Quantity, distribution and source of DOC in the soil profile in fresh and dried samples

Ms Rubeca Fancy1, Dr Brian Wilson1,2, Dr Ben Greatrex1, Dr Heiko  Daniel1, Dr Paul Kristiansen1, Dr Yui Osanai1

1University Of New England, Armidale, Australia, 2NSW Office of Environment and Heritage, Armidale, Australia

The role of dissolved organic carbon (DOC) in soil carbon (C) cycling has recently become the focus of considerable international research attention. DOC is a labile fraction of soil organic matter and plays an important role in soil C dynamics contributing to soil C movement and storage following stabilization in the soil. Soil microbial biomass is considered to be a potential source of DOC and microbial metabolites constitute a significant proportion of DOC. Fluctuations in soil moisture create drying-rewetting events that affect microbial biomass as well as DOC quantity in the soil. We examined the quantity and distribution of DOC under different land-use systems (woodland, improved pasture and cultivated) in northern NSW, Australia. To determine the source of DOC under different soil conditions, DOC was extracted from fresh soils and from 40°C oven dry soils. DOC concentrations were higher in surface soils under woodland followed by improved pasture and cultivated soils and DOC diminished with depth in all soil profiles.  DOC concentrations were significantly higher in 40°C oven dry soils than fresh soil samples in every land-use system and site studied. DOC samples were further analysed using ¹H-nuclear magnetic resonance (NMR) spectroscopy to determine the source of DOC in soils. Peaks consistent with carbohydrates and aliphatic biomolecules were detected in ¹H-NMR spectra of DOC samples extracted from 40°C oven dried soils. However, in DOC samples derived from fresh soil samples, peaks for these organic compounds were undetectable.  This result suggests that drying of soils causes a liberation of organic compounds of microbial origin contributing significantly to DOC generation in the soil. Potential mechanisms and implications of these results are discussed.

Biography: Rubeca Fancy is a PhD student at the University of New England, Armidale, NSW, Australia. She is investigating the importance of dissolved organic carbon in soils to carbon storage and climate change mitigation.

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