What can isotopes tell us about the controls on the coupling/decoupling of soil nitrogen and carbon?

Dr Naomi Wells1, Ms Kate Summer1, Dr Jeff Baldock2, Dr Mark Farrell2

1Southern Cross University, East Lismore, Australia, 2CSIRO Agriculture & Food, Adelaide, Australia

The isotopic composition of both the nitrogen (δ¹⁵N) and C (δ¹³C) entrained in soil organic matter (SOM) are altered by local biogeochemical reactions. They thus represent a potential integrative measure of SOM function that could bridge the distance between low cost, low sensitivity measures of bulk SOM stoichiometry and highly sensitive, but high cost, emerging SOM characterisation techniques. Here we tested this potential by using archived soil samples to explore δ¹⁵N and δ¹³C variations, 1) between climate and agriculture regions, and, 2) between physically separated SOM fractions. The re-analysed soils were collected at two depths during two previous recent national soil surveys (BASE for soils from natural ecosystems (0-20, 20-40cm), SCaRP for soils from agricultural ecosystems (0-10, 10-20cm)). For (1), we analysed soils from 413 sites across New South Wales. For (2), we analysed the previously separated ‘humic’ and ‘particulate’ components in soils from 300 farm sites across Australia. We found that δ¹³C values were strongly aligned with land-use type (pasture, cropping, or un-cultivated). In contrast, δ¹⁵N values in un-cultivated lands were strongly influenced by climate (aridity index). Cultivated soils deviated from this apparent climate-driven relationship. This suggests that δ¹⁵N could be used to indicate where and when agricultural soils are ‘regenerating’ (moving closer to the expected relationship) or ‘degrading’ (moving farther from the expected relationship) through time. Overall our results suggest that measuring variations in soil δ¹⁵N and δ¹³C can add value to established metrics of SOM functionality.


Naomi Wells is a Lecturer in the Centre for Coastal Biogeochemistry at Southern Cross University. She is a biogeochemist who uses stable isotopes to understand how nitrogen moves through land and water. After realising that most reactive nitrogen in the world is bound in organic matter her research has expanded to try to integrate the carbon and nitrogen worlds. She received her PhD from Lincoln University (NZ) in 2014, an MSc from the University of Aberdeen, and a BA from Wellesley College.

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