Effects of increased deposition of atmospheric nitrogen on an upland moor: leaching of N species and soil solution chemistry.

This study was designed to investigate the leaching response of an upland moorland to long-term (10 yr) ammonium nitrate additions of 40, 80 and 120 kg N ha(-1) yr(-1) and to relate this response to other indications of potential system damage, such as acidification and cation displacement. Results showed increases in nitrate leaching only in response to high rates of N input, in excess of 96 and 136 kg total N input ha(-1) yr(-1) for the organic Oh horizon and mineral Eag horizon, respectively. Individual N additions did not alter ammonium leaching from either horizon and ammonium was completely retained by the mineral horizon. Leaching of dissolved organic nitrogen (DON) from the Oh horizon was increased by the addition of 40 kg N ha(-1) yr(-1), but in spite of increases, retention of total dissolved nitrogen reached a maximum of 92% and 95% of 80 kg added N ha(-1) yr(-1) in the Oh and Eag horizons, respectively. Calcium concentrations and calcium/aluminium ratios were decreased in the Eag horizon solution with significant acidification mainly in the Oh horizon leachate. Nitrate leaching is currently regarded as an early indication of N saturation in forest systems. Litter C:N ratios were significantly lowered but values remained above a threshold predicted to increase leaching of N in forests.

The influence of nitrogen deposition, competition and desiccation on growth and regeneration of Racomitrium lanuginosum (Hedw.) brid.

Racomitrium lanuginosum shoot growth was studied under the combined effects of N deposition (0, 20, 40 and 60 kg N ha(-1) year(-1)). competition with Festuca ovina, and a drought pre-treatment. Moss regeneration from shoot fragments was also investigated. Growth was initially stimulated at the 60 kg N level. However, after 6 months, growth was lower in all N treatments than in the 0 kg N control. Reductions in shoot growth first became apparent in the pre-desiccated moss, while moss shoots grew longer when surrounded by a F. ovina canopy. Optimum regeneration occurred at 20-40 kg N on bare soil, and at 0-20 kg N under a F. ovina canopy. These results suggest that current N deposition in upland Wales is already detrimental to growth of this species, and to regeneration under certain conditions. This species may be affected under predicted climatic scenarios of increased summer drought in Britain.

The effects of nutrient limitation on the response of Plantago major to ozone.

Most natural and semi-natural communities are limited by major nutrients such as nitrogen (N) or phosphorus (P), but most experiments on the effects of ozone on wild plants have used nutrient-rich composts or soil. In order to investigate interactions between ozone and low nutrient supply, two artificially selected lines (ozone-resistant and ozone-sensitive) of two populations Plantago major ssp. major L. were grown on a sandy loam, with (HN) and without (LN) addition of fertilizer. The soil was from a semi-natural grassland that has never been fertilized. Plants were exposed to either charcoal/Purafil®-filtered air (CF=<5 nl O3 l-1 ) or 70 nl O3 l-1 7 h d-1 from the seedling stage to seed production. Poor growth (c. 25% of that in HN) of the low-nutrient plants, and leaf concentrations of N and P showed that the LN plants were severely nutrient-limited. In addition to affecting the total dry mass of the plants, the nutrient supply altered seed production, reproductive effort (number of seeds per total mass) and root-to-shoot allocation. Exposure to ozone had significant effects on physiology, growth, and seed production that varied with population, selection line, time, and plant development. There also were significant interactions between ozone effects and nutrient regime. In the Lullington Heath population, ozone reduced plant dry weight at 4 wk only in the LN treatment, and in the sensitive line of the Bush population, seed production was reduced by ozone only in LN. Therefore, contrary to what was expected, in the present experiment, plants given the LN treatment were often more sensitive to ozone than those grown under the high-nutrient regime. This increase in sensitivity was despite the fact that the LN treatment reduced stomatal conductance and ozone flux. It is concluded that there are potentially important interactions between ozone and low nutrient supply that need further investigation, particularly under field conditions.

Effects of elevated CO2 and temperature on growth and allometry of five native fast-growing annual species.

