Environ. Sci. Technol. 1993, 27, 2878-2884
Geographical and Seasonal Variation in Deposition of Selenium to Vegetation Philip M. Haygarth,’3t Anthony F. Harrison,* and Kevin C. Jones7 Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K., and Institute of Terrestrial Ecology, Merlewood Research Station, Grange-over-Sands, Cumbria, LA 11 6JU, U.K.
A field experiment was established to test the hypothesis that there are geographical and seasonal differences in the deposition of Se to vegetation. One soil was subdivided into replicate trays, sown with Lolium perenne, and transported to seven sites around England and Scotland chosen to represent a range of deposition environments. The herbage was harvested quarterly, and Se was determined using radiochemical neutron activation analysis (RNAA). Statistically significant differences in the Se content of herbage were found both geographically and seasonally. The concentration of Se in herbage a t a site in an industrial zone was an order of magnitude higher than that found in a rural district. Levels in the winter were 3-6-fold greater than in the summer due to washout in wet deposition. The study suggests that regional differences in grazing animal Se deficiency may be strongly influenced by atmospheric deposition with the atmosphere contributing up to 82% of total uptake. Introduction
Factors controlling the levels of Se in vegetation are important because of potential transfer through the food chain to grazing animals and humans. In most parts of Europe, Se deficiency poses a problem because of low levels of the element in grazing pasture (for livestock) (1)and wheat and other crops (for humans) (2). Conversely, high amounts of the element can have toxic effects on livestock and humans (3). The factors which control the Se inputs to vegetation may be (i) soil input; (ii) anthropogenic factors (fertilizer, sludge, organic waste, etc.); and (iii) atmospheric deposition. This study focuses on atmospheric deposition as a source of input to vegetation, with particular emphasis on geographical and seasonal differences. The global cycling of Se is strongly influenced by the atmosphere, and there are a number of emission sources, natural and anthropogenic (4-6). Natural emissions are generated by volatilization of Se from soils, plants, fresh and marine waters, and sediments, brought about by methylation from bacteria and fungi (e.g., Penicillium and Alternaria) of dimethylselenide [(CH&Se] and related chemical species (7). Marine inputs will be of particular significance in the U.K. because of the large coastal area. Anthropogenic emissions of Se are estimated to constitute about 20-40% of the U.K. atmospheric burden and are generated mainly from coal combustion and primary metal production (8). It is speculated that they will be emitted in the gas phase as SeOz [Se(IV)I (9,lO). Germani and Zoller (11)measured Se emissions from a coal-fired plant and found that 59% of total Se emissions were in the gas phase.
Atmospheric deposition has been shown to substantially alter the biogeochemical fluxes of Pb, Cd, As, and Se in Scandinavian ecosystems (12-14). In general, Pb, As, and Cd concentrations in soils are roughly an order of magnitude greater in Southern Norway than in Northern Norway, inferring geographical proximity to pollution sources from Northern and Eastern Europe. Se is also found to be enriched in the south and southeast of Norway, but levels of Se in the humus layers of soils were also shown to be higher close to the coast than at sites inland (15). These studies were empirically based and focused mainly on deposition to soils, mosses, and humus. However, there still remains very little understanding of the factors which affect levels of Se in herbage and, therefore, the deficiency of Se in grazing livestock. In the U.K., the geographical pattern of the Se deficiency syndrome cannot be correlated solelywith parent materialsoil geochemical relationships (16). Equally, there is evidence that atmospheric deposition has caused longterm temporal variation in the elemental concentration of terrestrial ecosystems for a range of atmophilic elements, includingse,and such work has implied that geographical as well as temporal variation in source warrants investigation (17-21). In view of this, we established the hypothesis that location and proximity to sources may be important in affecting the influence that atmospheric inputs of Se have on vegetation surfaces and, therefore, grazing animals. Seasonal trends in deposition were also investigated. Materials and Methods
* Address correspondence to this author at his present address: Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon, EX20 2SB, U.K. + Institute of Environmental and Biological Sciences. t Institute of Terrestrial Ecology.
