Greenhouse gas fluxes from grassland on organic soil used for beef grazing in the Irish midlands

Irish grasslands act as a significant carbon store, containing approximately 1.5 billion tonnes of carbon in grasslands under mineral and organic soils. Histosols, recognized as substantial carbon sinks, typically contain between 1000-4000 tC ha^-1, with carbon sequestration influenced by hydrological status, vegetation type, and associated management. It is currently estimated that more than 350,000ha of these soils are drained for permanent pasture in Ireland. Due to their high carbon stocks, these soils emit large quantities of carbon dioxide (CO₂) upon drainage, which is further accelerated by farm management to an estimated ~8 Mt CO2 eq yr^-1.

This study measured field-scale fluxes of carbon dioxide (CO₂) and methane (CH₄) to establish land-use and land management emission factors for grasslands situated on shallow-drained nutrient-rich histosols in 2023. This site has been actively rewetted through partial drain blocking, presenting a potential greenhouse gas (GHG) mitigation tool. The research contributes to more robust emission factors for key greenhouse gases, enhancing our understanding of the drivers of net ecosystem exchange (NEE). Additionally, it explores the relative impacts of changes in water table management on NEE and assesses how farm management influences the annual carbon budget.

The eddy covariance technique is employed in this study to gauge the impact of water table management on a grassland with organic soil used for beef grazing in the Irish midlands. In early 2023, a weir was installed at the field's border to limit water loss. This study examines the impact of this change on CO₂ and CH₄ fluxes, incorporating meteorological observations and farm management data. Early results indicate that the site was a net source of CO₂ in 2023, with variations in NEE throughout the year influenced by changes in water table height, meteorological conditions, and farm management. This investigation builds upon existing work in Ireland and other similar sites, comparing differences in management practices and evaluating their relative impact on a site's carbon balance. Furthermore, it addresses site-specific considerations when utilizing this data for national inventories or policy implementation.