Changes in agricultural soil carbon at a continental scale – results from Australia’s Soil Organic Carbon Monitoring Project

Australia’s Soil Organic Carbon Monitoring (SOC-M) project is a landmark study that revisited 308 sites 10-12 years on from the original baseline survey in 2009-12, yielding >3600 individual data points. It provides the first comprehensive quantification of SOC stock change (0-30 cm) as a result of pedoclimatic and management factors at the continental scale. Sites are on active commercial farms across Australia, with a focus on the major land-uses of dryland cropping and pasture. Because of the reduced sampling scope possible within SOC-M, a hybrid model-based site-selection framework approach to site selection ensured broad environmental and management representativeness while accommodating logistical constraints. Soil samples were analysed using methods consistent with the baseline survey for SOC stock and fractionation into functionally discrete particulate, humus, and resistant carbon pools. A number of other parameters were also quantified including total major/minor elements, pH, clay, and electrical conductivity. Rich data on management were combined with modelling approaches to estimate plant C inputs to soil over the period between the two sampling times. As an average across all sites, a modest increase in SOC stock was observed, although this was unevenly distributed (median +0.06 Mg C ha⁻¹ y⁻¹, 80% between -0.85 to +1.04 Mg C ha⁻¹ y⁻¹). While climate and soil type were dominant drivers of SOC dynamics, stubble retention, reduced tillage intensity, and maintenance of perennial pastures were consistently linked to positive SOC stock change. Indeed, carbon inputs—influenced by climate, soil, and management—were a key driver of SOC stock change.