The impact of carbon on microbial community dynamics in Irish grasslands

Grasslands are recognised as a major European sink for the long term sequestration of carbon (C), helping to offset greenhouse gas emissions. Such a scenario is particularly attractive in an Irish context as grasslands comprise 90 % of the land devoted to agricultural production. Thus a greater understanding is required of how C influences both the composition and functionality of soil microbial community, as this represents the most important biological component of these systems. The results of this thesis demonstrated that both the physical protection of carbon (soil aggregates) as well as carbon addition through agricultural management (i.e, slurry addition) impacts on the soil microbiome in Irish agricultural grasslands. Specifically it was demonstrated that (i) aggregate size was a significant determinant of both bacterial and fungal community composition down the soil profile of Irish grasslands. (ii) The functionality of the soil microbiome responds in a staged manner to slurry derived carbon at the field scale while the composition of the microbiome is more influenced by seasonal succession. (iii) Over a 1 year experimental period, both aggregate size and slurry applied C significantly influenced potential extracellular enzymatic rates with this activity being temporally dynamic. A thorough comprehension of the spatial and temporal influence of both soil aggregates and agricultural fertilisation regimes on the constitution and activity of the soil microbiome is critical if we are to elucidate the microbially mediated mechanisms of terrestrial carbon cycling in Irish grasslands.