Improved methodologies for SOC measurement, estimation and reporting its density changes in agricultural soils

Dr Mohammad Ibrahim Khalil1, Dr Bruce Arthur Osborne1

1UCD SBES and CRAES-EI, University College Dublin, Dublin 4, Ireland

Reduction of greenhouse gas (GHG) assessment uncertainties and improvement in the quantification of sinks and offsetting mechanisms are required to develop appropriate mitigation measures aimed at keeping global temperature <2oC. The key factors that are needed to fulfil these objectives are a precise, verifiable estimation of soil organic carbon (SOC) and its variation at field scales. For SOC measurements, land use (LU)/soil type-specific and consistent sampling protocols (e.g., method and timing of samplings) together with a consideration of other factors (e.g., soil moisture and carbon) that influence soil mass and volume, are required. For accurate estimations, the determination of SOC by ‘mass by volume’ on an equal soil mass basis, in a defined but adjustable soil layer and reporting is essential. The Intergovernmental Panel on Climate Change proposes proportional (%) approach, as a SOC density (here)/stock change factor (DCF), for application across key agricultural LUs, managements and inputs. Methodologies developed with higher spatial resolution databases, coupled with two-phase modelling and GIS approaches, could provide robust estimates. However, the DCF factors overestimated the SOC density changes for organo-mineral (12-19%) and organo-mineral plus organic soils (33-81%) compared to mineral soils. This resulted in a corresponding increase in national SOC stock estimates by, on average, 14 and 20%. The corrected estimates showed a sequestration rate of 0.04, 0.12 and 0.42 t C ha-1 yr-1 in Irish agricultural soils, and a potential GHG offsetting of 1.20, 2.93 and 5.41 Tg C yr-1 for the 0-10, 0-30 and 0-100 cm soil layers, respectively. These findings suggest the replacement of the apportioning approach, including 4‰ concept, by a ‘mass by area’ (depth-specific) one for more precise estimations. This will include the disaggregation of soil types, and the calculation of country-specific DCFs and weighting factors for individual LUs, management practices and inputs for upscaling to regional/international level.


Dr. M. Ibrahim Khalil is a Sr. Environmental Scientist and Modeler attached with University College Dublin, Ireland, and leading a multi-disciplinary research group (Climate-Resilient Agri-Environmental Systems, CRAES). Dr. Khalil is an agricultural graduate with honours, double masters and PhD in environmental soil science, completed with a Belgian scholarship. He was an awardee of IAEA traineeship and did post-doctoral research with the prestigious fellowships awarded by the Royal Society (UK), Alexander von Humboldt Foundation (Germany) and Japan Society for the Promotion of Science (Japan). Dr. Khalil  has been leading and coordinating a large number of externally-funded research projects, and published more than 150 scientific papers in peer-reviewed journals/book chapters/proceedings; member of editorial board and reviewer  for international journals. He has expertise in the key research areas of Biogeochemistry of Carbon and Nitrogen Cycles, Monitoring, Modelling and Mitigation of Greenhouse Gases/trace gases and SOC density/stocks; Climate Change and Adaptation.

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