Ammonia :: Neutral Grassland

Effects and implications

[Ammonia is one of the key pollutants that contribute to nitrogen deposition. Please read the Nitrogen deposition - Neutral Grassland  record to understand the full impacts effects of nitrogen deposition including ammonia.]

• Increase in proportion of productive grasses at expense of fine leaved grasses.
• Reduces species richness, especially among forbs
• May reduce productivity
• Changes in species composition
• Loss of rare or endangered species.
• Loss of annuals and forbs will have implications for sustainability, due to the short lived nature of the seed bank (Bossuyt et al 2005) and the isolation of many of these small areas of grassland.
• Loss of floral diversity, especially forbs/ herbs can reduce nutritional quality, nutrient levels, for grazing animals (Hopkins & Holz 2006).

Below ground changes

• Reduction in pH in the surface top cms of soil.
• Increase in acid cations e.g. Al and Mn
• Likely to increases rates of mineralization and nitrification

Overview: evidence, processes and main impacts

We are not aware of any in situ manipulation studies of ammonia impacts on this ecosystem. The comments below are based on what is known about the components of the ecosystem.

These mesotrophic grasslands represent semi-natural swards dominated by grasses with associated dicotyledonous herbs occurring on lowland clays/loams, pH 5.5-7, that tend to retain the moisture, and can often flood during the winter. Some examples include forb rich hay meadows which are managed by low input farming operations e.g. spring grazing (organic source of fertilizer), hay cut followed by autumn grazing. These ecosystems are generally nutrient depleted, because of long-term agricultural use with low levels of manure and removal of plant parts by grazing or hay-making. Their area has reduced hugely as agriculture has intensified and farming methods have changed. They are characterized by many species of low stature and of nutrient-poor soils. Appropriate management is crucial to maintaining their existence (Vickery et al 2001). Evidence from the UK Countryside Survey (Haines-Young et al. 2000) has shown a decrease in the floristic diversity of hay meadows that has been shown to be correlated with anthropogenic N deposition but there is no data on ammonia effects. Lichens and bryophytes are very sensitive to NH₃ but are not a key component of this ecosystem.

These communities are at risk from a range of anthropogenic influences: nutrient run off, selective herbicides, lack of or unsuitable management, all round horse grazing (poaching) since many are close to suburbia, neglect, age of owners and thus change of land ownership, isolation through urban and communication encroachment. Many occur within rural areas and are thus surrounded by potential sources of ammonia. Protection is afforded through implementation of the new Critical Levels for NH₃ (UNECE 2007). Because these systems’ existence is dependent on maintaining their nutrient poor status, through grazing and mowing off-take, they will be susceptible to NH₃-N deposition. The contribution by NH₃ to the total N deposition to unfertilized and semi-natural vegetation is likely to be more significant than the effects of direct exposure to NH₃, unless slurry or fertiliser are  applied regularly to nearby fields or the fields support a heavy stocking density or the grassland lies close to a point source. P availability is also likely to be low and may be important in controlling eutrophication responses in these ecosystems. Probable effects include: increase in proportion of productive grasses at expense of fine leaved grasses; reduced species richness, especially among forbs; reduced productivity; change in species composition through loss of rare or endangered species. The loss of annuals and forbs will have implications for sustainability, due to the short lived nature of the seed bank (Bossuyt et al 2005) and the isolation of many of these small areas of grassland. Likewise loss of floral diversity, especially forbs/ herbs can reduce nutritional quality, nutrient levels, for grazing animals (Hopkins & Holz 2006).

Pollutant deposition type and risk areas

Indicators of NH₃ enrichment

• Visible direct toxic effects unlikely

• Increase in proportion of productive, tall grasses
• Reduction in proportion and diversity of fine leaved grasses
• Loss of forb species.
• Loss of species with a preference for wetter conditions.
• Decrease in soil pH
• Accumulation of NH₄⁺ in soil solution

Examples of species specific responses

Some examples of specific responses are given in the table below. This does not represent a comprehensive review of all species impacts.

What factors modify N deposition impacts?

• Grazing management: too low grazing intensity will exacerbate effects of N stimulated nitrophilous grass growth
• More frequent mowing or harvesting may be necessary to remove N to compensate for the additional anthropogenic N.
• Some soils may be P limited, so increasing P availability will increase the impact of N on growth, eutrophication effects via competition for other limiting resources, changing the balance between species and ecosystem stability.
• Likelihood of flooding which can potentially replace nutrients removed by grazing and lead to a system that will promote growth in the presence of N deposition.