When does organic carbon induce aggregate stability in vertosols?

• Aggregate stability is positively related to organic carbon content in vertosols.
• Macroaggregate stability is positively related to soil organic carbon content.
• Organic carbon influences soil chemistry and consequently microaggregate stability.
• Trees and shrubs provide complex organic carbon inputs to soil ecosystems.
• Undisturbed natural ecosystems provide an important erosion mitigation service.

Two percent organic carbon (OC) (grams C per gram soil) provides a threshold for soil stability; below this threshold, soils become highly erodible as macroaggregates slake to form microaggregates. However, it is a long-held belief that a degree of slaking upon rapid wetting of aggregates is an inherent trait of vertosols, regardless of OC content. This is attributed to their physico-chemical attributes (e.g. high clay content, shrink–swell capacity, cation exchange capacity and pH). Studies investigating the erodibility of vertosols have concentrated on cropping soils, usually with low OC content (≤2%). Therefore, the importance of OC in maintaining structural stability and minimising erosion in vertosols is often dismissed. This study examined vertosol macroaggregate and microaggregate stability in natural ecosystems where OC can be >2%. We found a positive relationship between macroaggregate stability and OC content in vertosols, especially when OC was ≥3.5% in the surface soil (0–5 cm). Microaggregate stability was attributed to the dominance of Ca2+ over Na+ on clay-exchange sites. OC was positively correlated with Ca2+ and negatively correlated with Na+ and ESP. OC may play a role in microaggregate stabilisation through its capacity to lower soil pH and increase the availability of Ca2+. We demonstrate that OC can stabilise vertosol aggregates, and is therefore important in preventing erosion on this soil type.