The distribution of different ecological groups of lichens (acidophytes, ‘nitrophytes’, indifferent species) was compared on 1–24 year-old twigs of Abies sibirica sampled in the ‘pristine’ West Sayan and the polluted East Sayan Mountains (Krasnoyarsk District, South Siberia, Russia) to test their value as indicators of current pollution effects. Bark pH of twigs and bark chemistry (N, S, Ca, Mg, Al, Fe) were measured, and a preliminary estimate of emissions in the Krasnoyarsk District from livestock animal populations was calculated. In both regions, an unusually high twig bark pH and an abnormal species composition for A. sibirica canopy were found (e.g. Physcia aipolia, P. dubia, P. tenella, Phaeophyscia sp., Melanelia exasperatula and Candelariella vitellina), with P. tenella (East Sayan) and M. exasperatula (West Sayan) as dominants. The results confirm that the distribution of lichen species on Abies sibirica twigs is a valuable indicator of current changes in atmospheric conditions.

Dr Willem Asman concluded that the major global sources of atmospheric NH3 are excreta from domestic animals and fertilizers. A question raised was: how reliable are the emission estimates and extrapolations? The answer was that emission estimates are surrounded by uncertainty, which is a major handicap to sound modelling of NH3 dry deposition and, consequently, to obtaining good estimates of critical load exceedences.

Major uncertainties in emission estimates seem to be related to the use of simple emission factors, many of which are highly empirical or have been derived from measurements carried out under conditions which deviate considerably from those following modern practices of handling and applying manure and fertilizers. An example is provided by the commonly used emission factors for synthetic fertilizers (see e.g. Bouwman et al. (1997)), which are much higher than recent micrometeorological assessments seem to suggest. Thus, emission from urea, the most widespread fertilizer used in the world (currently around 55% of world N consumption) can be completely avoided if the fertilizer is incorporated into the upper soil layers. Similarly, a growing crop can reduce losses to well below 10% of the applied amount of urea-N, i.e. to less than half of the generally used emission factors of 15% for Europe and 25% for the tropics. The emission factor for NPK-fertilizer is set at 4%, whereas that for pure calcium-ammonium-nitrate, the same N compound as is present in NPK-fertilizers, is assumed to be only 2%.

Using field plots, three species (Mercurialis perennis L., Rubus fruticosus L., and Trientalis europaea L.) were tested for their potential to emit gaseous ammonia to the atmosphere. Canopies were misted with 5 mM methionine sulphoximine (MSO) to inhibit glutamine synthetase (GS), the enzyme of ammonium assimilation. Leaf tissue NH4+ concentration of control plants was 0·03–0·1 μmol g−1 f. wt. Although NH4+ accumulated in the leaf tissue of MSO-treated plants of all three species to similar concentrations (6-10 μmol g−1 f. wt after 4 d), emissions were only detected from the leaves of M. perennis, with potential rates of 2·5 nmol m−2 leaf s−1. Experiments carried out in a controlled environment confirmed this rate of emission over 9 d, during which time leaf tissue ammonium increased to 66 μmol g−1 f. wt. Comparisons with Hordeum vulgare grown under the same conditions showed that tissue NH4+ concentration reached a plateau of about 40 μmol g −1f. wt after 2 d. Emissions of NH3 during the 5 d of treatment reached a maximum rate of 10 nmol m−2 s−1 by the third day.

Apoplastic pH of the plants was determined, and it is suggested that this is an important factor explaining the differences in NH3 emission between species. The higher the apoplastic pH, the greater the likelihood of loss of NH3 from sub-stomatal spaces to the atmosphere. T. europaea (non-emitter) had an apoplastic pH of 5, R. fruticosus (non-emitter) a pH of c. 5·6, whereas that of M. perennis (emitter) was c. pH 6·3. The apoplastic pH is thought to be dictated in part by the N nutrition of a species, nitrophilous species tending to have high pH. Without NO3− fertilization, H. vulgare had an apoplastic pH of 6·8 but this increased to 7·3, 3 d after feeding with NO3−.

Short-term fumigation (2 h) of shoots of H. vulgare with 60 μg (≈32 mg NH3 m−3) of labelled gaseous 15N-NH3 (in the absence of MSO) showed that a substantial proportion (60%) of the applied label was found in the leaves, as well as in stems and roots (3%). There was also a change in amino acid pools, with an increase in shoot amino acids and a decrease in those in the root, while tissue NH4+ was very low in both shoots and roots. This provided indirect evidence that some of the applied label had been incorporated into an organic form. Following the fumigation treatment, emissions of NH3 were collected for 3 h, then c. 6·5 μg of N was recovered, of which c. 17% was 15N-labelled. Some of this label could have resulted from desorption of NH3 from leaf surfaces, but it was more likely that the remaining 14N isotope was from sub-stomatal emissions of NH3.

It is argued that non-nitrophilous plants tend to rely on mixed sources of N (NO3−, NH4+ or organic-N) and are more likely to favour root rather than shoot assimilation. Under these circumstances, their apoplastic pH is relatively low (compared with that of nitrophiles, which tend to assimilate NO3− mainly in their shoots), and at atmospheric concentrations most wild species are likely to be net assimilators, rather than emitters, of atmospheric ammonia.