Experimental Sites. Grass plants growing in the same soil were transported to a range of atmospheric environments (22-24) around England and Scotland. The sites were chosen to represent a range of atmospheric Se sources. A control was established in a filtered air ‘solar dome’ (glasshouse) at Lancaster University. The locations were chosen from two established research networks (Figure 1). Three sites called Wraymires, Chilton, and Styrrup have been monitored for particulate Se and bulk deposition by scientists from the Atomic Energy Research Establishment (AERE), Harwell, U.K., since the early 1970s (ref 24 and references cited therein), and the published information on Se already available made these contrasting sites an obvious choice. Sites at Goonhilly, Jenny Hurn, and River Mharcaidh were already established by scientists from the Warren Spring Laboratory’s United Kingdom Precipitation CompositionSecondaryMonitoring Network (23) and are established for the measurement of sulfate, nitrate, and ammonium. This background information gave insight into the likely depositional environment for Se at each of the sites. The seventh site was the control site conveniently located in a filtered-air glasshouse at Lancaster University. A detailed description of the sites is given in Table I.
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Envlron. Scl. Technol.. Vol. 27, No. 13, 1993
0 1993 American Chemical Society
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5. Chilton
6. Coonhilly
Figure 1. Location of the seven experimental sites used to study the geographical trends in Se deposltlon around the U.K.
Soil and Seed Tray Preparation. The soil used in the trays was sampled from the top 20 cm of a grassland at location SD 501 849 in south Cumbria, northwest England. The soil was chosen from under pasture from a farm where Se livestock deficiencies had occurred and was a typical brown earth of the Denbigh 1 Association described in Furness and King (29) and Jarvis et al. (301, with a mean total Se content of 0.68 mg k g l , which is typical of U.K. soils. The soil was homogenized in a concrete mixer and sieved to remove stones. The growth trays were supplied by Ward Darlston (Staffs), having internal dimensions of 35 cm X 21 cm X 6 cm (deep), giving an internal volume of 4410 ~ m and - ~a surface area of 735 cm-2. Aliquots of 4.37 kg (dry weight) of the homogenized soil were added to each tray to give a final dry density of ca. 1 g cm-3, equal to that found at the originalgrassland sample site. The trays were sowed with Hercules Perennial Ryegrass (Loliurn perenne). The application rate of 3.25 g m-2 is recommended to farmers by the British Seed Atlas, but an application rate of 5 g m-2 was chosen to counter effects of possible low germination rates. A rectangular arrangement was chosen for the trays. The trays were sunk into the ground so that the natural grass surface was flush with the surface of the trays to limit artificial air turbulence over the surface. The trays were delivered to the sites during April 1990 after having been sown with L. perenne. The first sampling took place in July 1990 and quarterly thereafter during October, January, and April with the final samples taken during late July 1991. Four trays were left at seven sites including the control during April 1990 for ca. 15months. The samples were analyzed quarterly for total Se, and the results were compared to assess the contribution of the atmosphere.
Four trays were used for sampling at each site, but the four samples were bulked together in 6/7 sites and on 4/5 sample events. In order to establish the degree of error for the work generally, it was decided to maintain full replication at one site throughout the full year. River Mharcaidh was chosen because it had a reasonably consistent yield. Additionally, full replication across all the sites was maintained throughout July 1991 to determine the degree of geographical error. Sampling and Analysis. The herbage was sampled by cutting with sheep shears to 1 cm above the grass surface, taking care not to contaminate with soil. The samples were transported to the laboratory inside sealable plastic bags and air-dried at room temperature. Green matter not identified as L. perenne was removed and discarded after drying. The washing of the leaves was deliberately avoided. Dry yield of each tray was determined by weighing. Total Se was determined by radiochemical neutron activation analysis (RNAA) using a method developed by Steinnes (31). This involved irradiation at 2 X 10l2n cm-2s, radiochemical separation using 5 mL of HzS04 and 15 mL of HN03 in the presence of a 2-mg stable Se carrier, and detection at the 0.41 MeV photopeak. The method was chosen because it had a very low working detection limit (0.5 ng on solid material), was reproducible, and was not subject to digestion problems and interferences common with other methods (32, 33). The quality control was maintained through analysis of certified reference material (CRM ryegrass at 0.028 mg k g l ) (34). A mean value of 0.020 mg k g l (mean of 11)was achieved with a coefficient of variation of 8.9%. Results
