Investigating the Impact of Climate and Extreme Weather on Greenhouse Gas Emissions from Irish Soils: An Evaluation of the ECOSSE Model

Soil is a complex material capable of storing and releasing large amounts of carbon, making it an integral part of the global carbon cycle. As soil is included in national inventory assessments where countries quantify their emissions, it is important that estimates of the soil carbon flux are as accurate as possible. Measuring the flux of carbon from every surface on the planet is unrealistic, therefore modelling emissions is the next best method of emissions assessment. Among modelling approaches, process-based modelling allows for the highest specificity of input data. Process-based models split soil carbon into pools with different decay rates influenced by environmental (biotic and abiotic) factors and management. The Estimation of Carbon in Organic Soils – Sequestration and Emissions (ECOSSE) model has previously been recommended as the optimum process-based model for simulating greenhouse gas (GHG) emissions from Irish soils. Results from the site-specific evaluation of the ECOSSE model for an Irish arable site showed a temporal offset between measurements and model outputs which could not be explained by model parameters, motivating further in-depth analysis of the model. This subsequent analysis indicated that the rate modifiers, which control the release of carbon from different pools in the soil, were functioning as intended, but highlighted issues with the model simulation of soil water. The modelled soil dried out fully when in reality it was above field capacity, attributable to erroneous simulation of evaporation on this well-drained sandy soil. In parallel, a spatial process-based model (GlobalECOSSE) was employed to derive national emissions estimates for multiple GHGs and CO2 equivalents for designated agricultural land uses on the island of Ireland. The justification for using the spatial process-based model was to provide an assessment of a model that had fewer input requirements and operated on a monthly timescale to simulate areal emissions. Results showed soil carbon emissions are enhanced during warmer months and lessened during colder ones, and showed cropland and grassland to be CO2 sources on an annual timeframe. The importance of examining emissions holistically is emphasised as some Irish grasslands are GHG sinks when other GHGs such as methane and nitrous oxide are included, and all croplands are GHG sources. To assess the susceptibility of Irish soils to unusual or extreme weather events, statistical sampling of past weather events was employed to generate inputs for GlobalECOSSE. The response of Irish soils to extreme weather events shows warmer temperatures enhance respiration, while drought conditions restrict it. Warm and wet conditions enhance respiration most as respiration increases with temperature and is not limited by moisture availability in this scenario. Including all GHGs produces similar patterns with most Irish soils being C sinks under normal climate and hydrological conditions, and more areas becoming sources during hot, very hot and hot & wet conditions (derived from extremes in the observed record). These results are presented in the context of the uncertainties in observations (large ranges across datasets and methods of partitioning fluxes) and in modelling (different models giving different magnitudes and even directions of change). More observations, experiments and modelling studies across land-use and climate types are needed before confident projections can be given. Observational networks should be placed strategically to determine responses from common soil and land-use types, and moisture regimes. Current action should focus on maintaining the carbon already in soils by avoiding disturbance, and promoting sensible sequestration practices such as cover-cropping, minimum/no tillage and incorporation of straw/manure