Improving estimates of soil organic carbon (SOC) stocks and their long-term temporal changes in agricultural soils in Ireland

The Paris Agreement emphasises a need to reduce greenhouse gas (GHG) assessment uncertainties, together with the improved quantification of sinks and offsetting mechanisms, in order to develop appropriate mitigation measures aimed at keeping global temperature below 2 degrees C. Although the soil organic carbon (SOC) pool has the potential to act as a major source or sink of GHGs, depending on land use (LU) and soil type, it remains largely unquantified. In order to provide improved SOC assessments, depth-distribution models and pedotransfer functions were refined, and soil (National Soil Database and Indicative Soil Type) and land use (Land Parcel Identification System: 2000-2014) maps overlaid using ArcGIS to identify and categorize long-term LU changes for different soil types/categories. The range of SOC density (SOCp) for the different LUs was in the order, rough grazing (R) > grassland (G) > rotation/ley (GT) > tillage (T), and was highest in the organic soils. For the 0-30 cm soil layer, the SOCp value (t C ha(-1)) measured in 2006 was 242 (R), 207 (G), 162 (GT) and 80 (T). The reference year (1990) values (SOCpref), predicted using two-phase models, was 238 (R), 198 (G), 166 (GT) and 99 (T), t C ha(-1). Based on the SOCp ref values, G and R were sinks, whilst T and GT were sources. Regardless of LU, management and input, the IPCC default density change factor (DCF, %) overestimated the annual rate of SOCp change for the organ-mineral (42-124%) and organic (73-156%) soils compared to mineral soils. The corrected annual carbon sequestration rate for the four agricultural LUs combined, over 25 years, was 0.23, 0.42 and 0.53 t C ha(-1) yr(-1) for the 0-10, 0-30 and 0-100 cm layers. The corresponding national agricultural SOC stock (SOCs) for 2006 were 316, 838 and 1679 Tg. The long-term projections resulted in a carbon sink of 1.24, 3.09 and 5.48 Tg C yr(-1), respectively, demonstrating a potential to offset 24, 59 and 106% of the total GHGs emitted from Irish agriculture. These results show significant deficiencies in the IPCC methodology and that integrated approaches can provide robust estimates of SOCp/s and their long-term changes for disaggregated LUs and different soil categories. This study suggests replacement of the apportioning approach by depth-specific "mass by area" values for precise estimations of SOCp/s and their changes. It also emphasises the need for country/region-specific DCFs for individual LUs/management interventions, together with the use of specific weighting factors.