Method Variability. The study revealed seasonal and geographical differences in Se concentration in vegetation (Table 11). However, the clarity of the differences is often occluded by variation. Factors which are likely to have contributed to the variability are leaf contamination, soil change, and difference in plant yield. These issues are considered before the wider implications for the Se cycle are discussed. It was deliberately decided not to wash samples before analysis as this may remove particulate material freshly deposited from the atmosphere. Implicit in this decision is the assumption that all plant contamination, whether taken into or onto the plant or derived locally (from soil), technically constitutes an atmospheric contribution to the ryegrass. Nevertheless, it is important to acknowledge the relative importance of contamination from locally derived soil particles as opposed to other long-rangetransported (LRT) Se. These mechanisms have been reviewed in detail by others (35-38). The purpose of the experimental design was to keep the supply of Se from the soil constant across the sites. However by July 1991 the soil pH and organic matter content at the seven different locations had become statistically significantly different. In the extreme, pH at Chilton was ca. 6.4 and pH at Wraymires was ca. 5.5. The organic matter (loss on ignition) ranged from ca. 8% at River Mharcaidh to ca. 10% at Wraymires. Both these differences were tested with a one-way ANOVA, and the differences for both pH and organic matter were statistically significant (p = 0.001). This implies that, by changing the atmospheric environment, the soil climate Envlron. Scl. Technol., Vol. 27, No. 13, 1993 2879
Table I. Site Descriptions
grid reference
altitude (m)
River Mharcaidh NH 876 052
274
Wraymires
SD 362 974
84
Styrrup
SK 562 906
53
Jenny Hurn
SK 816 986
4
Chilton
SU 468 861
130
Goonhilly
SW 723 214
108
control
total 1990 rainfall (25) (mm)
comment
1164 (Insh, Spey)
the most ‘rural‘ of the sites, on the western flank of the Cairngorm Mountains surrounded by grassland, heather moorland, and forestry, ca. 5 km to the east of Loch Insh, 10 km south of Aviemore; No maritime or known industrial emission sources are within 100 km 1944 (Hawkshead) regarded as a rural site located on the east side of Esthwaite Water between Windermere and Hawkshead in the Lake District, Cumbria; relatively uncontaminated and a moderate maritime effect (24);higher than average annual rainfall, 25 km from the Irish Sea, 40 km north of the Heysham oil refinery, and 32 km to the northeast of Barrow-in-Furness, where there is ship-building industry; there is no local arable farming which may contribute to local soil dusty (24,26) 764 (Sheffield Weston Park) along with sites 2 and 5, Styrrup has been used by Cawse (24) for measurements of total Se in air and bulk deposition; the site is ca. 5 km to the southeast of Maltby, South Yorkshire, in the grounds of Sandbeck Park, a rural setting, ca. 25 km to the east of the Sheffield-Rotherham conurbation, where the use of metals in manufacturing and coal burning is common; more locally, the Dinnington, Bircotes, and Maltby collieries lie within a few km of the site, and fossil fuel combustion for power generation is prevalent in the surrounding area 401 (Gainsborough situated on the east bank of the River Trent ca. 12 km south of Sewage Works) Scunthorpe and 9 km north of Gainsborough in South Yorkshire; the site is about 40 km to the east of Styrrup (site 3) and apart from being further away from the Sheffield-Rotherham conurbation and closer to the North Sea, Jenny Hurn has broadly similar surroundings to Styrrup; it is immediately surrounded by cereal agriculture where stubble burning was observed during the experiment, which may contribute to air particulate loadings 446 (Didcot Sewage Works) a rural site ca. 90 km from the English Channel and ca. 25 miles south of Oxford, on the westerly perimeter of AERE at Harwell, Oxfordshire; the surroundings are predominantly rural and mainly devoted to cereal cultivation (24,273 771 (Lizard) rural surroundings situated in the perimeter of the British Telecom Satellite Earth Station 10 km south east of Helston close to Lizard Point; the marine influence is of undoubted significance, since the site is situated on the Cornwall peninsular, only 5 km from the Atlantic Ocean; no significant industrial or alternative emission sources are known to be within the district at the biological field station on the south side of the Lancaster University campus; trays situated in a ‘solar dome’ greenhouse but having a filtered (i.e. clean) air input achieved by filtration through charcoal and phormisol. Charcoal has been shown to be an efficient trapper of gas phase Se (28);the trays were watered with deionized water on a regular basis
Table 11. Total Se (mg k g l ) in Unwashed Herbage at Six Sites around the U.K.s
date
River Mharcaidh
Wraymires
Styrrup
Jenny Hurn
Chilton
Goonhilly
control
quaterly meanb
July 1990 Oct 1990 Jan 1991 Apr 1991 July 1991 site meana
0.052 (0.004) 0.058 (0.006) 0.200 (0.009) 0.041 (0.005) 0.032 (0.001)
0.076 0.127 0.319 0.046 0.087 (0.016)
0.088 0.467 0.609 0.052 0.093 (0.003)
0.205 0.084 0.077 0.041 0.086 (0.019)
no sample 0.074 0.126 0.032 0.048 (0.005)
0.076 0.125 0.290 0.045 0.080 (0.012)
0.046 0.038 0.044 0.028 0.078 (0.018)
0.09 (0.024) 0.14 (0.056) 0.24 (0.073) 0.041 (0.0031) 0.072 (0.0086)
0.077 (0.031)
0.13 (0.021)
0.26 (0.12)
0.099 (0.028)
0.070 (0.021)
0.12 (0.044)
0.047 (0.084)
a Standard error shown in parentheses, value derived from mean of four trays. Where there is no standard error shown, the value is derived from bulked sample from four trays. The difference between the field sites (Le., control data excluded due to variance) was tested using a one-way analysis of variance (ANOVA) and an F value of 1.42 ( V I = 5, v 2 = 23) was computed. It was concluded that the difference between the sites was not statistically significant QJ = 0.05). The difference between the months was tested (using a one-way ANOVA) from replicate trays at River Mharcaidh this differene was statistically significant QJ = 0.01) (F = 148, v 1 = 3, v 2 = 12). Means derived from the mean of each row/column.
