Attribution of N2O sources in a grassland soil with laser spectroscopy based isotopocule analysis

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 δ¹⁵N^α, δ¹⁵N^β, 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-${\mathrm{NO}}_{\mathrm{3}}^{-}$ values and concentrations of ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and ${\mathrm{NH}}_{\mathrm{4}}^{+}$ 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.