Fate and cost effectiveness of soil carbon sequestered using supplementary nutrients applied to crop residues under field conditions

Changes in carbon (C) and nitrogen (N) associated with fine fraction (FF) soil were determined to a depth of 180 cm at a field site that was managed with supplementary nutrients (2007–2012) to sequester soil C from crop residues. Soil C and N stocks were monitored for a further 3 years (to 2015) with supplementary nutrients maintained, and then for 5-years after they were ceased (2015–2020). The increase in FF-C established in the first phase of nutrient addition (9.6 t C ha − 1 ) was maintained during the ongoing 3-yr period of nutrient addition (9.8 t C ha − 1 ) but declined to 3.0 t C ha − 1 during the subsequent 5-years when nutrient addition ceased. The decline in FF-C stocks was attributed to nutrient mining through increased N mineralization because of insufficient N being applied to fully satisfy the N removed in crops. The addition of supplementary nutrients to the residue influenced crop yield and seed protein in some years, but the responses were crop and season dependant and there was no overall difference in the amount of C in crop biomass returned to the soil. An economic analysis using long-term fertiliser and grain prices suggested that the strategy to sequester C in FF-SOM through nutrient addition to residue was essentially cost neutral. Better tailoring of nutrient inputs to achieve the required stoichiometric ratios in SOM, and an assumed payment for the sequestered C (AU$40 t − 1 CO 2 equivalent) provided a beneficial economic outcome during the period of nutrient addition. However the economic benefit was lost when nutrient addition ceased, and losses were significant if fertiliser prices increased or the C price declined. Our results confirm that addition of supplementary nutrients to C-rich crop residues is an effective means to sequester soil C that can persist for several years even after nutrient addition ceases. However, ongoing nutrient inputs in excess of crop removal are required to ensure long-term sequestration of the C from crop residue.