Physical and biochemical characterisation of soil organic carbon in topsoil and subsoil of Irish grasslands

Soil plays a key role in the global carbon (C) cycle, since it holds 3.5 times more C than the atmosphere, and is the largest C pool after the oceans. Therefore, changes in soil organic carbon (SOC) can contribute to important emissions or sequestration of CO2 from the atmosphere. Grassland soils have are one of the world biomes with a greater C sequestration potential. More than 46% of SOC can be found below 30 cm, and the dynamics and factors affecting subsoil SOC are different than the topsoil. The aim of this thesis is to physically and biochemically characterise SOC in topsoil and subsoil of Irish grasslands, up to 1 m depth, and to understand if there is a soil type effect on SOC sequestration potential. A subset of soils sampled for National Soil Survey project (the Irish Soil Information System) in 2012/2013 were selected, with the aim of being representative of the main soil types occurring under grassland systems in Ireland. For each horizon, a 1 kg sample was taken, and several analysis were carried out: 1) separation of four aggregate sizes (Large and small macroaggregates, microaggregates and silt and clay), 2) isolation of fractions within small macroaggregates (particulate organic matter, microaggregates and silt and clay within macroaggregates), and 3) biochemical characterisation: non-hydrolysable C and N, as a measure of the biochemically recalcitrant fraction, and hot water extractable C and N, as a measure of the most labile fraction. Topsoil only showed differences between soil types in the case of percentage of microaggregates within macroaggregates, which was higher in Typical Sufrace-water gley than in Typical Brown Earth, while all the other analysed characteristics were equal between soil types. Subsoil horizons showed significant differences in many of the fractions. Differences in SOC characteristics were mainly influenced by clay content and stagnation properties.