Effects of particle size fractions and clay content for determination of soil organic carbon and soil organic matter

Soil organic matter (SOM) and soil organic carbon (SOC) influence soil behaviour, quality, and applications. While conversion equations exist, the GlobalSoilMap Science Committee (2015) determined that no universal equation is viable due to regional soil variability. Instead, region-specific equations are required. This study presents SOM and SOC data from 75 sites for soils at 0–5 cm and 5–15 cm depth intervals, aligning with GlobalSoilMap specifications, allowing for global applicability of research findings. While this study develops region-specific SOM-SOC conversion equations for southeastern Queensland, the findings contribute to the broader understanding of how soil properties influence SOM-SOC relationships. The methods used, particularly clustered regression based on clay content and particle size, can be applied in other regions worldwide to refine soil carbon estimates in diverse landscapes. Regression analysis showed that the selected particle size fractions (<2 mm vs. <0.5 mm) had minimal effect on the dry combustion (DC) method but significantly influenced loss on ignition (LOI) results. A regression slope of 0.827 indicated that < 2 mm samples yielded higher SOM values than < 0.5 mm samples, introducing uncertainty in LOI-derived estimates. Clay content further impacted conversions: samples with < 15 % clay produced robust equations with high coefficients of determination, whereas those with > 15 % clay exhibited increased variance, lower slopes, and reduced accuracy, necessitating caution in their application. This research improves understanding of SOM-SOC relationships across key depth intervals and particle size fractions. It highlights the influence of clay content and particle size on LOI and DC methods, refining the accuracy of SOM and SOC estimation. These findings are valuable globally for soil science, agriculture, hydrology, and engineering, aiding more precise soil carbon assessments in various environmental and land management applications. • Selection of particle size fractions affect LOI results. • High clay content decreases certainty of SOM to SOC conversion. • Tailored SOM to SOC conversion equations are presented. • Results align with depth intervals specified by the GlobalSoilMap.