Examining correlations between organic carbon chemistry and δ15N abundance in soils across an aridity gradient

Dr Mark Farrell1, Ms Janine McGowan1, Mr Steve Szarvas1

1CSIRO, Glen Osmond, Australia

Storage of carbon and nitrogen (collectively soil organic matter; SOM) in soil is heavily influenced by climate change. Generally, more SOM is stored in soils due to greater inputs by plants in areas with higher rainfall. Organic nitrogen however, is mineralised by microbes into the more accessible nitrate which would show a tendency to leach out of soil in high rainfall. The effects of moisture changes on soil organic C and N across a 900 km aridity gradient in South Australia was examined. This transect was established under the Terrestrial Ecosystem Research Network (TERN) and our aim was to understand the transformations of soil organic C and N within it. Samples were collected from 42 long-term monitoring sites following the mainland portion of the Adelaide geosyncline, from the Fleurieu Peninsula in the south to Murnpeowie Station in the north. The sampling was carried out in the 2016 Austral autumn, comprising a composite of 20 individual surface soil samples from within a 25×25 m plot established at the corner of each site. Soil organic matter chemistry was quantified by 13C-CP/MAS NMR and δ15N was measured by EA-IRMS. The alkyl/o-alkyl (A/OA) ratio derived from NMR measurements is a measure of how much SOM is turned over or decomposed. The A/OA ratio was weakly positively correlated with aridity (R2 = 0.148), whereas δ15N showed a much stronger relationship (R2 = 0.515). Taken together, these data indicate greater cycling of SOM and fewer fresh inputs as aridity increases.


Janine McGowan is a Research Officer at CSIRO in Adelaide and has 30 years’ experience in soil organic matter characterisation through analytical expertise in soil fractionation, carbon and nitrogen analysis and NMR spectroscopy.

Janine has a Bachelor of Science degree (Hons) in Organic Chemistry from The University of Adelaide.

Steve Szarvas is a Research Support Officer at CSIRO and has over 20 years of experience in various analytical methods involving nitrogen and carbon, particularly related to soil and organic matter (SOM) turnover and more recently, food provenance. His expertise includes the use of N-15 tracers and subsequent isotopic analysis using Isotope-ratio mass spectrometry techniques. Steve has an Associate Diploma in Analytical Chemistry from the University of South Australia.

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