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Journal of Environmental Quality 32:33-39 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Atmospheric Pollutants and Trace Gases

Stable Sulfur Isotope Ratio Indicates Long-Term Changes in Sulfur Deposition in the Broadbalk Experiment since 1845

F. J. Zhao*, J. S. Knights, Z. Y. Hu and S. P. McGrath

Agriculture and Environment Division, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom

* Corresponding author (Fangjie.Zhao{at}bbsrc.ac.uk)

Received for publication April 4, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Archived wheat (Triticum aestivum L.) grain and straw, and soil samples from the control plot of the Rothamsted Broadbalk Experiment, located in southeastern England and established in 1843, were used to investigate the effects of dramatically changing SO2 pollution inputs on the concentrations and stable isotope ratios ({delta}34S) of S in the samples. Representative coal samples from UK major coal fields were also determined for {delta}34S. Concentrations of S showed no clear trends in either grain or straw over the 155 years from 1845 to 1999. However, grain and straw {delta}34S decreased rapidly from 6 to 7{per thousand} in 1845 to -2 to -5{per thousand} in the early 1970s, and since then have increased to 0.5 to 2{per thousand} in the late 1990s. This pattern mirrored the trend of UK SO2 emissions over the 155 years. Both grain and straw {delta}34S correlated strongly and negatively with UK SO2 emissions (R2 > 0.89), but the relationships were different for the pre- and post-1970 data sets. Soil {delta}34S also decreased considerably, from 8.2{per thousand} in 1865 to 3.7 to 4.5{per thousand} during 1965–1999. A negative {delta}34S value was inferred for the anthropogenic S deposited at the experimental site before 1970, and further confirmed by negative {delta}34S values (-6 to -10{per thousand}) found in the coal samples from southeastern England and southern Wales. Based on the S isotope ratios, we estimated that anthropogenic S contributed 62 to 78% of the S uptake by wheat at the peak of SO2 emissions, and accounted for 28 to 37% of the topsoil S in 1965.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ELEMENTS IN terrestrial ecosystems are traditionally grouped as atmospherically derived (e.g., C and N) or rock derived (e.g., P, K, Ca, and Mg), although recent studies have shown the distinction to be less clear than previously thought (Chadwick et al., 1999). Sulfur is a special case in that it is derived from both atmosphere and rocks. Atmospheric S originates from natural and anthropogenic sources. In 1990, anthropogenic SO2 emissions were about three times those derived from natural processes globally (Rodhe et al., 1995). Over the last two centuries, human activities have had a dramatic effect on the global S cycling. Due to industrialization, global SO2 emissions from anthropogenic sources have increased by about 20-fold since 1850 (Brimblecombe et al., 1989). The increase in emissions was most rapid between 1940 and 1970 in Europe and North America, but the trend has been reversed since the early 1970s due to pollution control. The S concentrations of ice cores from Antarctica and Greenland have been shown to reflect the dramatically increased S up to around the 1970s and 1980s (Neftel et al., 1985; Brimblecombe et al., 1989). Because excessive S deposition can cause soil and water acidification, it is important that the fates of atmospheric S and the processes of S cycling are understood. Ironically, recent rapid decline in S emissions and deposition in western Europe has led to increased S deficiency in many agricultural crops, which remove substantial amounts of S from the soil annually (McGrath and Zhao, 1995; Zhao et al., 1999).

The stable S isotope ratio ({delta}34S) has been used extensively to identify sources and the fates of S in the environment (e.g., Krouse, 1977; Nriagu et al., 1991; Mayer et al., 1995; Ohizumi et al., 1997; Zhao et al., 1998; Alewell et al., 1999; Novák et al., 2000). This approach requires that different sources of S have distinctive isotopic ratios. If the S isotope ratios from different sources are known, then it is possible to apportion the contributions of S from these sources to a receptor. The {delta}34S values found in nature vary mostly within the range from -40{per thousand} to +40{per thousand} (Thode, 1991). Sulfate in sea water is enriched in 34S, with a {delta}34S value of approximately 21{per thousand}. Atmospheric S derived from marine aerosols bears the isotopic signature of sea water sulfate (Ohizumi et al., 1997; Novák et al., 2001a). Because the {delta}34S values of coals from different regions vary widely, it is often difficult to quantify the contributions of S to a receptor from coal-burning sources (Nielsen et al., 1991; Alewell et al., 2000). However, coals produced from certain regions may have characteristic {delta}34S values. For example, coals produced in southwestern China contain high levels of S and have negative {delta}34S values, whereas coals produced in northern China have low S contents and high {delta}34S values (Mukai et al., 2001). Furthermore, these authors showed that the {delta}34S values of atmospheric S in eight Chinese cities broadly reflected the regional differences in the {delta}34S values of coals. Ohizumi et al. (1997) also used the average {delta}34S of northern Chinese coals to estimate the contribution of S released by combustion of coals in northeastern Asia to the atmospheric deposition collected from the Niigata Prefecture in Japan. In Europe, it has been shown that natural background deposition is relatively enriched in 34S (higher {delta}34S) compared with anthropogenic deposition (Novák et al., 1995; Pichlmayer et al., 1998; Novák et al., 2001b). Thus, analysis of {delta}34S in vegetation and soil samples collected from long-term experiments may reflect the changing anthropogenic deposition of S over the last century.

