Abstract
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.
Highlights
Nitrous oxide (N2O) is the third most important greenhouse gas (GHG), accounting for 6 % of the total anthropogenic radiative forcing (Myhre et al, 2013), and is far the dominant stratospheric-ozone-depleting substance emitted in the 21st century (Ravishankara et al, 2009)
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
Soil extracted NH+4 and nitric oxide (NO)−3 values in this period were 0.27 to 8.32 and 0.12 to 3.15 mg N L−1, respectively. This period was characterized by the lowest N2O flux rates (f (N2O)), i.e. the mean f (N2O) of all five chambers was below 70 μg N m−2 h−1
Summary
Nitrous oxide (N2O) is the third most important greenhouse gas (GHG), accounting for 6 % of the total anthropogenic radiative forcing (Myhre et al, 2013), and is far the dominant stratospheric-ozone-depleting substance emitted in the 21st century (Ravishankara et al, 2009). While it is well known that natural and agricultural soils are the major N2O sources on a global scale, the relative contributions of individual microbial and abiotic N2O production and consumption pathways remain largely uncertain because different N2O-producing and N2O-consuming processes are active simultaneously in a soil. Atmospheric N2O isotopic composition provides important information about N2O production and consumption processes because distinct microbial and abiotic process pathways exhibit characteristic isotopic signatures (Toyoda et al, 2017; Decock and Six, 2013b; Verhoeven et al, 2019; Denk et al, 2017). Apart from 14N14N16O, representing 99 % of total atmospheric N2O, the three most abundant N2O isotopocules are 14N15N16O (15N at central α position), 15N14N16O (15N at terminal β position), and 14N14N18O (Toyoda and Yoshida, 1999; Kato et al, 1999).
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