Whereas much is known of the short-term growth response to elevated atmospheric CO2 concentrations, [CO2 ]elev , there is relatively little information on how the response of native species is modified by temperature, despite the fact that an increase in global mean temperature is expected to accompany the rise in [CO2 ]. In this study, five functionally related annual native species were exposed to different combinations of ambient and elevated [CO2 ] and temperatures in order to assess their response in terms of growth and allometry. Fast-growing annuals were selected for the study because their growth responses could be assessed over a major portion of the plant's life cycle and in as short a period as 8 wk. Plants were grown in eight hemi-spherical glasshouses, programmed to track outside ambient conditions and provide a replicated experimental design. Treatments comprised (i) current ambient [CO2 ] and temperature, (ii) elevated [CO2 ] (ambient+34 kPa), and ambient temperature (iii) ambient [CO2 ] and elevated temperature (ambient+3°C) and (iv) elevated [CO2 ] and elevated temperature (T°Celev ). All five species responded positively to [CO2 ]elev , although the response was statistically significant for only one, Poa annua L. Averaged over all five species, [CO2 ]elev increased total plant biomass by 25% (P=0·005) at 56 d, reflecting a proportionally greater increase in leaf and stem mass relative to root weight. Elevated [CO2 ] had no effect on leaf area, either at the individual species level or overall. Elevated T°C, by contrast, had little effect on shoot growth but increased root mass on average by 43% and leaf area by 22%. Few interactions between elevated [CO2 ] and T°C were observed, with the CO2 response generally greater at elevated than ambient T°C. Both [CO2 ]elev and T°Celev resulted in a transient increase in relative growth rate, (rgr), during the first 14 d exposure and a 3°C increase in temperature had no effect on the duration of the response. CO2 stimulation of growth operated through a sustained increase in net assimilation rate. (nar), although the potential benefit to rgr was offset by a concurrent decline in leaf area ratio (lar), as a result of a decrease in leaf area per unit leaf mass (sla). The response to T°Celev was generally opposite of that to [CO2 ]elev . For example, T°Celev increased lar through an increase in sla and this, rather than any effect on nar, was the major factor responsible for the stimulation of rgr. Allometric analysis of CO2 effects revealed that changes in allocation observed at individual harvests were due solely to changes associated with plant size. Elevated T°C, by contrast, had a direct effect on allocation patterns to leaves, with an increase in leaf area expansion relative to whole plant mass during the initial stages of growth and subsequent increased allocation of biomass away from leaves to other regions of the plant. No change in the allometric relation between roots and shoots were observed at either elevated [CO2 ] or T°C. We conclude, therefore, that allocation of biomass and morphological characteristics such as sla, are relatively insensitive to [CO2 ], at least when analysed at the whole-plant level, and where changes have been observed, these are the product of comparing plants of the same age but different size.

Increasing concentrations of nitrogen dioxide pollution in rural Wales.

Monitoring of nitrogen dioxide pollution was carried out in rural environments throughout Wales during a 1-year survey to quantify any changes in background concentrations and distribution of the pollutant since an earlier survey in 1986. There were 23 sites in the present survey of which 16 had been monitored during the 1986 survey. The remaining 7 sites were based on moorland in mid-Wales within map squares for which critical loads for soil acidification are expected to be exceeded by the year 2005. All sites were chosen so as to be remote from major local sources of NO(2) and the values obtained were deemed to be minimum concentrations for the different regions. Measurements were made using diffusion tubes which aimed to provide mean concentrations of NO(2) for 2-week exposure periods. Concentrations of NO(2) were found to be higher in the winter months for most sites and this is probably related to a greater use of fossil fuels for heating buildings at this time of year. The exception was the high concentrations of NO(2) in May and June for several sites in North Wales, and in July and August for a site on Mount Snowdon. These high summer concentrations in North Wales are thought to be related to increased traffic associated with tourism. It is apparent that there has been a substantial increase in rural concentrations of NO(2) throughout Wales since the earlier survey of 1986. As an average of all 16 sites used in both surveys, there was a 53% increase in the annual mean concentration of NO(2). Also, it is evident that, since 1986, there has been a substantial increase in the area of south-eastern Wales which has a background level in excess of 10 ppb NO(2) and a notable reduction in land area with concentrations below 6 ppb NO(2) as an annual mean concentration. The possible future impact of increasing rural concentrations of NO(2) on Welsh vegetation is discussed with references to estimates of critical levels of NO(2) for adverse effects on plants.

An improved Solardome system for exposing plants to elevated CO2 and temperature.

Ventilated Solardomes (hemispherical glasshouses) have been used for 20 yr for studying the effects of gaseous pollutants on plants. This paper describes a computer-operated facility for studying the effects of CO2 × temperature regimes on plants. The eight chambers were set up for factorial design experiments - with two levels of CO2 (ambient and ambient + 340 ppmv), two levels of temperature (ambient and 3 °C tracked continuously above ambient) and two replicates of each CO2 × temperature treatment. Monitoring of environmental conditions within the chambers over a 2 yr period has shown highly effective control of CO2 and temperature regimes. Even with high-quality and u.v.-B transmitting glass, the irradiance in the PAR region Was reduced by 18% within the domes. Variation in temperature across the radii of the domes increased with higher photosynthetic photon flux density (PPFD). Vapour pressure deficits CVPDs) in the ambient temperature domes compared well with outside conditions hut were higher in the elevated temperature domes. The watering regime within the domes affected intermittently the relationship between 'dome' and 'outside' VPDs. The Solardome facility has been used extensively for studies of the impacts of climate change within the UK Programme on Terrestrial Initiative on Global Environmental Research (TIGER).

