This report describes a two-part field study, which applied bioindicator methods in parallel. The use of biomonitoring methods is an approach which could provide an early warning of sites at risk from N deposition. The conservation agencies are required to identify, monitor and protect sites designated for nature conservation under UK and European legislation. The ability to determine the impacts of enhanced N concentrations and N depositions is important for assessing effects on site condition and integrity.

Summary Atlantic bryophytes are of European conservation importance, yet the effect on them of excess atmospheric nitrogen is relatively unknown. This study assesses the effects of increased atmospheric N deposition on the growth and tissue N of epiphytic Atlantic bryophytes, and their potential to recover following a decline in N deposition. The N received in stemflow by bryophytes at two sites was measured and compared to model predictions. Four species of epiphytic bryophytes (Isothecium myosuroides, Dicranum scoparium, Frullania tamarisci and Ulota crispa), typical of Atlantic Oak woods, were studied in a 12‐month reciprocal transplant experiment between a pristine Oak woodland receiving a modelled atmospheric deposition of 12 kg N ha−1 year−1 and a polluted one receiving 54 kg N ha−1 year−1. Tissue N concentration increased and growth declined following an increase in atmospheric N deposition in all species except Ulota crispa. Conversely, tissue N concentration decreased and growth increased in Frullania tamarisci following a decrease in atmospheric N deposition, with similar non‐significant patterns in the other species. The reciprocal transplants indicate a detrimental effect of increased N deposition on the bryophyte species studied. The study indicated recovery following a decrease in atmospheric N deposition, but the responses caused by decreased N deposition were smaller than those due to increased N deposition. This suggests that the time‐scale for recovery of bryophytes from excess N deposition is longer than the timescale of nitrogen impacts.

Increased deposition of atmospheric N largely from intensive agriculture is affecting biodiversity and the composition of natural and semi-natural vegetation in Europe. The value of species based bioindicators such as the Ellenberg N index and measurements of total tissue N and free amino acids in key plant species, is described with reference to a mixed woodland downwind of a livestock farm in the Scottish Borders, operated for over 20 years with a measured spatial gradient of ammonia concentration (29–1.5 μg m−3). All the indicators examined showed a relationship with N deposition and provided some indication of vegetation change. Total tissue N and arginine concentrations were most closely linked with ammonia concentrations and N deposition, with r2 values of >0.97 and >0.78 respectively.

The impacts of increased N deposition (wet; NH4+ and dry; NH3) on ombrotrophic mire vegetation were compared in a two-year dose-response study. Vegetation from an ombrotrophic mire in SE Scotland was exposed in open-top chambers to a range of wet N deposition at 0 (deionised water), 8, 16, 32, 64 & 128 kg N ha(-1) y(-1) applied as NH4Cl and dry N deposition as gaseous NH3 at concentrations of 2, 6, 20, 50 90 mug m(-3). N concentrations in the foliage of all species showed a significant linear increase in response to dose when N was applied as wet deposited NH4+ in both 1999 and 2000. Comparison of the linear regression for foliar N versus NH4+ and NH3 in the dose-response relationships showed significant differences (p<0.05) between the form of N applied and %N concentration for two species in 1999 (C vulgaris and P. commune) but not in 2000. There was a significant increase in C vulgaris shoot growth in the NH3 treatments compared to NH4+ treatments when expressed per unit deposited N. Effects of nitrogen deposited as gaseous NH3 are generally greater than those of wet deposited NH4+ on moorland species, when expressed per unit deposited N.

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.

The impacts of increased N deposition (wet; NH4+ and dry; NH3) on ombrotrophic mire vegetation were compared in a two year dose response study. Vegetation from an ombrotrophic mire in SE Scotland was exposed in open-top chambers to a range of wet N deposition at 0 (de-ionised water), 8, 16, 32, 64 & 128kg N ha-1y-1 applied as NH4CI and dry N deposition as gaseous NH3 at concentrations of 2, 6, 20, 50 & 90 ug m-3. N concentrations in the foliage of all species showed a significant linear increase in response to dose when N was applied as wet deposited NH4+ in both 1999 and 2000. Comparison of the linear regression for foliar N versus NH4+ and NH3 in the dose-response relaitonships showed significant differences (p,0.05) between the form of N applied and %N concentration for two species in 1999 (C. vulgaris and P. commune) but not in 2000. There was a significant incrase in C. vulgaris shoot growth in the NH3 treatments compared to NH4+ treatments when expressed per unit deposited N. Effects of nitrogen deposited as gaseous NH3 are generally greater than those of wet deposited NH4+ on moorland species, when expressed per unit deposited N.

