Plant biomass inputs and soil organic carbon dynamics in woodlands and pastures of central Queensland

Mr Stuart Irvine-Brown1, Dr Usha Pillai-McGarry2, Professor David Mulligan2, Prof Damian Barrett3

1Queensland Dept. Agriculture and Fisheries, Nambour, Australia, 2Sustainable Minerals Institute, The University of Queensland, St Lucia, Australia, 3Commonwealth Scientific and Industrial Research Organisation, Canberra, Australia

Above and below ground plant biomass inputs to soil and their effect on soil organic carbon (SOC) dynamics within single and mixed species woodland and pasture areas of Central Queensland was investigated.  The major pathway by which carbon is naturally sequestered in soil is through the delivery and decomposition of plant derived inputs.  Uncertainty remains about the source and turnover of plant biomass inputs to soil from woodland and pasture vegetation types found in Central Queensland.  This study reports on the quantification of above and below ground plant biomass inputs (excluding coarse woody debris), its decomposition over 48 weeks, and its subsequent influence on SOC and constituent fractions within single and mixed species woodland and pasture systems common to scrublands and rangelands of Central Queensland.  In situ decomposition of plant components (leaves, sticks/twigs/stems, bark-flowers and fruits, and roots) was followed by SOC analysis for carbon fractions.  Site specific field measurements were used to improve existing default parameter settings to calibrate the FullCAM model for predicting change in carbon stocks from different types of plant inputs into soil over time using long-term weather records.  Results indicate there is greater SOC content in mixed woodland in comparison to single species woodland, and in single species pasture in comparison to mixed species pasture, although the difference was not statistically significant (p>0.05).  SOC decreased, particularly in the ratio of particulate organic carbon (POC) to humus organic carbon (HOC), over the duration of litter decomposition.  This study improves the understanding of how different forms, quantities, and quality of plant biomass inputs relate to increases and decreases in SOC at a site specific scale.  Comparison of 45-year (1970 to 2015) FullCAM model simulations with measured field study values showed encouraging carbon estimation in woodland but large disparity with quantified pasture values.


I am a soil scientist and horticulturist working across the state of Queensland on horticultural agronomy research and industry development for extension on issues of sustainability, resource base conservation and improvement to catchment scale water quality. My interests lie in sustainable international agriculture and natural resource management with a key focus on innovation and diversity in sub-tropical and tropical horticultural production systems linked to soil science.

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