and the soil chemical parameters also changed, which may have affected the uptake of Se from soil. However, Johnsson (39) observed that Se uptake to spring wheat grain (Triticum aestiuum) and winter rape (Brassica napus) ‘was insignificant or nonexistent’ in the pH range 5.0-7.0. It is therefore concluded that changes in soil pH and organic matter content over the ranges shown are 2880
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No. 13, 1993
very unlikely to account for the substantial differences that were found in the Se herbage content. Se in Herbage with Plant Yield as Covariate: Main Effects Model. Statistical testing has been shown to be restricted by high variances, an inevitable product of fieldbased experimental work, where a number of factors have apparently interacted to make the interpretation of the
0.6
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2. There is no evidence for an influence of plant yield, which implies that the relationship between yield and Se concentration (Figure 2) is spurious. 3. There is no evidence for an overall site effect. However, given the apparent difference of the Styrrup site in respect to the other sites, it was decided to take the analysis a stage further, examining Styrrup with respect to all the other sites. A contrast between Styrrup and the other sites was fitted, effectively decomposing the sum of the squares for the site into two components. It was concluded that Se concentration in L. perenne at Styrrup, using yield as covariant and with season and site as interaction terms, is significantly different (p = 0.016).
I
0.5
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5
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0.4
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e; 0.3e 0
ln
0.2
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Discussion
0.0
+ 0
I
100
I
m
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Yield (grams of herbage per square metre)
Figure 2. Relationship between Se concentration and herbage yield (log +log e transformation in inset).
initial hypothesis more difficult. In this data set, Se concentration of herbage is a function of location, season, and, perhaps most confusingly, productivity. The relationship between Se concentration and yield is less easily resolved. A main effects model was employed to overcome these variations. Ehlig et al. (40) observed a dilution effect of higher yields on Se uptake from soil to timothy grass (Phteum pratense),although the issue was admittedly complicated since a positive relationship between dry matter production and concentration was observed for alfalfa (Medicago sativa). In contrast, Pinder et al. (41)reported a positive relationship between plant biomass and dry interception of particulate Se from the atmosphere, implying that greater biomass causes a proportionately greater atmospheric uptake. In this data, higher yields generally associate with lower concentration, illustrating the dilution effect noted by Ehlig et al. (Figure 2). The relationship becomes closer to linear after undergoing a log e-log e transformation (Figure 2, inset). An analysis of covariance, based on this general linear model and using a log e transformation was undertaken to gain insight into the aforementioned interactions. Criteria for homogeniety of variances need not be fulfilled since interactions are an integral part of the model. The model may be expressed as log e (Se concentration in L. perenne) = [ a + b log e yield] + [site effect] + [season effect] where a and b are model parameters, the yield is the covariate with Se concentration, and [site effect] and [season effect] are factors. The model was extended to allow for interaction terms such as that between log e yield and site. The missing data from Chilton, July 1990, due to no productivity made it necessary to conduct an unbalanced analysis, using the Glim 4 statistical package. The results of the analysis, which used Se concentration with yield as covariate and season and location as interaction terms, made it possible to conclude that: 1. There is a statistically significant interaction with the seasonal effect (p = 0.03).
