Mineral removal and its influence on thermal step-ramping estimates of resistant organic carbon in fine soil fractions

There is a growing interest in characterising distinct forms of soil organic carbon (SOC) with contrasting turnover times. In Australia, a well-established approach combining physical size fractionation (coarse >50 µm and fine <50 µm) with chemical characterisation has been developed to quantify a resistant organic carbon (ROC) fraction, commonly referred to as ‘char-like’ carbon. This fraction is typically characterised by ¹³C nuclear magnetic resonance (NMR) spectroscopy. However, hydrofluoric acid (HF) pre-treatment is often needed to remove paramagnetic soil minerals that interfere with magnetic resonance measurements and to concentrate carbon sufficiently to generate a detectable ¹³C NMR signal. It is recognised that this pre-treatment, while necessary to enable NMR analysis, may affect SOC chemistry, generating artefacts. While the combined size-fractionation scheme followed by NMR analysis has evolved over nearly two decades, increasing attention is now being given to comparisons between ‘char-like’ carbon estimates derived from combining size fractionation with ¹³C NMR analysis and those obtained using thermal step-ramping methods. In this study, we hypothesised that removing mineral particles (clay + silt) from the fine fraction does not significantly affect estimates of ROC derived using stepwise thermal ramping, quantified between 400°C and 600°C. To this end, we analysed a total of 111 soil samples collected from diverse soil types across Australian agricultural production regions. Topsoil samples were collected at three depth intervals (0-10 cm, 10-20 cm, and 20-30 cm). Fine-fraction ¹³C NMR-based ROC estimates were directly compared with the thermal step-based ramping ROC estimates, analysed both before and after HF pre-treatment of the fine fraction. Key findings of this research will provide underpinning knowledge to assess whether the removal of the mineral phase by HF treatment affects estimates of the ROC fraction derived from stepwise thermal ramping. The results further enable evaluation of similarities and differences between ROC or ‘char-like’ carbon concentrations estimated using ¹³C NMR and thermal step-ramping methods for the fine fraction of soils, with important implications for method application and inter-model comparisons.