Bio-indicators of nitrogen pollution in heather moorland

Long-term nitrogen and phosphorus enrichment alters vegetation species composition and reduces carbon storage in upland soil

Reactive nitrogen (N) deposition can affect ecosystem processes, particularly in oligotrophic upland habitats. Phosphorus (P) addition has been proposed to reduce the effects of N enrichment on N leaching and acidification, since P limitation can reduce biomass production and consequent sequestration of reactive N. However, biodiversity is often reduced in more productive ecosystems and P limitation may protect against this effect. Responses to P availability in instances of high N deposition are poorly understood. This study investigated the ecosystem response to alleviation of P limitation, using a long-term nutrient addition experiment (1996-2012) three years after ceasing N inputs and 15years after a single P application. Substantial differences were observed in the structure and composition of vegetation species and above-ground vegetation biomass. Vegetation height was greater in the N+P addition treatments (+38% cf. control), with increased cryptogam cover (+47%), whereas N addition increased graminoid species cover (+68%). Vegetation diversity was significantly reduced by the addition of P (-21%), indicating that P limitation is likely to be an important mechanism that limits biodiversity loss in upland habitats exposed to chronic N deposition. Significant differences in soil C and N contents were also observed between treatments. Relative to control, the addition of N increased soil C (+11%) and N (+11%) pool sizes, whereas the addition of N and P reduced soil C (-12%) and N (-13%) pool sizes. This demonstrated the importance of P availability for upland ecosystem processes, and highlights the long-term effects of P addition on vegetation species composition and C storage. Thus, the addition of P cannot be endorsed as a method for reducing impacts of N deposition. Capsule: Phosphorus limitation is a major mechanism governing ecosystem processes in situations of high atmospheric nitrogen deposition.

Long-term nitrogen deposition increases heathland carbon sequestration

The large increases in reactive nitrogen (N) deposition in developed countries since the Industrial Revolution have had a marked impact on ecosystem functioning, including declining species richness, shifts in species composition, and increased N leaching. A potential mitigation of these harmful effects is the action of N as a fertiliser, which, through increasing primary productivity (and subsequently, organic matter production), has the potential to increase ecosystem carbon (C) storage. Here we report the response of an upland heath to 10years of experimental N addition. We find large increases in plant and soil C and N pools, with N-driven C sequestration rates in the range of 13-138kgCkg^-1. These rates are higher than those previously found in forest and lowland heath, mainly due to higher C sequestration in the litter layer. C sequestration is highest at lower N treatments (10, 20, and 40kgNha^-1yr^-1 above ambient), with evidence of saturation at the highest N treatment, reflecting a physiologically aged Calluna vulgaris (Calluna) canopy. To maintain these rates of sequestration, the Calluna canopy should be managed to maximise it's time in the building phase. Scaling our results across UK heathlands, this equates to an additional 0.77Mt CO2e per annum extra C sequestered into plant litter and the top 15cm of heathland soil as a result of N deposition. The bulk of this is found in the litter and organic soil horizons that hold an average of 23% and 54% of soil C, respectively. This additional C represents around 0.44% of UK annual anthropogenic GHG emissions. When considered in the context of falling biodiversity and altered species composition in heathland, policy focus should remain on reducing N emissions.

How will the semi-natural vegetation of the UK have changed by 2030 given likely changes in nitrogen deposition?

Nitrogen deposition is known to have major impacts on contemporary ecosystems but few studies have addressed how these impacts will develop over coming decades. We consider likely changes to British semi-natural vegetation up to the year 2030 both qualitatively, based on knowledge of species responses from experimental and gradient studies, and quantitatively, based on modelling of species relationships in national monitoring data. We used historical N deposition trends and national predictions of changing deposition to calculate cumulative deposition from 1900 to 2030. Data from the Countryside Survey (1978, 1990 and 1998) was used to parameterise models relating cumulative N deposition to Ellenberg N which were then applied to expected future deposition trends. Changes to habitat suitability for key species of grassland, heathland and bog, and broadleaved woodland to 2030 were predicted using the MultiMOVE model. In UK woodlands by 2030 there is likely to be reduced occurrence of lichens, increased grass cover and a shift towards more nitrophilic vascular plant species. In grasslands we expect changing species composition with reduced occurrence of terricolous lichens and, at least in acid grasslands, reduced species richness. In heaths and bogs we project overall reductions in species richness with decreased occurrence of terricolous lichens and some bryophytes, reduced cover of dwarf shrubs and small increases in grasses. Our study clearly suggests that changes in vegetation due to nitrogen deposition are likely to continue through coming decades.

