Understanding and quantifying whole soil-profile organic carbon transfer using an environmental tracer

Context. Quantifying soil organic carbon (SOC) depth distribution and its vertical transport is needed for both improved understanding of soil properties and behaviour as well as enhanced organic carbon sequestration. This is a global issue, that if better understood, could result in both more agriculturally productive soils as well as enhanced environmental outcomes. Aims. Quantify whole soil-profile SOC and down-profile movement at a series of sites in south-east Australia. Methods. Soil is sampled at regular intervals using cores and assessed for SOC and environmental tracer (137Cs) concentration. Key results. Soils that have a high clay content (Vertosols) and crack (i.e. self mulching) have the highest SOC content. In high clay content soils, 137Cs is present at depths well below that at which it would be present by diffusive processes. Conclusions. Surface soil, labelled with 137Cs is moving down the soil profile by advective processes to depths well below that possible by diffusive processes alone. Using local erosion rates and carbon input, it is estimated that less than 1% of SOC is delivered to the cracking soils by erosional processes and that the majority of SOC must be produced in situ. Implications. Given that 137Cs is a relatively new environmental tracer (1945 onwards), this suggests that surface labelled soil is reaching depths of up to 80 cm at decadal time scales. The methods and findings here have global applicability and provide insights into potential enhancement of carbon sequestration in both cropping and grazing landscapes.