Mrs Elaine Mitchell1, Professor Peter Grace1, Dr. David Rowlings1, Professor Francesca Cotrufo2, Professor Richard Conant2, Dr. Clemens Scheer1
1Queensland University of Technology, Brisbane, Australia, 2Colorado State University, Fort Collins, USA
Soil organic matter (SOM), the largest terrestrial carbon (C) pool, is fundamental to soil and ecosystem functions across a wide range of scales, from site-specific soil fertility and water holding capacity to global biogeochemical processes that influence carbon-climate feedbacks. Although management practices such as crop residue retention and organic amendments result in SOM accrual, their contribution to mitigating climate change may be offset by increased greenhouse gas (GHG) emissions, particularly in finer textured soils due to enhanced microbial activity. Understanding the balance between ‘new’ SOM from residue inputs and how this interacts with native SOC and GHG fluxes is critical to assessing the effectiveness of land management practices as a climate change mitigation strategy. We tracked the fate of above-ground residues into functionally different SOM pools (persistent mineral-associated OM versus unprotected OM) and GHG fluxes using isotopically labelled residues (13C and 15N) over 12 months in a pasture soil in sub-tropical Australia. Residues were placed on three different soil types with varying texture and mineralogy within close proximity (< 2 km2) to each other. Residue management was simulated by either placing residue on the soil surface (i.e., no tillage) or incorporating it with the top 10 cm of soil (i.e., tillage). Soils with greater clay content resulted in a greater amount of SOM formation in more persistent mineral-associated SOM fractions. However, the greater SOC accrual in finer textured soils was offset by C priming of native SOC, resulting in a net C source. The incorporation of residue resulted in ~ 4 to 5 fold increase in SOC formation, highlighting that no-till systems might not always promote C sequestration. Overall results demonstrate the need to consider both soil properties and residue management as they affect the climate change mitigation potential of residue amendment to a grassland soil.
Elaine is currently a student at Queensland University of Technology. Her PhD focuses on using isotopically labelled biomass to trace the fate of above-ground residues to SOC and GHGs. Elaine completed her undergraduate and Masters education in the UK (Durham University and Cambridge University). Prior to her PhD, Elaine worked in the EU-CDM framework, initiating and maintaining agro-forestry projects in developing countries.