Securing Biodiversity in Breckland - European Commission

Nitrogen Deposition: Potential Impacts and MitigationGrass-heath communities depend on low nutrient conditionsEricoid (heather) heathland vegetation, in common with chalk grassland, dune and grass-heathvegetation, depends on the low nutrient status of the soil for the characteristic stress tolerant plantcommunities to persist. With increased nutrients, less competitive stress-tolerant species arereplaced by more vigorously competitive species. The diversity and composition of vegetation isfundamentally altered, generally resulting in a loss of scarce species, low growing species, lichens andbryophytes.Experimental work conducted in the Netherlands has shown the susceptibility of heather heathlandvegetation to increased nitrogen deposition, with dwarf shrub heather species being replaced byperennial grass species as nitrogen loads increase (Berendse et al. 1993). Effects include thereplacement of ling heather Calluna vulgaris by wavy-hair grass Deschampsia flexuosa on dry heaths(Heil and Diemont 1983) as well as replacement of both ling heather Calluna vulgaris and crossleavedheather Erica tetralix by purple moor grass Molinia caerulea on damp or wet heaths (Heil andBruggink 1987; Aerts et al. 1990; Berendse 1990; Berendse et al. 1993).Experimental work shows that addition of either phosphorous or nitrogen to calcareous Brecklandgrass-heath vegetation results in greater dominance and growth of perennial grasses and loss oflichens and other low-growing stress-tolerant plants, together with a reduction in vascular plantdiversity in the sward (Davy and Bishop 1984). Similar effects have been demonstrated onecologically similar habitats , including sand dune vegetation, with a decline in annual species,mosses and lichens and increase in red fescue Festuca rubra (Willis 1963; Boorman and Fuller 1982),and on calcareous grassland in which Festuca rubra became vigorous and dominant (Smith, Elstonand Bunting 1971).In Breckland, nitrogen appears to be the primary limiting nutrient (Davy and Bishop 1984), as bothsand dune soils (Boorman and Fuller 1982) and chalk grasslands (Bobbink et al. 1989). Unpublishedwork carried out by P. Dolman and S. Lake showed that diverse lichen-heath sub-communities of U1acid grassland occurred on soils that were higher in phosphorous than grass dominated subcommunities,as long as their nitrogen status was very low. The effects of nitrogen enrichment arepartially linked with those of grazing (Berendse 1985). Work by both Davy and Bishop (working atDeadman's Graves, Breckland) and Boorman and Fuller (working on dune vegetation on the Norfolkcoast) showed that grazing was not sufficient to offset the impacts of nitrogen addition on lichen richgrass-heath.Increased nitrogen deposition in the UK and BrecklandThere has been a very large reduction in the emissions of nitrogen attributable to domestic coal sincethe peak of that source in c.1900. Despite this, overall reduced nitrogen (NH 3 and NH 4 +) emissionshave continued to increase throughout 1900-2000 due to large increases from the agricultural sector(Fowler et al. 2004). Oxidised nitrogen emissions (NO 3 - , NO 2 and HNO 3 ) form c80% of total emissionsand have decline since a peak in 1980. Overall emissions have thus declined in recent decades(Fowler et al. 2004).98