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Tracing the sources of nitrate using isotopes: 10 years of progress for New Zealand freshwater1.19 MB
CC BY V4.0, Open Access
CC BY V4.0, Open
Conference paper
Tracing the sources of nitrate using isotopes: 10 years of progress for New Zealand freshwater
figshare
NZHS, Rivers Group & NZFSS Joint Conference (Invercargill | Waihōpai, 01/12/2020 - 04/12/2020)
08/01/2021
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Abstract
Introduction
The use of nitrogen and oxygen isotopes in nitrate (δ15N-NO3 and δ18O-NO3, respectively) has developed over the past 30 years to provide powerful tracer value, identifying sources and tracking processes related to nitrogen fluxes through the atmosphere, biosphere and hydrosphere. Globally, the high δ18O-NO3 values associated with atmospheric sources and oxidation processes has been most powerful in source identification for polluted northern hemisphere regions.
In these regions, the signal in δ18O-NO3 is immense but highly variable, and therefore obscures nuanced studies of nitrogen cycling. Over a decade of intensive New Zealand studies have shown signatures dominated by nitrogen cycle processes without substantial atmospheric inputs, therefore offering unique potential to drill into the detail of controls on ecosystem nitrogen retention.
Methods
A range of programmes and activities have collected nitrate isotope samples with a 100 mL target size, preserved for indefinite storage at pH 3 with 1 mL 10% HCl. Sulfanilic acid is added to remove nitrite prior to nitrate analysis, which
is performed according to a modified Cd-Azide two step reduction method for analysis as N2O at GNS Science’s Stable Isotope Laboratory in Gracefield. Precision and reproducibility is generally ≤0.3 ‰ for samples over 30 ppb NO3-N with lower limits for analysis ~3-5 ppb. Collections were generally aligned with monitoring or experiments measuring nitrate or total oxidized nitrogen using standard methods.
Results and Discussion
New Zealand’s observed pattern of dual-isotope nitrate suggests that many pastoral soils have good nitrate retention, such that added urine or urea intensifies nitrate leaching that remains buffered through the soil organic nitrogen pool, as indicated by δ15N-NO3. Within a variability of 1–2 ‰, nationwide studies find nitrate matching these conditions sits on a 1:1 line in bi-plots (see Figure 1) from δ15N-NO3 = 4 ‰ and δ18O-NO3 = 0 ‰ along a gradient of increasing agricultural intensity to δ15N-NO3 = 8‰ and δ18O-NO3 = 4 ‰. This defines a denitrification and mixing (DAM) line for integrated soil processes characteristic of good ecosystem nitrogen retention.
Details
- Title
- Tracing the sources of nitrate using isotopes: 10 years of progress for New Zealand freshwater
- Creators
- Troy Baisden - University of WaikatoTimothy Clough - Lincoln UniversityClinton RissmannNaomi Wells - Southern Cross UniversitySimon Stewart - University of WaikatoAndy Phillips - GNS ScienceSephrah Rayner - Lincoln UniversityRachel Murray - University of WaikatoChris Eager - University of WaikatoClaire Eyberg - University of WaikatoMeti Yulanti - University of Waikato
- Conference
- NZHS, Rivers Group & NZFSS Joint Conference (Invercargill | Waihōpai, 01/12/2020 - 04/12/2020)
- Publisher
- figshare
- Identifiers
- 991013154279402368
- Academic Unit
- Faculty of Science and Engineering; Science
- Language
- English
- Resource Type
- Conference paper