The influence of fine fraction content on storage and retention of soil organic carbon in Vertisols of subtropical Australia

• Soil organic carbon (SOC) was unrelated to fine fraction content in Vertisols. • SOC coverage was low and patchy regardless of mineral surface area. • Occluded particulate organic matter did not increase with fine fraction content. • Increasing fine fraction did not enhance SOC retention under long-term cropping. The proportion of fine mineral particles in soils is a broadly used predictor of soil organic carbon (SOC) storage based on the potential enhancement of organo-mineral associations with increasing mineral surface area. However, the influence of increasing fine fraction content on SOC pools remain understudied in high clay soils (>50 % clay). We investigated SOC storage across paired native and cropped Vertisols from subtropical Australia, spanning a range of fine fraction (soil minerals < 53 µm) contents from 481-927 g fine fraction kg −1 . The free particulate organic matter (fPOM), occluded POM (oPOM) and fine mineral-associated organic matter (fine-MAOM) were isolated by density and particle-size fractionation to assess SOC storage along the fine fraction gradient. The role of other mineral properties, such as specific surface area, was also examined. Overall, native soils revealed SOC in the unfractionated soil and the SOC fractions were poorly related to fine fraction content. Moreover, specific surface area increased substantially with fine fraction content, but SOC coverage on mineral surfaces was low (15–23 %), suggesting that MAOM formation is unrelated to the mineral surface area. By comparing native and cropped soils, we found that greater fine fraction contents did not enhance SOC retention in any fraction in the paired-cropped soils, indicating SOC retention is decoupled from fine fraction content in these Vertisols. We conclude that although fine-MAOM is important for SOC storage (storing 46–85 % of SOC in these soils), improved soil practices should prioritise the management of POM to maintain SOC in the fine-MAOM fraction of fine-textured Vertisols.