Soil organic carbon (C) is an essential component of the global C cycle. Processes that control its composition and dynamics over large scales are not well understood. Thus, our understanding of C cycling is incomplete, which makes it difficult to predict C gains and losses due to changes in climate, land use and management. Here we show that controls on the composition of organic C, the particulate, humus and resistant fractions, and the potential vulnerability of C to decomposition across Australia are distinct, scale-dependent and variable. We used machine-learning with 5,721 topsoil measurements to show that, continentally, the climate, soil properties (for example, total nitrogen and pH) and elevation are dominant controls. However, we found that such general assessments disregard underlying region-specific controls that affect the distribution of the organic C fractions and vulnerability. This can lead to misinterpretations that prejudice our understanding of soil C processes and dynamics. Regionally, climate is mediated through interactions with soil properties, mineralogy and topography. In some regions, climate is uninfluential. These results highlight the need for regional assessments of soil C dynamics and more local parameterization of biogeochemical and Earth system models. Our analysis propounds the development of region-specific strategies for effective C management and climate change mitigation.
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The data sets generated during and/or analysed during the current study are not publicly available but are available from the corresponding author on reasonable request. The digital soil maps of the particulate, humus and resistant fractions are available for download from https://doi.org/10.25919/5ca56d1d0166b.
The code used for the machine-learning modelling is available from the corresponding author on reasonable request.
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We thank the Australian Government Department of the Environment and Energy for funding work, as part of their progressing improvements of the FULLCAM model and the National Greenhouse Gas Inventory System, which led to the development of this research. We also thank S. Tuomi, P. Leppert, M. Virueda and G. Navarrette for their help with the spectroscopic measurements. We thank SCaRP for the collection of soil samples and their analysis. SCaRP was funded by the Climate Change Research Program of the Australian Department of Agriculture and the Grains Research and Development Corporation. We also thank the NGSA team and Geoscience Australia for the sampling, preparation and provision of the NGSA samples.
The authors declare no competing interests.
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Supplementary Description, Supplementary Figs. 1–6 and Tables 1–6.