The biogeochemical cycling of the essential micronutrient selenium (Se) is explored in a naturally seleniferous area of West Limerick, Ireland where selenium toxicity in livestock is a historical problem. Plant, soil and rock samples were collected to determine the distribution of selenium and other deleterious elements. Seleniferous soil horizons were selected for speciation, microbial, organic and sequential extraction analysis. The selenium distribution in soils was inhomogeneous with highly seleniferous concentrations of up to 1266 mg kg-1 identified in close proximity to deficient values. Overlying pastures contained concentrations toxic to livestock and translocation was highest in Trifolium repens (white clover) with 296 mg kg-1 Se in the leaves. The Brassica oleracea species (cabbage and kale) accumulated up to 972 mg kg-1 Se in the leaves showing phytoextraction potential. Selenium was present in the soil predominantly in a reduced phase as elemental Se (Se0) and the organic species SeMet (Se-II) which is bioavailable. SeMet was also the main species in the pastures and Se accumulator plants. Microcosm experiments showed anaerobic conditions decreased selenium solubility and bioavailability but the opposite occurred with molybdenum due to the reductive dissolution of FeIII(hydroxy)oxides. The microbial community remained relatively stable to changing physiochemical conditions which was dominated by Proteobacteria, Actinobacteria and Acidobacteria. Selenium positively correlated with the total organic carbon except for the soil profile overlying the Clare Shale Formation. Selenium was fractionated mainly as elemental selenium, in fulvic and humic acids and also recalcitrant organic matter. Soils were abundant in lipid moieties, in particular prist-1-ene which originates from chlorophyll and suggests selenium is associated with the burial of plant material. The Clare Shale samples contained up to 18 mg kg-1 Se and weathering and transport has caused enrichment in the low lying soils and pastures. The findings in this thesis show how variations in plant species, redox conditions, soil organic carbon content, and geology influence the distribution of selenium species in soils and thus the bioavailability and toxicity to livestock and human health.