Nitrogen deposition :: Calcareous grassland

Effects and implications

• Reduces species richness and/or species diversity/changes species composition
• Loss of subordinate vascular plants e.g Thymus
• Loss of rare or endangered species.
• Loss of characteristic calcicolous mosses and lichens at risk from shading and N accumulation.
• Increase in non-native calcifuge species
• Increase in Hypochaeris radicata (Carroll et al 1997). Negative effects on nitrogen fixing legumes.
• Increased risk of drought effects.
• Loss of annuals and forbs will have implications for recovery, due to the short lived nature of the seed bank (Bossuyt et al 2005).

Below ground changes

• Reduction in pH in the surface top cms of soil.
• Increase in acid cations e.g. Al and Mn
• N deposition increases rates of mineralization and nitrification, (Phoenix et al 2003) although as soils acidify with the release of protons nitrification rates may slow leading to higher NH₄⁺ concentrations, which can be toxic to many calcicoles.
• Increase in soil N pool, which may have implications for the future habitat quality
• These soils tend to be on the dry side with low denitrification rates, leaching losses are also low leading to N retention within the system (Phoenix et al 2003).

Overview: evidence, processes and main impacts

These low productivity grasslands occur on shallow, well buffered soils (with a calcium carbonate content of ~10%), where the availability of both N and P is low (co-limited). The co-limitation by P, which reflects the higher soil pH and its influence on P solubility, has an important modifying influence on N deposition effects. This is because growth stimulation will be species specific, because of insufficient P, and many of the eutrophication / competition effects associated with N will be more muted, making the ecosystem less vulnerable to N deposition.  

Calcareous grasslands, partly because of their low productivity, tend to be very species rich, containing many rare or endangered species and some of these species may be highly sensitive to N deposition effects. N deposition will favour those species that can tolerate low P availability under conditions of improved N supply and can also effectively scavenge and monopolise that additional N through storage e.g. Brachypodium pinnatum with underground rhizomes (Bobbink et al 1998). The expansion of the rough grass Brachypodium pinnatum in calcareous grasslands has been attributed to excess atmospheric ammonia contributing to N deposition (Baxter and Farmer 1994). This is quite a robust grass and will change the canopy structure of the grassland at the expense of lower plants and especially lichens. The increased grass cover will also suppress the growth of annuals via effects on germination. Species that form mycorrhizal associations might be expected to flourish, as these will improve P foraging. In addition to high pH, these systems also tend to be classified as dry (Bobbink & Hettelingh 2011). Increased growth may give rise to biological drying of these thin shallow rooting soils as the potential for evapotranspiration increases. Thus species of Carex (sedges) and Juncus (rushes) will decline as grasses e.g. Lolium perenne and Holcus lanatus increase (Jeffrey & Piggott 1973).

Studies in Europe and the chalk grasslands of southern England provided much of the early data on N responses (Bobbink et al 1998). The longest UK N addition experiment was established in 1995 (Morecroft et al 1994; Carroll et al 2003; Phoenix et al 2003) White Peak area of Derbyshire, Wardlow Hay Cop, a limestone hill NNR, UK (NGR SK 1773). The calcareous vegetation is classified as a CG2d, Festuca-Avenula (Helictotricon).  Data are summarised in Phoenix et al 2012.

Pollutant deposition type and risk areas

Indicators of N enrichment

• Loss of forb species.
• Increase in proportion of tall grasses including Increase in cover of false brome grass (Brachypodium pinnatum) at the expense of overall species diversity.
• Increase in competitive species and plant productivity inferred from increased canopy height and specific leaf area.
• Increase in Ellenberg N
• Loss of species with a preference for wetter conditions.
• Decrease in soil pH
• Accumulation of NH₄⁺ in soil solution
• Increased rates of root surface phosphomonoesterase activity to increase P mineralisation rates.

Example evidence 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). Grazing, mowing or harvesting can help to offset N enrichment, as losses from gaseous emissions or leaching of inorganic N are relatively small. Similarly grazing or mowing will help maintain a more open canopy structure keeping the robust grasses in check.

• Grazing management: too low grazing intensity will exacerbate effects of N stimulated nitrophilous grass growth
• More frequent mowing or harvesting will help to remove N.
• These soils are P limited so changing 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.