Foliar injury in young Betula pendula roth., Salix purpurea L. and Ilex aquifolium L. trees and in propagated Taxus baccata L. shoots exposed to intermittent fog at a range of acidities.

The occurrence of various visible symptoms of foliar injury was assessed on a 5-point scale for each of the species. There was a distinct increase in the incidence of most types of injury in plant material which has been exposed to pH 2.5 acid fog. No clear pH-related trend in foliar injury occurred in plants exposed to acid fog at pH 3.5, pH 4.5 and pH 5.6, indicating that a threshold for a range of different types of visible injury may exist between pH 2.5 and pH 3.5 for all four species.

Effects of acidic gases and mists on the reproductive capability of three fern species.

The effects of exposure to 40 nl litre(-1) SO2 + 40 nl litre(-1) NO2 on the reproductive biology of Polypodium interjectum (Shivas), Dryopteris affinis (Lowe) Fraser-Jenkins and Phyllitis scolopendrium (L.) Newman were investigated after 14 weeks exposure in a closed chamber fumigation system. The numbers of sori per pinna were reduced in response to SO2 and NO2 for D. affinis but were unaffected for the other species. Numbers of sporangia in sori and spore viability were reduced in P. interjectum and P. scolopendrium but not in D. affinis in response to the SO2 and NO2 treatment. Spore size was not affected by the pollution treatment. A separate experiment tested viabilities of spores collected from the three species in response to daily spraying with simulated mists at pHs of 2.5, 3.5, 4.5 and 5.6. For all three species, there was little or no spore germination in the pH 2.5 treatment and significantly reduced germination in response to the pH 3.5 as compared to the pH 4.5 and pH 5.6 treatments.

Effects of rural roadside levels of nitrogen dioxide on Polytrichum formosum Hedw.

Polytrichum formosum Hedw. was exposed to 60 nl litre(-1) (122.4 microm(-3)) NO2 for 37 weeks in a closed chamber fumigation system. This concentration was chosen to simulate roadside levels in rural areas. Over an initial winter period (October-January) growth of existing shoots was stimulated by NO2. When new growth was recorded in April and May, NO2 pollution over winter and spring had resulted in a 36% reduction in new (< 1 cm) shoot production, and a 46% reduction in old shoots showing new growth. It is concluded that plants of Polytrichum formosum Hedw. growing near to roads may be adversely affected by NO2 pollution. Adverse effects of NO2 on plants and possible synergistic effects with other pollutants could cause growth reductions in sensitive species, thus affecting species composition of roadside vegetation.

Exposure of two upland plant species to acidic fogs.

A system is described for exposing large numbers of plants to acidic fogs. The system allows low volumes of treatment solutions to be provided at particle sizes chiefly in the 5-30 microm range (equivalent to fog/cloud droplets). Plants of Poa alpina L. and Epilobium brunnescens were propagated from material collected in Snowdonia, North Wales and exposed to fog treatments at pH values of 2.5, 3.5, 4.5 and 5.6. There were 3 x 4 h exposures per week which provided a total of 6 mm deposition. Supplementary watering was with pH 4.5 simulated acid rain (24 mm per week). After 21 weeks, there was increased lowering and a greater dry weight for plants of E. brunnescens exposed to the pH 2.5 fog in comparison with other treatments. Also, the plants used assimilated material to form shoots rather than roots. A similar increase in dry weight accumulation in the pH 2.5 treatment was found in P. alpina after 63 weeks but this was not associated with changes in assimilate partitioning.

Effects of nitrogen dioxide pollution on the growth of three fern species.

The effects of exposing plants of Dryopteris filix-mas (L.) Schott, Phyllitis scolopendrium (L.) Newman and Polypodium vulgare L. to 60 nl litre(-1) (122 microg m(-3)) NO(2) for 37 weeks were investigated in a closed chamber fumigation system. There was no effect of NO(2) on the numbers of fronds produced for any species at any time during the exposure period. However, at the end of the study, there was a lower dry weight yield of green shoots of D. filix-mas and P. scolopendrium and a higher yield of green shoots of P. vulgare for plants in the NO(2) treatment as compared to control plants. These differences in shoot dry weights were not accompanied by an effect of NO(2) exposure on total plant dry weights.

Growth responses of three legume species exposed to simulated acid rain.

