Physicochemical protection is more important than chemical functional composition in controlling soil organic carbon retention following long-term land-use change

Understanding the mechanisms that control soil organic carbon (SOC) persistence is central to soil management and climate change mitigation. In the present study, we utilised a chronosequence of Vertisols which have undergone land use change from native vegetation to cropping for up to 82 y in subtropical Australia. We examined whether the marked changes in SOC concentrations were associated with changes in the physicochemical protection of SOC in aggregate structures (occlusion) and mineral surfaces (adsorption) or with changes in chemical functional composition. Soil samples were fractionated using density and physical fractionation to isolate the free particulate organic matter (fPOM), occluded POM (oPOM) and fine mineral-associated organic matter (fineMAOM) to assess the impact of land use change on soil organic matter (SOM) fractions with differing degrees of physicochemical protection. The impact of long-term cropping on SOC functional group composition across soil fractions was assessed using synchrotron-based near edge X-ray absorption fine structure (NEXAFS) analyses. We found that although long-term cropping caused a loss of 43 % of bulk SOC after 20 y, this marked loss over time was not associated with a change in C functional group composition. Furthermore, although the SOC retention in the various fractions differed up to 60-fold (fPOM-C decreased by 78 % after cropping for 20 y, whilst fineMAOM decreased by 25 %), there were only comparatively minor differences in SOC functional group composition between these fractions. Together, these findings suggest that the differences in C retention between fractions were less related to SOC functional group composition and more related to SOM's physicochemical protection.