What change does the quantity of root mass and seasonality make to soil organic carbon?

Miss Ivanah Oliver1, Dr Brian Wilson1,2, Dr  Oliver Knox1, Dr Richard Flavel1

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

In recent years, considerable research interest has focused on soil carbon (C), particularly with respect to the potential for soils to store additional C in the context of climate change mitigation. To fully understand this potential it is necessary to determine the contribution and stability of C from the various sources. Roots and exudates have received little attention with regard to their contribution to the soil organic C pool.


We aimed to determine the contribution of ‘new’ C inputs from the root system and their stability down the soil profile across multiple growth and decomposition phases.  Specifically, our objectives were to investigate 1) whether the amount of ‘new’ C reflects the amount of root biomass found across the depths, and 2) if multiple growth and decomposition phase’s results in an accumulation of ‘new’ C.


Rhodes grass (Chloris gayana), was grown in a pot experiment using a soil with a history of C3 plant growth. The study ran for two seasons, with each season consisting of a growth phase and decomposition phase of three months. The above-ground biomass was removed at the end of the growth phase. After each phase a 10 pot subset was destructively harvested for soil and root analysis, at four depths. Isotope Ratio Mass Spectrometry (IRMS) δ13C analysis of soil and roots was used to determine the distribution of ‘new’ C inputs through the soil profile and between treatment phases.


The results showed that even though the upper depth had approximately double the root mass compared to all other depths, there was no significant difference in the soil δ13C. The relationship between soil δ13C and treatment phase was unexpected, particularly in the second growth phase where the soil became depleted of 13C despite the ‘new’ C inputs from the C4 plants.


Ivanah is a PhD candidate at the University of New England. Her research focus is on root carbon inputs into the soil.

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