Whole Ecosystem Nitrogen Manipulation: An Updated Review
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.
- 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
- The design of experimental manipulation studies has been
carefully evaluated, and only studies which meet specific criteria
have been considered in this review.
- 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
- 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.
- 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.
- 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.
- 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
- 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.
- 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
- 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
- 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.
- 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.
- 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.
- Bryophytes and lichens appear to be particularly sensitive
components of most ecosystems.
- 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.
- The importance of other limiting nutrients, particularly
phosphorus, and interactions with habitat management, are major
issues for both grasslands and heathlands.
- 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.
- 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.
- 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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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