Nitrogen deposition :: Neutral Grassland

Effects and implications

• Increases in proportion of productive grasses at expense of fine leaved grasses.
• Reduces species richness, especially among forbs
• Possible reductions in 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 increase rates of mineralization of organic N, conversion to NH₄⁺ and then nitrification and possibly loss as the GHG N₂O.

Overview: evidence, processes and main impacts

In order to understand how N deposition will affect these grasslands we need to put their fertility status into context: semi-natural swards dominated by grasses with associated dicotyledonous herbs without the calcifuge / calcicole element on lowland clays / loams, ie. mesotrophic, moist  and potentially high yielding when fertilized, pH 5.5-7, e.g. forb rich hay meadows, generally managed by low input farming operations e.g. spring grazing (organic fertilizers), hay cut, grazing autumn, possibly winter flooding . These ecosystems are generally poor in nutrients, because of long agricultural use with low levels of manure and removal of plant parts by grazing or hay-making. They are characterized by many species of low stature and of nutrient-poor soils. Vickery et al (2001) reports on the management of some of these communities with respect to optimising conditions for birds, but including how management (grazing, mowing fertilizing) and the timing of such activities affect native flora and soil fauna and the consequences of N inputs as fertilizer which are relevant for predicting N deposition effects in the absence of limited data.  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.

Major threats to these communities include nutrient run off, selective herbicides, lack of or unsuitable management, all year 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.

Because these systems are generally nutrient poor as result of grazing and mowing off-take, they will be sensitive to N deposition. N deposition has the potential to both eutrophy and acidify these grasslands. NPK fertilization produces species poor stands dominated by a few highly productive crop grasses (Park grass experiment Rothamsted). Acidification will be damaging, because this might lead to high concentrations of potentially toxic NH₄⁺ ions and increase the solubility of toxic cations e.g. aluminium. P availability is also recognised as being important in controlling eutrophication responses in these ecosystems.

Pollutant deposition type and risk areas

Indicators of N enrichment

• Increase in canopy height, 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?

These systems require specific management to prevent natural succession to woodlands (Dierschke 1985). Appropriate type, level and timing of grazing and or mowing are crucial to their sustainability, especially under conditions of enhanced N deposition. Grazing, mowing or harvesting (hay crop) will help offset N enrichment. N deposition will eutrophy and, depending on the availability of P, promote growth and expansion of faster growing grasses. Grazing or mowing will help maintain a more open canopy structure keeping robust grasses in check.

• 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 improving 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.