Deposition of fixed atmospheric nitrogen and foliar nitrogen content of bryophytes and Calluna Vulgaris (L.) Hull

Atmospheric deposition of fixed nitrogen as nitrate and ammonium in rain and by dry deposition of nitrogen dioxide, nitric acid and ammonia has increased throughout Europe during the last two decades, from 2-6 kg N ha-1 year-1 to 15-60 kg N ha-1 year-1. The nitrogen contents of bryophytes and the ericaceous shrub Calluna vulgaris have been measured at a range of sites, with the objective of showing the degree to which nitrogen deposition is reflected in foliar plant nitrogen. Tissue nitrogen concentrations of herbarium bryophyte samples and current samples of the same species collected from the same sites were compared. No significant change in tissue nitrogen was recorded at a remote site in north-west Scotland where nitrogen inputs are small (< 6 kg N ha-1 year-1). Significant increases in tissue N occurred at four sites ranging from 38% in central Scotland to 63% in Cumbria where nitrogen inputs range from 15 to 30 kg N ha-1 year-1. The relationships found between the estimated input of atmospheric nitrogen and the tissue nitrogen content of the selected bryophytes and Calluna at the sites investigated were found to be generally linear and fitted the form Ntissue = 0.62 + 0.022 Ndep for bryophytes and Ntissue = 0.83 + 0.045 Ndep for Calluna. There was thus an increase in total tissue nitrogen of 0.02 mg g-1 dry weight for bryophytes and 0.045 mg g-1 dry weight for Calluna for an increase in atmospheric nitrogen deposition of 1 kg ha-1 year-1. The lowest concentrations were found in north-west Scotland and the highest in Cumbria and the Breckland heaths of East Anglia, both areas of high atmospheric nitrogen deposition (30-40 kg N ha-1 year-1). The implications of increased tissue nitrogen content in terms of vegetation change are discussed.Changes in atmospheric nitrogen deposition with time were also examined using measured values and values inferred from tissue nitrogen content of mosses. The rate of increase in nitrogen deposition is not linear over the 90-year period, and the increases were negligible over the period 1880-1915. However, during the period 1950 to 1990 the data suggest an increase in nitrogen deposition of 2 kg N ha-1 every 10 years

Climate change and pollutant impacts on Scottish vegetation

By the 2050’s the UK is projected to be about 1.6°C warmer, when the atmospheric CO2 concentration will be 525 ppmv. These changes will have profound effects on the Scottish flora and fauna. Vegetation primary productivity will increase, except in dry regions, and the productivity of upland forest plantations may increase by several Yield Classes. The spread of plant species may be less than expected, but a number of slow-growing ‘stress-tolerant’ species, including montane/alpine species, are likely to be lost. Nitrogen deposited as a result of emission of NOx from vehicles and NH3 from agriculture is now a major source of acidity, and problems of acidification and eutrophication are linked. Despite reductions in sulphur emissions, critical loads of acid deposition are likely to be exceeded for soils in most of the Scottish uplands until at least 2005. Critical levels affecting tree growth may be exceeded where forests are in cloud for 10% of the time in areas of the Great Glen. Much of the Scottish uplands receives 25-30kg N ha-1 yr-1, which may be causing change in species composition. Background tropospheric ozone concentrations are increasing. Much of the Scottish uplands experiences mean summer ozone concentrations exceeding those in southern England, but with fewer exceedances of critical levels. However, many crops and some sensitive native species are probably being adversely affected

The mass budget of atmospheric ammonia in woodland within 1 km of livestock buildings

International audienc

Bioindicator and biomonitoring methods for assessing the effects of atmospheric nitrogen on statutory nature conservation sites

Bioindicators provide a range of techniques to assess the impacts of air pollution from reactive nitrogen (N) compounds on statutory nature conservation sites. They complement physical monitoring of atmospheric concentrations and deposition and risk assessment based on the critical loads approach by providing site-based information on atmospheric N concentrations, N deposition and/or ecological impacts. Appropriate bioindicators for N may be applied by sampling at one time to compare results between different locations. In particular, local-scale transects can help identify the impacts of a nearby point source of reactive N emissions to the atmosphere. The repeated application of bioindicator methods over time provides the basis for biomonitoring. In general, biomonitoring reflects changes over periods of several years, although short-term changes can also be monitored (over several weeks and months). This report reviews the wide range of bioindicator and biomonitoring methods for N and incorporates the results of a field test of several of the methods. In addition, datasheets are provided that summarize the key characteristics, advantages and limitations of the different methods. Bioindicator methods can be grouped into several contrasting approaches: Biochemical methods (based on an accumulation of N or a chemical/physiological response to N), Species composition methods (based on previously characterized species preferences) and Transplant methods (based on the response following transplanting of either locally native species or standardized plants). Nitrogen accumulation methods include measurement of plant tissue N concentration, amino acids, substrate N and foliar ammonium. The accumulation methods provide the closest link to atmospheric N deposition. Results show that the smaller and more available the chemical pool, the larger the magnitude of response, with increasing responses from: total N < substrate N < foliar ammonium. Biochemical response methods include analysis of enzymes such as nitrate reductase and emissions of nitrous oxide from soils. These methods are useful to demonstrate physiological effects, but tend to be less well correlated with atmospheric N deposition due to interactions with environmental conditions. Species composition methods are of particular interest to the statutory conservation agencies since they relate directly to changes in plant communities due to excess atmospheric N. 'Ellenberg' N preference scores for higher plant and bryophyte species can be used to score the overall community for nitrogen. The limitation of this approach is that a wide range of other factors may also affect species composition. Lichens are particularly sensitive to atmospheric reactive N, particularly ammonia. Detailed approaches are available to score lichen responses to N, but require more development for UK conditions. There is also the potential to refine simple methods that can be applied by non-experts. The use of standardized grass plants has been shown to provide a robust method for monitoring the deposition and effects of N. The method can be applied in situations of complex terrain where physical estimates of deposition are difficult and as a graphic demonstration of impacts to stakeholders. It has a key advantage that exposure periods of only a few weeks are necessary. Transplanting native species between sites is useful to demonstrate impacts at polluted sites and conversely the benefits of clean conditions. These methods have been shown to work well for lower plants, and have the benefit of being able to demonstrate recovery following a reduction in deposition where this occurs. Overall, recognizing the limitations and benefits of the different methods, it is concluded that bioindicators provide a practical site-based approach for assessing N concentrations, deposition and impacts. Each of the above mentioned approaches are have merits, with different techniques matching to the range of questions being addressed. The most robust results are to be obtained by implementing several complementary techniques simultaneously, where possible in combination with low-cost physical monitoring of atmospheric concentrations

Heather-grass competition on Scottish moorlands: interacting effects of nutrient enrichment and grazing regime

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