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

Application of organic amendments such as livestock manures and compost is commonly listed amongst strategies with potential to sequester soil organic carbon (SOC) in agriculture and contribute to climate change mitigation. However, quantifying this potential is hampered by the paucity of data on amounts and characteristics of organic amendments applied to land, and limited understanding of the carbon dynamics during storage, processing and following land application. The objective of this study was to evaluate this potential for the State of Queensland, Australia, by collating and analysing information on organic amendments and modelling SOC sequestration in illustrative cropping locations. An estimated 2.7 million tonnes (Mt) dry matter (dm) of organic amendments has likely been land applied in Queensland in 2015/16, supplying significant quantities of carbon (C) to the soil. Simulations with Australia’s national inventory modelling tool predicted that, in a favourable location, high annual applications of manure and compost (10 t / 15 t fresh matter (fm) per hectare and year (ha−1 yr−1) could result in SOC increases of 0.9% and 0.55%, respectively, per year averaged over 20 years of continuous cropping, exceeding the aspirational goal of the United Nations Framework Convention on Climate Change 4 per 1000 Initiative. In less favourable conditions, C stocks may continue to decline but at a slower rate than without organic amendments. Based on regional analysis and review of current understanding of the dynamics of organic matter in soils, we identified a set of research priorities to enable more accurate assessments of the C sequestration potential to support development of policies and frameworks for use of organic amendments in agricultural soils for climate, food security and waste management benefits.