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

  • Kirkham F
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1Samples 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 (NÂ :Â 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. 2Sites 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. 3None of the plant tissue variables tested was significantly correlated with NHx deposition and only Nardus NÂ :Â 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 NÂ :Â P ratios for Calluna, Vaccinium and Nardus. 4Most of the significant relationships were recorded with material sampled in 1997, rather than in 1996. 5In 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. 6Increases 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. 7Significant relationships between N deposition and NÂ :Â 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.

Author-supplied keywords

  • Atmospheric nitrogen deposition
  • Calluna
  • Nutrient limitation
  • Uplands

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