Geographical Variations. With reference to Figure 3, the proximity to the pollution source appears to be important in affecting the Se levels in herbage. Campbell et al. (22,23)have monitored 32 sites for sulfate, nitrate, and ammonium in wet deposition around the U.K. and found that the greatest deposition occurs around the South Yorkshire-Nottinghamshiredistricts, with least deposition in Northwest Scotland. This compliments the findings of our studies for Se. Cawse (24) has studied air particulate concentration and deposition of heavy metals to bulk deposition samplers at different locations around the U.K.For Se, one of the most heavily polluted sites is at Styrrup near Maltby in South Yorkshire, with up to 3.8 ng m-3 particulate Se and bulk deposition of 520 pg of Se m-2 year-l (24). Atmospheric Se was reported to be lower in the rural sites of Chilton and Wraymires (24,26). Seasonal differenceswere also apparent, with the degree of difference dependent on geographical location. Harrison and Chirgawi (42) used moss bags colocated with plants at 21 sites around the U.K.to estimate the atmospheric contribution of Zn, Cd, Ni, Cr, and P b to vegetation, comparing a ‘background’ site to a ‘polluted’ site and found geographical differences. In our experiments, the control site has the lowest mean Se concentration (0.047 mg kgl), and this is to be expected since the only atmospheric emission sources will be volatilization from the other soils and plants (7,43)growing in the solar dome. Styrrup had the highest mean Se concentration in herbage (0.26 mg kg-l) (found to be significantly different to the other sites at p = 0.016). This is largely explained in terms of the high particulate emissions associated with coal burning and metal production prevalent in the surrounding district. Dust from the local spoil heaps from nearby mines such as the Maltby mine may also contribute to the airborne Se burden. Cawse (24) found that this same site had the highest Se air and bulk deposition levels in Great Britain, and Campbell et al. (22,23)found that this area had the highest levels of atmospheric S. These trends broadly continued during the time span of the experiment (44,45). Levels at Jenny Hurn (0.099 mg k g l ) are, perhaps surprisingly, much lower than at Styrrup despite their close proximity. This may be due to lower precipitation at Jenny Hurn, resulting in more efficient washout of Se pollution from the atmosphere at Styrrup. Moreover, Jenny Hurn may be more influenced by the maritime air mass from the North Sea than from the industrial zone. Goonhilly (0.12 mg kg-l) and Wraymires (0.13 mg kg-1) have similar herbage levels with concentrations second Envlron. Sci. Technol., Vol. 27, No. 13, 1993 2881
se (mgflrg) 0.7
-
0.6 ' 0.5
-
0.4 -
0.3 02
-
05
-
0.0
Flgure 3. Quarterly threedimenslonal plot of Se in vegetation at seven sites around the U.K., July 1989-July 1990.
Table 111. Theoretical Calculated Quarterly Se Deposition Flux (pg m-a) to Herbage at Six Sites around the U.K.*
River Mharcaidh
Wraymires
Styrrup
Jenny Hurn
Chilton
Goonhilly
residual supply from soil
July-Oct 1990 5 23 112 12 9 23 10 Oct 1990-Jan 1991 41 72 147 9 21 64 11 Jan-Apr 1991 3 5 6 3 1 4 7 Apr-July 1991 0 2 4 2 0 1 20 total annual flux 49 102 269 26 32 92 49 % uptake from atmosphere 39 64 82 53 33 61 Based on the following assumptions: (1)Differences observed in experimental work were wholly due to atmospheric deposition. (2)Dry aerial biomass of 260 g m-2. (3)Residual contribution from soil derived from control (clean air) site.
