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

Abstract

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.

Determining the depth and rate of soil movement down the soil profile using an environmental tracer: a hillslope scale assessment

“…The rate and pattern of soil component movement down the profile will be location specific based on site geology, soil mineralogy, climate, vegetation and land use. The findings here suggest that soil materials can move relatively rapidly down the soil profile which suggests the potential for movement of SOC to depth (Hancock 2023a).…”

“…The method has been extensively employed in the study region to examine both soil erosion and soil organic carbon (SOC) transport (Martinez et al 2009(Martinez et al , 2010Wells and Hancock 2014;Hancock et al 2015Hancock et al , 2019. Due to its uniqueness in the environment and its delivery to the environment between 1945 and 1972, it can be used to provide an age constraint on soil material (Hancock 2023a). The University of Newcastle houses two p-type high purity germanium (HPGe) gamma ray detectors that were installed in 1992.…”

“…As an anthropogenic product and a relatively new addition to the environment, 137 Cs allows an approximate age of the labelled soil material to be determined (Hancock 2023a(Hancock , 2023b. For the Upper or top hillslope site, the maximum depth of 137 Cs labelled soil is 80 cm and that it 137 Cs can be assumed that arrived on the soil surface between 1945 and 1972.…”

“…Soil materials can move vertically down the soil profile by processes that include diffuse clay illuviation and solutes moving along a wetting front. More immediate or direct advective type processes such as soil cracks can provide direct access to the soil subsurface (Wells et al 2012(Wells et al , 2013Wells and Hancock 2014;Hancock 2023a). Diffusive processes are mostly controlled by soil texture with clay content being a major influence.…”

“…The study focusses on a section of hillslope where cattle are commercially grazed and an adjacent area where has been fenced off from cattle for 20 years to provide ecological services. The work builds on that of Hancock (2023a).…”