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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Abstract

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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