The effects of air‐borne nitrogen pollutants on species diversity in natural and semi‐natural European vegetation

Abstract

Summary The effects of increased atmospheric nitrogen inputs, from both NOy and NHx, on diversity in various semi‐natural and natural ecosystems are reviewed. The severity of these impacts depends on abiotic conditions (e.g. buffering capacity, soil nutrient status and soil factors that influence the nitrification potential and nitrogen immobilization rate) in the particular system. The sensitivity of fresh water ecosystems, wetlands and bogs, species‐rich grasslands, heathlands and field layer of forests, all of which have conservational value, are discussed in detail. The most important effects of nitrogen deposition are: (i) accumulation of nitrogenous compounds resulting in enhanced availability of nitrate or ammonium; (ii) soil‐mediated effects of acidification; and (iii) increased susceptibility to secondary stress factors. Long‐term nitrogen enrichment has gradually increased the availability of nitrogen in several vegetation types, leading to competitive exclusion of characteristic species by more nitrophilic plants, especially under oligo‐ to mesotrophic soil conditions. Soil acidification (with losses of buffering capacity and increased concentrations of toxic metals) is especially important after nitrification of ammonium in weakly buffered environments: acid‐resistant plant species then become dominant at the expense of the often rare plants typical of intermediate pH. The related change in the balance between ammonium and nitrate may also affect the performance of several species. The susceptibility of plant species to secondary stress factors (pathogens; frost and drought) may be affected by air‐borne nitrogen but data are only available for a few communities (e.g. dry heathlands). Most global biodiversity is contained within natural and semi‐natural vegetation. It is thus crucial to control emissions of nitrogenous compounds to the atmosphere, in order to reduce or prevent effects on diversity in these systems. Most research has focused on acidification in forestry stands and lakes and on the effects on trees. We highlight serious gaps in knowledge of other ecosystems.

Ecological issues related to N deposition to natural ecosystems: research needs

“…They reported a shift from forb-dominated to grassdominated communities, due to the greater capacity of alpine graminoids to sequester N. A reduction in species number and the increasing dominance of a few grass species, in response to N fertilization, have been well documented in the Rothamsted Park Grass experiment (UK) (Tilman et al, 1994) and at the Cedar Creek Long-Term Ecological Research Site in Minnesota (US) (Inouye and Tilman, 1995). On extremely nutrient-poor soils, on the other hand, species richness may increase as a result of N enrichment because of invasion by exotic species (Bobbink et al, 1998). The exotic species often become extremely good competitors with slight increases in fertility.…”

“…Moss cover at four wooded sites in the Scotland was found to decrease with increasing N deposition (Pitcairn et al, 1998). Declines in bryophytes and lichens (Bobbink et al, 1998;Lee and Caporn, 1998;Turkington et al, 1998) have been attributed to fertilization with N. Because lichens and bryophytes have little or no protective cuticle, they may be particularly susceptible to direct toxic effects. Thus, in a complex community, sensitivity to elevated N may vary among life forms, and the eventual results at the community level are difficult to predict.…”

“…C. vulgaris plants receiving N fertilization were more severely damaged by the heather beetle (Lochmae suturalis) than plants in nonfertilized plots (Heil and Diemont, 1983). Significant outbreaks of heather beetle can open large areas of canopy and increase light penetration, which leads to enhanced growth of understory grasses such as wavy hair-grass (Deschampsia jlexuosa) or purple moor-grass (Molinia caerulea) (Berdowski and Zeilinga, 1987;Pitcairn and Fowler, 1995;Bobbink et al, 1998).…”

“…Effects may be manifested through changes in species rich-ness, evenness or both (DiTomasso and Aarssen, 1989). For example, Bobbink et al (1998) reported that long-term N enrichment from acidic deposition led to competitive exclusion of naturally occurring species by more nitrophilic plants, particularly under nutrient-poor conditions. An increase in nitrophilic species, which are often tall and densely growing, can result in decreased competitive ability of small and slow-growing species (Falkengren-Grerup, 1986) and eventual declines in diversity.…”

Sphagnum can ‘filter’ N deposition, but effects on the plant and pore water depend on the N form

“…Understanding how Sphagnum removes inorganic N from precipitation and the 52 effects of increasing N inputs on this process is key to predicting N effects on peatland atmosphere (Bobbink et al, 1998;Van Breemen, 1995). The efficient N removal by…”