The potential for enhancing soil carbon levels through the use of organic soil amendments in Queensland, Australia

Application of organic soil amendments such as manure or compost is commonly listed amongst strategies with potential to sequester carbon in agricultural soils, and Australian farmers are able to earn carbon credits for increasing soil organic carbon (SOC) stocks through the use of organic amendments under the Government’s Emission Reduction Fund. Despite their assumed contribution to enhancing SOC levels and inclusion into climate change mitigation strategies, there has been little qualitative or quantitative assessment of the effects of organic amendments on SOC stocks and dynamics. We evaluated this potential for Queensland (Australia) by collating and analysing information on organic amendments and by modelling soil organic carbon sequestration with the FullCAM model in three different cropping locations. An estimated 2.7 million tonnes dry matter (dm) of organic amendments, containing up to one million tonne (dm) of organic carbon was likely land applied in Queensland in 2015/16. Simulations predicted that, in favourable locations, high annual applications of raw manure and compost (10 t and 15 t ha^-1 yr^-1 fresh matter, respectively) could result in annual soil organic carbon increases of 0.9% and 0.55%, respectively, averaged over 20 years of continuous sorghum cropping. In less favourable conditions and with less frequent or lower applications, carbon stocks may continue to decline but more slowly than without organic amendments. The paucity of key data for more accurate assessments of carbon sequestration potentials led us to identify research priorities that support development of frameworks for use of organic amendments in agricultural soils for climate, food security and waste management benefits.