Soil carbon market-based instrument pilot: The sequestration of soil organic carbon for the purpose of obtaining carbon credits

Increasing soil organic carbon (SOC) in Australian farming systems has the potential to offset greenhouse gas emissions. Even though methods for soil carbon (C) sequestration have been developed under the Australian Government's Emissions Reduction Fund, the scope for farm-scale soil C sequestration is poorly understood. A pilot scheme was developed in Central West New South Wales to trial the use of a market-based instrument to encourage farmers to change farm management to increase SOC. This paper reports changes to SOC stocks measured on farms that were successfully contracted in the pilot. The 10 contracted farms were those that submitted the lowest bid per Mg CO2-e. Four land uses were contracted in the pilot: (1) reduced tillage cropping (reference); (2) reduced tillage cropping with organic amendments (e.g. biosolids or compost); (3) conversion from cropping land to permanent pasture; and (4) conversion from cropping land to permanent pasture with organic amendments. At each site a minimum of 10 locations (sampling points) were sampled and analysed for total carbon (LECO elemental analyser) and bulk density calculated. The SOC stocks (0-0.3 m) were assessed before (2012) and after the pilot (2017; calculated on equivalent soil mass of 2012), with 60% of sites showing a significant increase. Pasture had a higher rate of SOC sequestration than reduced tillage cropping (1.2 vs 0.28 Mg C ha-1 year-1, 0-0.3 m); and organic amendments had higher rates of SOC sequestration than without (1.14 vs 0.78 Mg C ha-1 year-1, 0-0.3 m). The results of the pilot demonstrated increases in SOC, using quantification methods consistent with the current Measurement Method of the Australian Government's Emissions Reduction Fund policy used to generate Australian Carbon Credit Units. The results require careful interpretation as rates of sequestration are likely to be lower in the longer term than initial rates of change seen in this pilot (five years), and the pilot intentionally selected sites with initially low SOC, which ensured a greater opportunity to sequester SOC.