Abstract of PhD Thesis

The ecology of soil microbial communities in acidic upland grasslands

Deirdre Rooney (2006) - University College Dublin

Grasslands are a major ecosystem in temperate zones, including Ireland, and are valuable in terms of agriculture, environment and ecology. In Ireland, a common feature of upland areas such as the Wicklow Hills is plant-species rich semi-natural grasslands, dominated by Agrostis capillaris. The past few decades have seen a trend towards agricultural intensification, with ‘improvement’ of semi-natural grasslands a common occurrence. During improvement, semi-natural grasslands are converted to plant species-poor mesotrophic grasslands, dominated by Lolium perenne and Trifolium repens through re-seeding, fertilisation and liming. Although intensification is central to the productivity of Irish grasslands, concerns have been raised regarding the impacts of agricultural improvement on environmental processes, particularly the basic functioning of grasslands. In this study, a range of biochemical and molecular techniques were used to elucidate microbial community response to grassland management regimes, both in field and microcosms situations.

An area of typical Nardo-Galion (U4a) grassland at Longhill, Co. Wicklow was chosen for part of this study. This site comprised mainly unimproved U4a grassland (soil pH 4.0, approx.), and an area of agriculturally improved Lolio-Plantiginion (MG7b) grassland (pH 6.5 approx.) enclosed in one corner, with a gradient of improvement (zone of floristic transition) between. Community structure of the ammonia oxidising bacteria, a key functional group in N-cycling, was investigated. Differences in AOB community structure in U4a and MG7b grasslands and across the gradient were evident, implying a response to improvement practices. Phylogenetic analysis also revealed that the majority of AOB species present belonged to the genus Nitrosospira. Further AOB analysis involved the use of two microcosm experimental systems investigating: (1) NH4NO3 addition, plant species (A. capillaris and L. perenne) and lime; and (2) the effects of synthetic sheep urine, plant species (A. capillaris and L. perenne) and time on AOB community structure. Microcosm studies revealed that addition of ammonium nitrate (NH4NO3) had a major influence over AOB community structure, but plant species alone had little effect, except when plant species interacted with NH4NO3. Lime appeared to have relatively little effect on overall community structure, yet positively influenced the abundances of some AOB ribotypes. Synthetic sheep urine (SSU) addition also had an effect in determining AOB community structure, with many AOB ribotypes responding negatively to SSU application. Notably, plant species type was an important consideration as to the extent to which SSU affected AOB community structure.

Increased grazing pressure on grasslands is commonly concurrent with improvement. The effects of SSU addition, plant species and time on total bacterial and fungal communities were investigated. Application of SSU significantly increased soil pH, with pH typically increasing dependent on the concentration of SSU applied. SSU addition significantly altered bacterial community composition in grassland microcosms, but this response was dependent on which plant species were present, and the time after SSU application. Fungal communities also responded to SSU addition, with SSU and plant species both contributing to shifts in fungal community structure. Another aspect of improvement, phosphate addition, was studied using a separate set of microcosms. Phosphate addition slightly changed the composition of both bacterial and fungal communities, and was also a major determinant in both bacterial and fungal community structure. Significantly, the interactions with plant species and phosphate were often more important than the effect of plant species as a single variable.

In general, shifts in bacterial, fungal and AOB community structure associated with upland acidic grasslands appeared to be mediated through a complex set of interactions, involving both plant species and chemical treatments.