Seedlings of Vicia faba L., Phaseolus multiflorus L. and Pisum sativum L. were raised during exposure to simulated acid rainfall treatments of pHs 5.6, 4.5, 3.5 and 2.5 at a rate of 30 mm per week. All three species were found to be adversely affected by the more acid pH 3.5 and pH 2.5 treatments after 7-8 weeks of exposure. There were total plant dry weight reductions of 40% for V. faba, 31% for P. sativum and 28% for P. multiflorus exposed to the pH 2.5 treatment, as compared to those grown in the control (pH 5.6 treatment). In addition, V. faba was found to be sensitive to the pH 4.5 treatment with an 18% reduction in total plant weights (compared to plants grown in the pH 5.6 treatment). In P. multiflorus, reduction in the dry weights of shoots in response to increasing acidity of rain was not accompanied by reduction in root weights, indicating an interference in the partitioning of assimilates. It is concluded that these three species, and V. faba in particular, may be growing below their potential in much of the UK.

Rural concentrations of nitrogen dioxide pollution throughout Wales.

Monitoring of nitrogen dioxide was carried out at over 50 rural sites in Wales throughout 1986. All sites were chosen so as to be remote from any local sources of NO(2) and the values obtained were deemed to be minimum values for the different regions. Measurements were made using a diffusion tube technique which aimed to give mean concentrations of NO(2) for 2-week exposure periods. The results obtained have been used to generate pollution maps to show mean monthly levels of NO(2) for rural environments throughout Wales. It is apparent that levels of NO(2) are generally higher during the winter months. In addition, annual mean concentrations of the pollutant are greatest in the north-eastern and south-eastern parts of Wales with the lowest levels being found along the western coast. The work marks the completion of the first national survey of nitrogen dioxide pollution in Wales. The data are discussed in terms of the potential threat rural concentrations of NO(2) pose to crops and natural vegetation.

Growth responses of birch and Sitka spruce exposed to acidified rain.

Seedlings of birch and Sitka spruce were grown on a range of British soils for 2 years and exposed to simulated acid rainfall treatments of pHs 5.6, 4.5, 3.5 and 2.5. Both species developed visible leaf injury patterns when exposed to the pH 2.5 treatment. In Sitka spruce this leaf injury was followed by high needle loss during the first winter and greater mortality. Generally, height growth of Sitka spruce was unaffected by treatments, but acid rainfall at pH 2.5 increased the height of birch. Mean height of both species was strongly affected by soil type. Significant soils x treatment effects on the heights of both species indicated that on some soils plant growth responses to the treatments did not fit the general pattern. Hence, while the results indicate that generally ambient acidities of rainfall in the UK are unlikely to adversely affect the growth of birch or Sitka spruce, plants growing on some soils may be susceptible to injury.

The effects of simulated acid rain on the growth of three herbaceous species grown on a range of British soils.

Seedlings of winter barley, perennial ryegrass and white clover were grown on a range of British soils for 21-24 weeks and exposed to simulated acid rainfall treatments of pHs 5.6, 4.5, 3.5 and 2.5. Whilst leaves of white clover developed leaf lesions after 18 weeks of exposure to the pH 2.5 treatments, there were no signs of visible injury to the other two species. At harvest, it was noted, for all species, that there was a large amount of variation in the sizes of individual plants and this made it difficult to detect differences between the treatments for plants on an individual soil. However, in combined analyses for all soils, it was found that the treatments had substantial effects on the yields of plants. The yields of both winter barley and clover were highly correlated with rainfall pH, showing substantial reductions in the more acid rains as compared to the pH 5.6 (control) treatment. In contrast, plants of perennial ryegrass produced higher yields of shoots at the most acid (pH 2.5) as compared to the other treatments. It was noted that the pH 2.5 treatment resulted in a generally lower soil pH at the termination of the experiment.

Effects of ambient levels of air pollution on grass swards subjected to different defoliation regimes.

The urgent need to evaluate effects of air quality on plant growth in the field is discussed in relation to the difficulties which field-based studies pose in the interpretation of data. An initial study is described which aimed to overcome some of these difficulties and the data are discussed within the remaining limitations. The productivity of pure swards of Lolium perenne L. and Dactylis glomerata L., subjected to defoliation at different times of the year, was studied at field sites with different levels of sulphur dioxide pollution, for a period of three years. The field sites and propagation methods were chosen so as to minimise differences in environmental conditions other than the degree of air pollution. At the most polluted site, both species showed signs of visible injury during the first winter and subsequently produced much lower yields than grasses at the other sites. The effects of cutting regimes on the growth of D. glomerata and L. perenne differed for the three years of the study. However, there was a tendency for lower yields with frequent defoliation (every 6.5 weeks), as opposed to the less regular defoliation regimes. It was concluded that air pollution was at least partially responsible for the yield reductions recorded. However, it was considered that sulphur dioxide was not the only pollutant responsible.