Nitrogen deposition :: Acid grassland

Effects and implications

• Nitrogen deposition provides a fertilization effect on acid grasslands which are generally N limited.
• N deposition favours graminoids (grasses) at the expense of forbs and lower plants, especially where sites are surrounded by farmland.
• Nitrophilous grasses tend to shade out slower growing species.
• Nitrophilous grasses increase the amount of litter which falls on and shades out under-storey bryophytes (Berendse et al 1987).
• Lower plants especially mosses at risk both from N accumulation (Carroll et al 2003).

Overview: evidence, processes and main impacts

Some components of acid grassland are able to flourish and increase their share of the canopy space under enhanced N deposition, leading to loss of species that are not able to increase their rates of C assimilation, through shading. UK surveys  (Maskell et al 2010; Stevens et al 2004, 2010) show clear, nitrogen  driven declines in species richness, predominantly amongst the forbs e.g. Cerastium arvense, Viola canina  and the N fixing forb, the annual Vicia lathyroides (Spring vetch) (Dupré et al 2010; Maskell et al 2010; Stevens et al 2006). Functional diversity is lost as grasslands became more grass dominated, showing increasing canopy height, leaf area index and productivity (Stevens et al 2011).  The loss of forbs will have implications for pollinators and the aesthetic qualities of the ecosystem. Vulnerability to N deposition is enhanced where grasslands are surrounded by seed sources for nitrophilic species: agricultural areas provide pools of propagules of N responsive species particularly graminoids.

Pollutant deposition type and risk areas

Indicators of N enrichment

• Reduced forb cover (Stevens et al 2006) and increase in grass to forb ratio (Stevens et al 2009).
• Absence of nitrophobic species that characterise the habitat.
• greater presence of nitrophilous species and higher Ellenberg N.
• Increase in canopy height
• Reduced bryophyte cover (Carroll et al 2000).
• Soil acidification
• Increased rates of N mineralization, increasing NH₄⁺ concentrations in soil solution and potential for toxicity.
• Increased root surface phosphatase enzyme activity can be measured in response to nitrogen (Johnson et al 1999).

Examples of species specific responses

What factors modify N deposition impacts?

• Grazing management: reduced grazing intensity will exacerbate effects of N stimulated nitrophilous grass growth.
• High grazing intensity can exacerbate detrimental effects of N on ericoid components (Emmett et al 2001).
• Availability of other nutrients through their effects on growth responses.