Nitrogen deposition reduces plant diversity and alters ecosystem functioning: field-scale evidence from a nationwide survey of UK heathlands

Findings from nitrogen (N) manipulation studies have provided strong evidence of the detrimental impacts of elevated N deposition on the structure and functioning of heathland ecosystems. Few studies, however, have sought to establish whether experimentally observed responses are also apparent under natural, field conditions. This paper presents the findings of a nationwide field-scale evaluation of British heathlands, across broad geographical, climatic and pollution gradients. Fifty two heathlands were selected across an N deposition gradient of 5.9 to 32.4 kg ha(-1) yr(-1). The diversity and abundance of higher and lower plants and a suite of biogeochemical measures were evaluated in relation to climate and N deposition indices. Plant species richness declined with increasing temperature and N deposition, and the abundance of nitrophilous species increased with increasing N. Relationships were broadly similar between upland and lowland sites, with the biggest reductions in species number associated with increasing N inputs at the low end of the deposition range. Both oxidised and reduced forms of N were associated with species declines, although reduced N appears to be a stronger driver of species loss at the functional group level. Plant and soil biochemical indices were related to temperature, rainfall and N deposition. Litter C:N ratios and enzyme (phenol-oxidase and phosphomonoesterase) activities had the strongest relationships with site N inputs and appear to represent reliable field indicators of N deposition. This study provides strong, field-scale evidence of links between N deposition--in both oxidised and reduced forms--and widespread changes in the composition, diversity and functioning of British heathlands. The similarity of relationships between upland and lowland environments, across broad spatial and climatic gradients, highlights the ubiquity of relationships with N, and suggests that N deposition is contributing to biodiversity loss and changes in ecosystem functioning across European heathlands.

The legacy of nitrogen pollution in heather moorlands: ecosystem response to simulated decline in nitrogen deposition over seven years

Eutrophication and acidification of heather moorlands by chronic atmospheric nitrogen (N) pollution, is of major concern within these internationally important ecosystems. However, in the UK and Western Europe generally emissions of NO(y) and NH(x) peaked during the 20th century. Due to the history and scale of atmospheric N pollution, the legacy of these high levels of N deposition, through accumulation in soil, may hinder or prevent ecosystem recovery. Effects of N pollution on heather moorland were investigated throughout the ecosystem including; the dominant plant species, Calluna vulgaris, the bryophyte and lichen community and the soil system using a long-term experiment simulating wet N deposition. We observed an increase in C. vulgaris height, shoot extension and canopy density, litter mineral N, total N concentration, N:P and C:N ratios in response to N addition. Bryophyte species diversity, bryophyte and lichen frequency and the frequency of two individual bryophyte species (Lophozia ventricosa and Campylopus flexuosus) were significantly reduced by N addition. We developed an N recovery experiment, using a split-plot design, on the long-term N treatment plots to investigate ecosystem response to a simulated decline in N deposition. Two years after cessation of N treatment the only ecosystem component that responded to the recovery experiment was C. vulgaris shoot extension, however after seven years of recovery there were significant declines in litter total N concentration and mineral N and an increase in litter C:N ratio. Although bryophytes and lichens form a close relationship with atmospheric N deposition these organisms did not show a significant response to the N recovery experiment, two years after cessation of N treatment. These data indicate that low nutrient ecosystems, such as moorlands, have the capacity to respond to declines in N deposition however the accumulation of pollution may hinder recovery of sensitive organisms, such as bryophytes and lichens.

Andromeda polifolia and Oxycoccus microcarpus as pollution indicators for ombrotrophic bogs in the Western Sudety Mountains (SW Poland)

Concentrations of the elements Cd, Cr, Cu, Fe, Li, Mn, N, Ni, Pb and Zn in Andromeda polifolia, Oxycoccus microcarpus and in the peat in which these plants grew were measured in the Western Sudety (Karkonosze and Izerskie Mts., SW Poland). Of both the investigated plant fruit, O. microcarpus harvested from wild populations are commonly used as medicines. Samples from ombrotrophic bogs were investigated within the area influenced by exhausts of the former Black Triangle, one of the most heavily industrialized and polluted areas in Europe. A. polifolia and O. microcarpus growing at the highest elevations contained the highest Cu, Li, Ni, Mn and Zn concentrations and in addition O. microcarpus also contained the highest Cr concentrations. Both the investigated species have wide circumpolar distribution in ombrotrophic mires of the Northern hemisphere. As this type of mires is nourished solely by atmospheric deposition, the increased metal concentrations in A. polifolia and O. microcarpus may be an indication that their habitats receive an atmospheric input of long-range transported pollution. Our investigation proves that both species are able to accumulate elevated metal levels and may be used in the bioindication of the metal status in ombrotrophic mires. Controlling the collection of O. microcarpus fruit for consumption and medicinal purposes is recommended as this species can accumulate increased metal levels. However, further more detailed studies are necessary to verify the inner translocation of metals into fruit.