Increased deposition of nitrogenous compounds from the atmosphere may lead to the competitive dominance of aggressive grass species and reductions in species richness. This thesis aims to investigate the effects of atmospheric deposition of ammonia on species-rich limestone grasslands in the Cotswolds, and the role that management may have in modifying these effects. A field survey investigated species composition and quantified environmental variables including atmospheric ammonia concentration. Species richness and diversity were greatest on the more heavily grazed sites on deeper soils, but these diverse grasslands were composed of a high proportion of nitrophilic, competitive and ruderal species. No relationships between atmospheric ammonia and species composition were detected. The effects of grazmg and nitrogen on species composition were investigated by transferring turves between sites with contrasting atmospheric ammonia concentrations. Although grazed plots at the high ammonia site were of slightly higher diversity than grazed plots at the low ammonia site, these differences were reversed in fenced plots. In ungrazed plots, the development of a grass dominated, species-poor community was less pronounced at the low ammonia site, although the higher level of nitrophilic/competitive species here suggested that soil depth and soil phosphorous may also be important. A controlled greenhouse experiment investigated the effect of nitrogen addition and cutting on the competition between Brachypodium pinnatum and Bromopsis erecta. Above ground growth of both species was limited by nitrogen, although growth of B. erecta appeared to be limited by another resource at high densities. There was significant niche overlap between the two species, though cutting of high density pots reduced this niche overlap. The two grasses were equally matched in competitive ability, and neither treatment had any clear effect on the outcome of competition. The tillering response to cutting was greater in B. pinnatum than in B. erecta, although nitrogen addition increased tiller production by B. erecta. The effects of nitrogen addition and cutting treatments on nine grassland species was investigated in a greenhouse experiment. Nitrogen addition had no significant effects, but cutting reduced the above ground biomass of B. erecta and B. pinnatum and increased the above ground biomass and size of most of the other species. It is concluded that atmospheric ammonia deposition appears relatively unimportant in determining the species composition of species rich grasslands. However, experimental addition of nitrogen may increase grass growth, and grass dominance in the field can lead to reductions in species richness. In grazed grasslands, grasses may become more grazing resistant under conditions of enhanced nitrogen availability. However, phosphorous may be an important factor modifying the effects of enhanced nitrogen deposition.