Nitrogen uptake and nutrient limitation in six hill moorland species in relation to atmospheric nitrogen deposition in England and Wales

1 Samples of shoot tissue of six hill moorland species were obtained from upland sites in England and Wales in 1996 and 1997 and analysed for nitrogen (N) content, annual accumulated N per hectare and nitrogen to phosphorus (N : P) ratio. The species sampled were: Calluna vulgaris in four growth phases (pioneer, building, mature and degenerate), Vaccinium myrtillus, Molinia caerulea, Nardus stricta, Eriophorum vaginatum and Agrostis/Festuca. 2 Sites dominated by each species were sampled in eight upland regions from Northumberland in the north‐east to Dartmoor and Exmoor in the south‐west, and shoot N variables were related to atmospheric N deposition data by regression analysis. For each species and Calluna type in each year, separate analyses were carried out against oxidized N (NOₓ) deposition, reduced N (NHₓ) and total N deposition. Shoot N variables were also tested for the influence of altitude and temperature. 3 None of the plant tissue variables tested was significantly correlated with NHₓ deposition and only Nardus N : P ratios were correlated with total N deposition. However, for all species except Nardus, Eriophorum and Molinia, shoot N concentration and/or assimilated N per hectare were significantly correlated with NOₓ deposition in at least one of the two years, as were N : P ratios for Calluna, Vaccinium and Nardus. 4 Most of the significant relationships were recorded with material sampled in 1997, rather than in 1996. 5 In terms of increased shoot N concentration, the response of mature Calluna was lower than that shown by previous work encompassing a different range of locations in the United Kingdom. 6 Increases in the amount of N per hectare accumulated in the current season's Calluna shoots exceeded corresponding increases in estimated annual deposition rates by several‐fold, implying that N deposition has enhanced the accumulation of organic N in upland soils, leading to an increased pool of mineralizable N. 7 Significant relationships between N deposition and N : P ratios in shoot tissue suggested that this accumulation has changed a substantial proportion of Calluna‐dominated uplands from N‐limited ecosystems into P‐limited ones. This could favour species such as Molinia that are better adapted to P limitation.