentangling source and sink processes
in situ and found that at the Fendt site bacterial denitrification or nitrifier denitrification is the major source for N2O, while N2O
reduction acted as a major sink for soil-produced N2O.
Catch Crop Residues Stimulate N2O Emissions During Spring, Without Affecting the Genetic Potential for Nitrite and N2O Reduction
