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Sequestering soil organic carbon (SOC) has been identified as a critical tool to mitigate anthropogenic climate change. Therefore, the response of SOC to climate variability needs to be quantified to achieve carbon sequestration over decadal to centennial scales. This study assesses the temporal variability of SOC for a grazed catchment in Australia that has been exposed to periods of drought and flood over the last 100 years. The analysis utilises field-measured SOC data obtained 2006–2018, along with modelled SOC data derived from a linear regression model and BIOS2 model outputs (1908–2015). Catchment SOC did not vary significantly, with mean values of 3.40% (2006), 3.21% (2014) and 3.10% (2018), despite the catchment experiencing extreme drying and wetting events during the sampling period. This stability was further supported by the long-term modelling, which showed a mean SOC of 3.68% and coefficient of variance of 0.16%. SOC was believed to be buffered against short term fluctuations in climate that persist for less than the SOC turnover times (i.e., multi-decadal). This indicates that temporal changes in SOC reflects only long-term climate variability. Current SOC and sequestered SOC are therefore unlikely to become a carbon sink in the short-term due to climate variability. [ABSTRACT FROM AUTHOR]

The factors determining the spatial distribution of soil organic carbon (SOC) at large-scales closely align with bioclimate regions; reflecting climate, ecosystem and soil properties. Recent studies of the Köppen-Geiger climate zones of Australia have highlighted an extension of the hot, arid, steppe environment from central Australia into the southeast (SE) under future climate change scenarios (2071–2100 under RCP 8.5). As SOC concentrations are highest in Australia's SE, it is important the effect of this shift is quantified. This study assesses this and how changes in the factors that control SOC formation may alter SOC concentrations. Field measured SOC concentrations were compared to current climate, soil, topography, vegetation, and soil erosion variables for 12 grassland sites from SE to NW Australia. SOC concentrations ranged from 0.39% in northwest (NW) and Central Australia to as high as 6.88% in the SE. Using regression analyses; temperature, elevation and Normalised Difference Vegetation Index were found to be the only significant drivers (α = 0.95) of SOC across the sites. Partial correlation analyses then identified temperature, elevation and clay content as imparting a significant effect on the relationships between SOC and water availability variables. This indicates that an extension of the arid environment into SE Australia may lead to a decrease in SOC (up to 1.12%), as mean annual temperature exceeds threshold values that limit SOC concentration. This is significant as the majority of Australia's SOC is stored in this area and these environments exert a strong influence on global carbon cycling. • Temperature is a major control soil organic carbon at a national scale. • Field data was collected from 12 grassland sites across Australia. • Soil organic carbon concentrations are linked to Köppen-Geiger climate class. • Warming under climate change will shift Australia's climate zones. • Temperate sites will cross thresholds limiting soil organic carbon concentration. [ABSTRACT FROM AUTHOR]