Early season N<sub>2</sub>O emissions under variable water management in rice systems: source-partitioning emissions using isotope ratios along a depth profile

Abstract

Abstract. Soil moisture strongly affects the balance between nitrification, denitrification and N2O reduction and therefore the nitrogen (N) efficiency and N losses in agricultural systems. In rice systems, there is a need to improve alternative water management practices, which are designed to save water and reduce methane emissions but may increase N2O and decrease nitrogen use efficiency. In a field experiment with three water management treatments, we measured N2O isotope ratios of emitted and pore air N2O (δ15N, δ18O and site preference, SP) over the course of 6 weeks in the early rice growing season. Isotope ratio measurements were coupled with simultaneous measurements of pore water NO3-, NH4+, dissolved organic carbon (DOC), water-filled pore space (WFPS) and soil redox potential (Eh) at three soil depths. We then used the relationship between SP × δ18O-N2O and SP × δ15N-N2O in simple two end-member mixing models to evaluate the contribution of nitrification, denitrification and fungal denitrification to total N2O emissions and to estimate N2O reduction rates. N2O emissions were higher in a dry-seeded + alternate wetting and drying (DS-AWD) treatment relative to water-seeded + alternate wetting and drying (WS-AWD) and water-seeded + conventional flooding (WS-FLD) treatments. In the DS-AWD treatment the highest emissions were associated with a high contribution from denitrification and a decrease in N2O reduction, while in the WS treatments, the highest emissions occurred when contributions from denitrification/nitrifier denitrification and nitrification/fungal denitrification were more equal. Modeled denitrification rates appeared to be tightly linked to nitrification and NO3- availability in all treatments; thus, water management affected the rate of denitrification and N2O reduction by controlling the substrate availability for each process (NO3- and N2O), likely through changes in mineralization and nitrification rates. Our model estimates of mean N2O reduction rates match well those observed in 15N fertilizer labeling studies in rice systems and show promise for the use of dual isotope ratio mixing models to estimate N2 losses.

Attribution of N<sub>2</sub>O sources in a grassland soil with laser spectroscopy based isotopocule analysis

“…ues of the sample gas, for δ 18 O this is currently confined by the non-availability of suitable standard gases. Nonetheless, the implemented calibration procedure presents current best practice in particular as the linearity of the delta scale for QCLAS measurements was demonstrated already in 2008 (Waechter et al, 2008). This phase was followed by a series of four alternating S1 and ambient air sample (S) measurements.…”

“…However, IRMS instruments are usually not suitable for field deployment. Recently, quantum cascade laser absorption spectroscopy (QCLAS) (Waechter et al, 2008;McManus et al, 2015), cavity ring-down spectroscopy (CRDS; Erler et al, 2015), and off-axis cavity output spectroscopy (OA-ICOS; Wassenaar et al, 2018) have been introduced as alternatives for greenhouse gas (GHG) stable isotope analysis, with the capability for real-time, onsite analysis even at remote locations (Tuzson et al, 2011;Wolf et al, 2015;Eyer et al, 2016;Röckmann et al, 2016). Another advantage of spectroscopic techniques is their ability for direct selective analysis of intra-molecular isotopic isomers (isotopomers) such as 14 N 15 N 16 O and 15 N 14 N 16 O, while the determination of the SP using IRMS is only possible via a detour of measuring δ 15 N-NO + in combination with δ 15 N bulk and a correcting for scrambling (Toyoda et al, 1999).…”

Denitrification Is the Main Nitrous Oxide Source Process in Grassland Soils According to Quasi‐Continuous Isotopocule Analysis and Biogeochemical Modeling

“…According to those 542 T measurements, the long-term analytical repeatability of the measurements spanning the whole measurement period was 0.61‰, 0.55‰, 0.47‰, and 3.3 ppb for δ 15 N α , δ 15 N β , δ 18 O, and N 2 O concentration measurements, respectively. The accuracy of the applied technique was additionally assessed by triplicate in situ measurements of T, T1, and T2 undergoing identical treatment as the sample measurements by TREX-QCLAS and IRMS at ETH and at the Tokyo Institute of Technology (Tokyo Tech; Figure S7 and Tables S1 and S2 (Verhoeven et al, 2019). To this end, discrete air samples were collected from the chamber headspace through a sample port using a 60 ml syringe at 0, 20, 40, 60, 80, 110, and 130 min time intervals after chamber closure.…”

CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O fluxes along an altitudinal gradient in the northern Ecuadorean Andes: N<sub>2</sub>O consumption at higher altitudes

“…N Bulk ) using a gas preparation unit (Trace Gas, Elementar, Manchester, UK) coupled to an Isotope Ratio Mass Spectrometer 160 (IRMS) (IsoPrime100, Elementar, Manchester, UK). For measurement and calibration details seeVerhoeven et al (2019).https://doi.org/10.5194/bg-2020-105 Preprint. Discussion started: 3 April 2020 c Author(s) 2020.…”