• Energy Research

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.

Publication abstract: Nitrous oxide is a powerful greenhouse gas whose atmospheric growth rate has increased rapidly over the past decade. Causes of N2O emission dynamics from terrestrial ecosystems and their responses to climate change remain poorly understood. Based on high-resolution isotopic measurements we found that microbial functional gene abundances and soil moisture were the main drivers of variability in baseline emissions and pathways. After fertilization and, surprisingly, under drought, denitrification dominated grassland N2O production and dynamics, contributing 70% of emissions. Drought responses of N2O emissions were supported by a clear reversible enrichment in nitrogen-bearing organic matter compared to oxygen on the surface of soil microaggregates. We observed hysteresis for both total N2O flux and denitrification contribution during rewetting, which were higher than control and drought for the same soil moisture range. These results illustrate that the magnitude of feedbacks between climate change and N2O emission pathways is sufficient to account for increases in atmospheric growth rate in the past decade. We expect these effects will be amplified in the coming decades as climatic extremes are expected to increase in severity. Supplement to: Harris, Eliza; Diaz-Pines, E; Stoll, E; Schloter, Michael; Schulz, Stefanie; Duffner, C; Li, K; Moore, K L; Ingrisch, Johannes; Reinthaler, D; Zechmeister-Boltenstern, S; Glatzel, Stephan; Brüggemann, Nicolas; Bahn, Michael (2021): Denitrifying pathways dominate nitrous oxide emissions from managed grassland during drought and rewetting. Science Advances, 7(6), eabb7118

Isotopic measurements showed that N 2 O production during drought is unexpectedly dominated by denitrification of organic nitrogen.

This dataset includes flux and isotope data for 16 monoliths from the Kaserstatt Alm subject to an experimental drought. The treatments are designated C/W/D/DF/R for Control, Wet (+25% precipitation), Drought, Drought (finishes later) and Rain (drought monolith which received rain) respectively. Further information on the experimental set up is given in the associated publication.