Jacqueline A. Carroll

The interactions between plant growth, vegetation structure and soil processes in semi-natural acidic and calcareous grasslands receiving long-term inputs of simulated pollutant nitrogen deposition

Regular applications of ammonium nitrate (35-140 kg N ha(-1) year(-1)) and ammonium sulphate (140 kg N ha(-1) year(-1)) to areas of acidic and calcareous grassland in the Derbyshire Peak District over a period of 6 years, have resulted in significant losses in both overall plant cover, and the abundance of individual species, associated with clear and dose-related increases in shoot nitrogen content. No overall growth response to nitrogen treatment was seen at any stage in the experiment. Phosphorus additions to the calcareous plots did however lead to significant increases in plant cover and total biomass, indicative of phosphorus limitation in this system. Clear and dose-related increases in soil nitrogen mineralization rates were also obtained, consistent with marked effects of the nitrogen additions on soil processes. High nitrification rates were seen on the calcareous plots, and this process was associated with significant acidification of the 140 kg N ha(-1) year(-1) treatments.

Evidence that soil carbon pool determines susceptibility of semi-natural ecosystems to elevated nitrogen leaching

Deposition of reactive nitrogen (N) compounds has the potential to cause severe damage to sensitive soils and waters, but the process of ‘nitrogen saturation’ is difficult to demonstrate or predict. This study compares outputs from a simple carbon–nitrogen model with observations of (1) regional- and catchment-scale relationships between surface water nitrate and dissolved organic carbon (DOC), as an indicator of catchment carbon (C) pool; (2) inter-regional variations in soil C/N ratios; and (3) plot scale soil and leachate response to long-term N additions, for a range of UK moorlands. Results suggest that the simple model applied can effectively reproduce observed patterns, and that organic soil C stores provide a critical control on catchment susceptibility to enhanced N leaching, leading to high spatial variability in the extent and severity of current damage within regions of relatively uniform deposition. Results also support the hypothesis that the N richness of organic soils, expressed as C/N ratio, provides an effective indicator of soil susceptibility to enhanced N leaching. The extent to which current C/N is influenced by N deposition, as opposed to factors such as climate and vegetation type, cannot be unequivocally determined on the basis of spatial data. However, N addition experiments at moorland sites have shown a reduction in organic soil C/N. A full understanding of the mechanisms of N-enrichment of soils and waters is essential to the assessment of current sensitivity to, and prediction of future damage from, globally increasing reactive nitrogen deposition.

The effect of long-term nitrogen additions on the bryophyte cover of upland acidic grasslands

Abstract Regular applications of NH4NO3 (3.5–14 g N m−2 yr−l) and (NH4)2SO4 (14 g N m−2 yr−l) to areas of acidic grassland in the Derbyshire Peak District over a period of six years have resulted in marked changes in the abundance of the bryophyte species present on the site. A dose-related reduction in bryophyte cover, significant at all levels of nitrogen addition, was obtained after only one year of applications and this effect has been maintained over a further five years of treatment. (NH4)2SO4 additions at the same rate as the highest NH4NO3 input (14 g N m−2 yr−1) produced greater reductions in cover with losses of 80–95% compared with 45–55% for the 14 g NH4NO3 treatment. Soil pH measurements taken from cores collected in December 1994 showed a small reduction in the pH of the (NH4)2SO4 treated plots (4.03 compared with 4.41 for the controls), whereas the NH4NO3 treatments were unaffected. The mean stem density of the dominant bryophyte species Rhytidiadelphus squarrosus was significantly reduced at the lowest level of nitrogen addition and this was associated with marked increases in the total stem nitrogen content of this species. Pleurozium schreberi, present at much lower cover values, however showed no significant change except at the highest level of application, suggesting differential effects of the treatments on these two species. These results are indicative of significant losses in the bryophyte cover of acidic grasslands at atmospheric input rates of 5 g N m−2 yr−1 or less, and suggest that these species may be particularly at risk from the high atmospheric nitrogen deposition rates that are becoming a feature of many upland areas.

Bio-indicators of nitrogen pollution in heather moorland

Heather moorlands are internationally important ecosystems that are highly sensitive to eutrophication and acidification by reactive atmospheric nitrogen (N) deposition. We used a long-term experiment simulating wet-deposition of N on heather moorland to identify potential bio-indicators of N deposition. These indicators were subsequently employed in a survey covering a N deposition gradient ranging from approximately 7 to 31kg N ha(-1) yr(-1), at selected sites throughout the UK. In this regional survey litter phenol oxidase activity and bryophyte species richness were negatively associated with N deposition. Calluna vulgaris N:P ratios and litter extractable N were positively correlated with N deposition. The use of the suite of four bio-indicators has the potential to provide rapid assessment of the extent of N saturation of heather moorland sites and moorland ecosystem functioning, and has significant advantages over reliance on single measures such as soil N status or an individual bio-indicator species.

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.

IMPACTS OF VEHICLE EMISSIONS ON VEGETATION

This research describes a system constructed to provide stable, realistic urban atmospheres with pollutant mixtures at concentrations and proportions relevant to those found at roadsides in urban areas. The system has been used in conjunction with several field sites to assess the impacts of urban pollution mixtures on a broad range of plant species of contrasting morphological and functional types. Impacts of pollution treatments have been assessed in terms of visible injury symptoms, growth, rates of stomatal conductance, senescence, and leaf surface characteristics. The data from this work clearly shows that levels of pollutant mixtures typical of urban areas do have species-specific, direct effects on plant growth and may make plants susceptible to other environmental stresses.