Alternatives to Equivalent Soil Mass in Monitoring, Reporting and Verification of Changes in Soil Carbon

Sequestering carbon in soils is a key action to address climate change and food security. Schemes incentivising farmers to change land management practices to sequester more carbon in soils are underpinned by soil monitoring protocols. Accurate estimation of soil organic carbon (SOC) stocks is essential for the integrity of such carbon credit schemes. Common SOC estimation methods like sampling to fixed depth are prone to errors due to changes in bulk density over time, particularly under changing management practices. Equivalent Soil Mass (ESM), utilising a reference mass rather than a reference volume for SOC estimation, arguably alleviates this. In practice, the potentially large variation in sampled core lengths (and thus masses) still introduces substantial variability into SOC estimates. This work compares four approaches to SOC estimation: (1) ESM 10 , based on the 10th percentile of sampled masses, currently implemented in the Australian Soil Carbon Method; (2) ESM μ , based on the mean of sampled masses, and (3) EBD 10 and (4) EBD μ , built on the concept of equivalent bulk density (EBD), based on either the 10th percentile or mean of the average soil sublayer bulk densities. Variability in ESM and EBD and in resulting SOC estimates was quantified using soil cores from nine intensively sampled farms in eastern Australia. The proposed alternatives, particularly ESM μ and EBD μ offered more stable and accurate SOC estimates, reducing variance by up to 38%–86% compared to ESM 10 . These findings can be applied to support the evolution of improved methods of soil carbon monitoring, reporting and verification in Australia and internationally.