Organic carbon stocks in agricultural soils in Ireland using combined empirical and GIS approaches

Substitution of the Intergovernmental Panel on Climate Change (IPCC) default methodology by country-specific activity data is recommended for improved estimation of baseline soil organic carbon (SOC) stocks and their changes. In the Republic of Ireland (ROI), previous studies focused either predominantly on grassland or on all land cover types but were depth-limited. To improve the accuracy, Tier 2 approaches are proposed by the IPCC. This requires an analysis of high spatial resolution databases (such as the Irish NSDB – National Soil Database) and maps, collated for major land cover, soil types and land use areas in Ireland. In this study, data were overlaid using ArcGIS to derive information for disaggregated soil types and agricultural land use areas. Empirical models were developed using separate measurement data to estimate the NSDB-derived SOC concentrations for deeper layers, using a depth distribution function and the bulk density (ρd) using pedotransfer functions. The soil type specific models (R2 = 0.87–0.99) had an improved estimate of SOC densities when mineral and organic soils (peat) were treated separately. The estimated SOC densities for grasslands on mineral plus organo-mineral soils at the 0–10, 0–30, 0–50 and 0–100 cm depths were 52.2, 127.1, 170.9 and 213.8 t C ha− 1, respectively. For arable lands, the corresponding SOC densities were 29.9, 81.3, 117.6 and 167.5 t C ha− 1. Nationally, for all soil types, the corresponding stocks (the products of SOC density and land cover area) were estimated to be 246.9, 608.1, 829.5 and 1079.3 Tg for grassland, and 13.5, 36.7, 50.2 and 67.0 Tg for arable lands in the three soil layers. The total national SOC stocks were estimated to be 888 at 0–30 cm and 1832 Tg at 0–100 cm reference depths. For the complete soil profile, including peats > 100 cm depth, the national estimate was 2824 Tg. The combined empirical models and Geographical Information System technique provide robust estimates of SOC stocks for disaggregated land covers and soil types, enabling Ireland to consider moving from Tier 1 to Tier 2 accounting methodology. This improved national inventory of the ROI is important for estimates of the C stock related to the Land Use, Land Use Change and Forestry (LULUCF) categories.

► High spatial resolution Irish databases and maps were used to estimate SOC stocks.
► Data were overlaid using ArcGIS and empirical models were developed.
► SOC density of 0–30 cm was 127.1 t C ha− 1 for grassland and 81.3 for arable lands.
► For the complete soil profile, the national estimate for SOC stock was 2824 Tg.
► The coupled empirical models and GIS technique provide robust estimates of SOC stocks.