The marker variables, Ellenberg Nitrogen Index, nitrous oxide and nitric oxide fluxes and foliar nitrogen, were used to define the impacts of NH3 deposition from nearby livestock buildings on species composition of woodland ground flora, using a woodland site close to a major poultry complex in the UK. The study centred on 2 units in close proximity to each other, containing 350,000 birds. and estimated to emit around 140,000 kg N year(-1) as NH3. Annual mean concentrations of NH3 close to the buildings were very large (60 mug m(-3)) and declined to 3 mug m(-3) at a distance of 650 m from the buildings. Estimated total N deposition ranged from 80 kg N ha(-1) year(-1) at a distance of 30 m to 14 kg N ha(-1) year(-1) at 650 in downwind. Emissions of N2O and NO were 56 and 131 mug N m(-2) h(-1), respectively at 30 m and 13 and 80 mug N m(-2) h(-1), respectively at 250 in downwind of the livestock buildings. Species number in woodland ground flora downwind of the buildings remained fairly constant for a distance of 200 in from the units then increased considerably, doubling at a distance of 650 in. Within the first 200 m downwind, trends in plant species composition were hard to discern because of variations in tree canopy composition and Corer. The mean Ellenberg N Index ranged from 6.0 immediately downwind of the livestock buildings to 4.8 at 650 m downwind, The mean abundance weighted Ellenberg N Index also declined with distance from the buildings. Tissue N concentrations in trees. herbs and mosses were all large. reflecting the substantial ammonia emissions at this site. Tissue N content of ectohydric mosses ranged from approximately 4% at 30 m downwind to 1.6% at 650 in downwind. An assessment of the relative merits of the three marker variables concludes, that while Ellenberg Index and trace gas fluxes of N2O and No give broad indications of impacts of ammonia emissions on woodland vegetation, the application of a critical foliar N content for ectohydric mosses is the most useful method for providing spatial information which could be of value to policy developers and planners. (C) 2002 Elsevier Science Ltd. All rights reserved.

Measurements of landscape-scale methane emission were made over an aapa mire near Kaamanen in Finnish Lapland (69° 8′ N, 27° 16′ E, 155 m ASL). Emissions were measured during the spring thaw, in summer and in autumn. No effect of water table position on CH4 emission was found as the water table remained at or above the surface of the peat. Methane emission fluxes increased with surface temperature from which an activation energy of −99 kJ mol−1 was obtained. Annual emission from the site, modelled from temperature regression and short-term flux measurements made in three separate years, was calculated to be 5.5 ± 0.4 g CH4 m−2 y−1 of which 0.6 ± 0.1 g CH4 m−2 y−1 (11%) was released during the spring thaw which lasted 20 to 30 days.

Long-term and short-term N deposition effects on N2O and NO emissions from forest soils were compared. Long-term NH3 deposition (> 20 years) from a poultry farm to a downwind woodland (decreasing from 73 to 18 kg N ha-1 y-1, 30 to 110 m downwind of the farm) resulted in the re-emission of 6% and 14% of NH3-N deposited as N2O-N and NO-N, respectively. However, when in short-term (2-3 years) field experiments the atmospheric N deposition to mature conifer plantations was raised by fumigation with NH3 to 15 kg N ha-1 y-1 or by acid mist to 48 and 96 kg N ha-1 y-1 the N deposited was immobilised. In the acid mist experiment more than 2 years of acid mist (48 and 96 kg N ha-1 y-1) were required to significantly increase N2O emissions from -0.3 μg N2O-N m-2 h-1 (control) to 0.5 and 5.7 μg N2O-N m-2 h-1, respectively. This suggests, that N deposition simulation studies in soil ecosystems, which have previously not been exposed to high rates of N (by deposition or fertilisation), need to be long-term. Also, measurements of N2O and/or NO may be a non-destructive, quick indicator of the N status of the soil.

Wet deposition of nitrogen is reasonably well monitored throughout Europe, whereas the dry deposition inputs are provided largely by models. Recent long-term measurements of NO2 and NH3 fluxes to semi-natural vegetation have shown that rates of NH3 deposition exceed those of NO2, typically by an order of magnitude. Incorporating the results of these dry deposition measurements in regional deposition budgets shows that the inputs of reduced nitrogen contribute the dominant fraction of the total nitrogen inputs in most regions of the UK. The results are illustrated by comparing the atmospheric mass-budget for oxidized nitrogen over the UK. Of the annual UK emissions of NOx, amounting to 780 kt N (Salway et al., 1997), only 5% is dry deposited to terrestrial surfaces within the country while 15% is wet deposited, whereas for the reduced nitrogen, 42% of emissions (of the 260 kt N, Salway et al., 1997) are dry deposited and 46% are wet deposited. Even more striking are the relative contributions of oxidized and reduced nitrogen to semi-natural vegetation, which is a particularly efficient sink for NH3. The species composition of semi-natural vegetation is also regarded as very sensitive to nitrogen inputs. The distribution of nitrogen deposition among different land uses shows that the average input to forest in the UK is 33 kg N ha−1 annually of which 78% is reduced nitrogen. The other land uses receive about 15 kg N ha−1 of nitrogen of which between 55% and 65% is NHx. Critical loads for nutrient nitrogen are exceeded primarily in forested and moorland areas as a consequence of NH3 dry deposition and wet NH4+ deposition. For forests the area in exceedance of 20 kg N ha−1 year−1 critical load represents 70% of the forest area (1.4 × 106 ha−1) while for moorland the area in exceedance is 13% of the moorland area and occupies 1.04 × 106 ha−1).

Elevated N deposition caused by ammonia emissions from poultry and pig farms, and supplemented N concentrations in acid mist in field and chamber experiments increased soil available NH4+ and NO3− concentrations and emissions of N2O and NO. In a ‘pristine’ soil, not previously exposed to high N deposition rates, an initial threshold of 40 kg N ha−1 year −1 was required to increase N2O emissions. For all data described here on average 0.76% (range 0.2 to 15%) of the elevated N deposited was emitted as N2O. For soils exposed to long-term and large N deposition rates N2O losses >3% of the N deposition rate were calculated. This suggests that N2O losses of more than 3% of the N input can be indicative of soil ecosystems where the N input exceeds its demand. For NO a more limited data set showed losses ranging from 1.3 to 20% of the elevated N input. It was calculated that NH3 emissions from all intensive pig and poultry farms in Great Britain accounted for 18 t N2ON year−1 and that poultry farms accounted for less than 3 t NON year−1.