only to Styrrup. These sites are likely to have similar sources, with marine dimethylselenide gas (biological methylation) and sea particulate (associatedwith sea spray from turbulent seas) both contributing. The maritime effect is probably more significant at Goonhilly on the Cornwall peninsular receiving an incoming air mass from the Atlantic Gulf Stream, but Morecambe Bay and the Irish Sea will also have influenced Wraymires. The prevailing wind direction plays an important role in importing air masses which have passed over the Manchester-Liverpool conurbation to Wraymires. Chilton (0.070 mg kgl) and River Mharcaidh (0.077 mg kg-1) both had the lowest Se levels measured in herbage observed in this study (with the exception of the control). Anthropogenic inputs are thought to be low in these areas. River Mharcaidh has slightly higher levels than at Chilton, perhaps because it receives higher rainfall. Seasonal Variations. Seasonal variations are illustrated in Figure 3, and there is clearly a pronounced enrichment in the winter months. These can be summarized by calculating a seasonal enrichment ratio (winter/ summer = W/S ratio). Using summer values from the 2882
Envlron. Scl. Technol., Vol. 27, No. 13, 1993
mean of July 1990 and 1991,the calculated W/S ratios are River Mharcaidh = 4.76, Wraymires = 3.91, Styrrup = 6.72, Chilton = 2.63, Goonhilly = 3.72, and control = 0.71. The winter enrichment is explained in terms of the following: 1. Importance of anthropogenic sources: Combustion emissions are generally known to be higher in the winter months (24),and this will explain the high ratio at Styrrup. Furthermore, emissions from biological methylation of Se will be lower in the winter due to lower temperatures. For example, Thompson-Eagle et al. (46)showed volatilization of W e by Alternaria alternata was significantly greater at 30 than at 5 "C. 2. Meteorological factors; prevalence of inversions in the winter months (24) causing occult deposition and the effect of high seasonality in the rainfall: These factors are considered to be particularly important at Wraymires and River Mharcaidh. Seasonality at Goonhilly may be due to the greater incidence of turbulent seas in the winter months, providing a source of Se sea spray. Deposition Flux Calculations. Making the assumption that the geographical differences shown wholly reflect
Table IV. Estimated Se Deposition Characteristics at Three Sites
site
total annual total annual total calcd V,, depositionto deposition to particulate particle vegetation sampler % dry Sein the Seto (pg m-2 (pg m-2 depo- atmosphere herbage yea+) year-')? sition (ng rn-3)~ (cm 8-1)
Wraymirea Styrrup Chilton Q
:LO2 269 32
400 520 280
7b 15c 24'
1.2 3.8 1.6
0.019 0.034 0.015
From ref 24. From ref 26. After estimations from ref 47.
atmospheric deposition and assuming a constant biomass of 260 g m-2 (47), it is possible to make flux calculations for total deposition to grasslands at the six sites. The principle is to assume that the level in the control (clean air) site represents only soil uptake and that the difference at the field sites represents the contribution from the atmosphere. The calculated geographical flux is shown in Table 111. From the data, it can be implied that the importance of the soil contribution in relation to that from the atmosphere rises in the spring months in contrast to the role of the atmosphere which is very small, even at Styrrup. The relative importance of soil versus aerial inputs has also been calculated. The soikatmosphere ratio ranges from 67:33% at the Chilton semirural site to 18: 82% at Styrrup in the industrial zone. At four out of the six field sites, the percent contribution from the atmosphere was greater than that from the soil, and the mean contribution from the atmosphere for all the sites in the U.K. was %%I. Cawse (24) published data on the total air particulate Se and deposition at three of the sites (Wraymires, Chilton, and Styrrup) from studies conducted during 1972-1981, and this data can be used to build further estimations for our data. First, since deposition to samplers was used to determine the proportion of wet:dry deposition, the proportions of wet:dry deposition to vegetation can be inferred (Table IV). Further, assuming that all the dry deposited Se is in the particle phase, the dry deposition velocity (V,) of Se to the vegetation (48)can be estimated, using the total air particulate Se determined at these sites by Cawse (24). These are shown in Table IV and range between 0.015 and 0.034 cm s-l, which is an order of magnitude lower than values for dry deposition to arainfall sampler surface presented by Cawse (24). Conclusions
Levels of Se in herbage have been shown to be substantially influenced by atmospheric deposition. In the U.K., deposition in an industrial zone has caused levels to be an order of magnitude greater than in a site in a rural district. The deposition flux to herbage has also been found to be up to 6-fold higher in the winter than in the summer, indicating the importance of washout and rainout as a deposition process. A wider implication of the work is that regional and seasonal differences in grazing animal Se deficiency may be influenced by atmospheric deposition. Acknowledgments
We are grateful to the Natural Environment Research Council for financial support. Thanks also to Denise Wright and Alan Nelson for assisting with computing and
statistics and Jane Rushton for help with graphics. For provision of supportive data, access to field sites, and help with field sampling, we would like to thank Dr. Peter Cawse and Steve Baker (AERE, Harwell), Dr. Jimmy Irwin (Warren Spring Laboratory), Geoff Trevarthan, Simon Laity (both at Goonhilly), and Nigel Buxton (Nature Conservancy Council, Scotland).
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Received for review May 5, 1993.Revised manuscript received August 9,1993.Accepted August 30, 1993.' @
Abstract published in Advance ACS Abstracts,October 1,1993.
