Report 331
Whole Ecosystem Nitrogen Manipulation: An Updated Review
(2002)
Cunha, A.,Power, S.A., Ashmore, M.R., Green, P.R.S., Haworth B.J. and Bobbink, R.
Over the past two decades, concern over the effects of atmospheric nitrogen deposition on terrestrial ecosystems has led to the instigation of a number of field manipulation studies. These were reviewed in an earlier report, commissioned by English Nature. This review updates and revises that earlier report, based on a comprehensive review of all relevant published literature since 1997.

Executive Summary

  1. Over the past two decades, concern over the effects of atmospheric nitrogen deposition on terrestrial ecosystems has led to the instigation of a number of field manipulation studies. These were reviewed in an earlier report, commissioned by English Nature. This review updates and revises that earlier report, based on a comprehensive review of all relevant published literature since 1997.
  2. The design of experimental manipulation studies has been carefully evaluated, and only studies which meet specific criteria have been considered in this review.
  3. This revision includes an interpretation of new data from a number of important long term field experiments in the UK, as well as additional information on interactions between nitrogen deposition and habitat management. Furthermore, in light of recent assessments that UK nitrogen emissions have stabilised and, in the case of nitrogen oxides, even begun to decline, this updated review also considers studies which have looked at rates and indicators of ecosystem recovery.
  4. Nitrogen manipulation experiments in forest ecosystems have shown a relationship between deposition inputs and a range of effects. However, the effects of different manipulation studies are highly variable, depending on factors such as soil type, deposition history, stand age, and various biotic interactions.
  5. Reported responses include both increases and decreases in tree growth, improvements or imbalances in foliar nutrition, increased incidence of pests and pathogens, and changes in the root system. Soil biology and chemistry are also affected in forested ecosytems, with functional changes in nutrient cycling widely reported; acidification and eutrophication lead in several studies to increased leaching of base cations and increased availability of aluminium and hydrogen cations.
  6. Changes in the species composition of the ground flora, mycorrhizae and macro-fungi have been found in several studies, with a general increase in more nitrophilous species.
  7. Despite differences in both soil type and climate, responses of Calluna-dominated heathland and moorland ecosystems to N addition in the UK have been fairly consistent. Early responses include increased Calluna shoot growth, canopy height, canopy density, flowering and litter production, whereas prolonged exposure results in reduced root:shoot ratios and an acceleration of the Calluna life cycle.
  8. In both UK and continental European experiments, increased nitrogen deposition has been related to increased vulnerability to biotic and abiotic stresses, such as frost, drought and herbivory. Effects on mycorrhizae are varied, with both increases, decreases and no change reported in response to nitrogen. Observed increases in soil microbial activity and biomass, and consequent effects on decomposition and mineralisation rates, have implications for the turnover and availability of both nitrogen and phosphorus. Differences between immobilisation and mineralisation rates following N addition appear to reflect the degree of nitrogen saturation of a heathland.
  9. In grassland ecosystems, nitrogen additions over 5-10 years have resulted in changes in community composition. Lichens, bryophytes forbs and dwarf shrubs are frequently decreased, while grasses typically increase in dominance. These changes are generally associated with a reduction in species richness and diversity of grassland communities following nitrogen addition.
  10. Evidence that nutrient cycling may be affected by increased nitrogen availability has also been found for grasslands: Nitrogen mineralisation rates and soil bacterial biomass and activity have increased in response to nitrogen addition. Whilst many similarities in response exist between acidic and calcareous grasslands, the former are typically more vulnerable to the acidifying effects of nitrogen (particularly reduced N) inputs, while interactions with phosphorus availability are important in the latter.
  11. In bog ecosystems, nitrogen addition improves growth and survivorship of some moss species, and decreases that of others, with consequent shifts in bryophyte species dominance. In the longer term, it may also cause a reduction in diversity as bryophytes and other low-growing plants are out-shaded by taller species, including grasses. These changes may be related to penetration of nitrogen through the moss layer into the rooting zone, as the capacity to immobilise inputs is exceeded.
  12. The few data available on fen ecosystems indicate that high levels of nitrogen deposition cause changes in community composition, with a reduction in species diversity and bryophyte biomass. Changes in species composition and tissue chemistry are also reported for tundra ecosystems.
  13. Overall, there are some similarities in response between different ecosystem types, as well as some common mechanisms underlying the observed changes. Nitrogen deposition typically alters the competitive ability of many plant species, resulting in a shift towards more N-tolerant species. The response of a number of species groups is similar across a range of ecosystems, with a general pattern emerging for a reduction in forbs and dwarf shrubs and an increase in grasses.
  14. Bryophytes and lichens appear to be particularly sensitive components of most ecosystems.
  15. Although not all experiments have reported shifts in community composition in response to nitrogen addition, results consistently suggest a disruption of normal plant and microbial physiology and/or function. Changes in, for example, foliar chemistry or microbial activity may thus be early indicators of the potential for deleterious responses at the community or ecosystem level, in the longer term.
  16. The importance of other limiting nutrients, particularly phosphorus, and interactions with habitat management, are major issues for both grasslands and heathlands.
  17. Although there are relatively few experiments which have aimed to assess the rate of recovery from ecosystem eutrophication, there is convincing evidence that many ecosystem effects may persist for many years, with recovery only occurring over time scales of many decades, if at all. Indeed, the loss of species from more sensitive ecosystems (such as bogs or tundra systems) may prevent their re-establishment over any realistic time scale, in the absence of active re-introduction and restoration measures.
  18. The evidence from nitrogen manipulation experiments demonstrates a significant cause for concern over the impacts of nitrogenous pollutants on sites of nature conservation value. Given the very slow natural rates of recovery once changes in species composition have occurred, and the costs of management interventions to accelerate this process, there is a clear need for a precautionary approach to minimise the risk to sensitive communities across the UK.
  19. This review has identified a number of gaps in current knowledge and highlights the need for further research to improve understanding of the responses of the diverse range of (semi-)natural ecosystems in the UK to nitrogen, and to provide a more informed basis for assessment of appropriate critical loads to prevent long-term effects.
 
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108 pages
ISSN 0963 8091
 
Please cite as: Cunha, A.,Power, S.A., Ashmore, M.R., Green, P.R.S., Haworth B.J. and Bobbink, R., (2002), Whole Ecosystem Nitrogen Manipulation: An Updated Review, JNCC Report 331, 108 pages, ISSN 0963 8091