Agricultural improvement of seminatural grasslands has been shown to result in changes to plant and microbial diversity, with consequences for ecosystem functioning. A microcosm approach was used to elucidate the effects of two key components of agricultural improvement (nitrogen addition and liming) on ammonia-oxidising bacterial (AOB) communities in an upland grassland soil. Plant species characteristic of unimproved and improved pastures (A. capillaris and L. perenne) were planted in microcosms, and lime, nitrogen ( N H 4 N O 3 ), or lime plus nitrogen added. The AOB community was profiled using terminal restriction fragment length polymorphism (TRFLP) of the amoA gene. AOB community structure was largely altered by N H 4 N O 3 addition, rather than liming, although interactions between nitrogen addition and plant species were also evident. Results indicate that nitrogen addition drives shifts in the structure of key microbial communities in upland grassland soils, and that plant species may play a significant role in determining AOB community structure. 1. Introduction The impact of anthropogenic activities on soil biodiversity is central to our understanding of the links between soil functional diversity, species diversity, and overall ecosystem functioning. Agricultural improvement of natural upland pastures is widespread in NW Europe, with increased fertilisation, liming, and grazing producing a shift in the floristic composition of acidic upland grasslands [1, 2]. Such intensification practices result in a gradual shift from a plant species-rich Agrostis capillaris pasture to a species-poor grassland dominated by Lolium perenne [3], with concurrent changes in soil physicochemical properties [1, 4, 5], most notably soil nutrient status. Nitrogen pools in particular have been shown to be held in different ratios between unimproved and improved grasslands, with ammonium dominating unimproved pastures, while nitrate is prevalent in improved pastures [5]. Soil bacterial and fungal communities are also understood to undergo changes in response to agricultural management [4–7], so it is likely that specific functional groups, such as ammonia-oxidising bacteria (AOB), may be similarly affected. The initial step in nitrification—the conversion of ammonium to nitrite—is microbially mediated by ammonia oxidisers via the enzyme ammonia monoxygenase (AMO). In recent years, exploitation of the amoA gene as a molecular marker and the application of community fingerprinting techniques have revealed considerable AOB diversity [8]. While previous studies have
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