Reduced nitrogen has a greater effect than oxidised nitrogen on dry heathland vegetation

We investigated the effects of different ratios of reduced (NH4+) versus oxidised (NO3−) nitrogen in deposition on heathland and species-rich grassland vegetation at high nitrogen deposition levels in large mesocosms filled with nutrient-poor soils to which different NH4+/NO3− ratios were applied. The response of the forbs, Antennaria dioica, Arnica montana, Gentiana pneumonanthe, Thymus serpyllum, the grasses Danthonia decumbens, Deschampsia flexuosa, Nardus stricta and the shrub Calluna vulgaris was recorded. The forb A. dioica and the grass D.decumbens preferred low NH4+/NO3− ratios and were characterised by a negative correlation between NH4+/NO3− ratios and biomass and survival, whereas the grasses N. stricta and D. flexuosa showed no correlation with NH4+/NO3− ratios. Lime addition eliminated the negative effects of high NH4+ concentrations in deposition for A. dioica and the grass D. decumbens. The implications of these findings for heathland vegetations are discussed.