Most studies using S isotope ratio have focused on the S cycling in natural ecosystems, and few have been done on agricultural systems. In this study, we analyzed long-term changes (>150 yr) in the S concentration and {delta}34S in plant and soil samples from the world's oldest agricultural experiment, the Broadbalk Continuous Wheat experiment at Rothamsted, England. Representative coal samples from UK major coal fields were also analyzed for {delta}34S to help infer the isotopic signature of the atmospheric S deposited at the experimental site. The main objectives were to investigate whether the {delta}34S values of archived plant and soil samples reflect long-term changes in anthropogenic deposition of S, and whether the isotopic results can be used to estimate the proportions of S in plant and soil that were derived from anthropogenic sources.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Wheat and Soil Samples from the Broadbalk Experiment
To investigate long-term changes in the S isotope composition in vegetation, we chose 44 samples of wheat grain and 38 samples of wheat straw from the Rothamsted Archive, which spanned more than 150 yr. The samples were from the control plot (no fertilizers, continuous wheat section where wheat straw was removed after harvest) of the Broadbalk Continuous Wheat Experiment at Rothamsted. The experiment started in 1843 in a semirural environment in Hertfordshire, southeastern England, approximately 40 km north of London (0°21' W, 51°49' N, elevation 128 m), and has been run continuously for 159 yr. Because the experiment was started well before the advent of modern statistics, there were no replicates of each treatment, although this is partly compensated by the large size of the plots (0.19 ha). The soil belongs to the moderately well drained Batcomb series (Aquic Paleudalf) with a flinty silty clay loam textured topsoil on clay-with-flints overlying chalk parent material. The original objective of the experiment was to test the effects of different inorganic and organic fertilizers on wheat yield. However, the experiment and the associated large array of archived crop and soil samples have become a unique and invaluable resource for a wide range of agricultural, environmental, and ecological studies (Johnston, 1997). For more details of the experiment, see the web pages of the Electronic Rothamsted Archive (http://www.iacr.bbsrc.ac.uk/res/corporate/ltexperiments/tbwinterwheat.html [verified 7 Aug. 2002]) and Johnston and Garner (1969). Winter wheat is normally sown in October and harvested in August, and yields of grain and straw are determined. Samples of straw and grain were dried at 80°C for 16 h before being archived in sealed glass jars or tin boxes. In this study, samples were selected in approximate 5-yr intervals, starting from 1845. In addition, the yearly samples between 1970 and 1978 and between 1990 and 1999 were also included. The majority of the grain and straw samples were from the same year, except in a few cases when either straw or grain samples were missing from the archive and samples from the next available year were used.

In 1999, wheat leaves and stems were sampled from the control plot of the Broadbalk experiment on three occasions between May and July. The samples were dried at 80°C for 16 h and ground to pass a 0.5-mm sieve. These samples were used to investigate whether S assimilation from sulfate to organic S in wheat is associated with an isotopic fractionation.

Soil was sampled from the control plot (0- to 23-cm depth) in the following years: 1865, 1881, 1904, 1914, 1944, 1966, 1987, and 1996. Twenty to thirty cores of soil were collected with an auger and bulked into one sample. The samples were air-dried, ground to <2 mm, and stored in sealed glass jars in the Rothamsted Archive. In May 1999, the soil was sampled again, and treated in the same way as before. Also in 1999, three lichen samples were collected from stone walls and from a tree, which were located within 200 m of the Broadbalk experiment. The lichen samples were cleared of debris, air-dried, and ground to pass a 0.5-mm sieve.

Coal Samples
Thirty coal samples were used in this study, including 26 from all the major coal fields in the UK and four from other countries for comparison. The samples were from the UK Coal Research Establishment (CRE) Coal Bank, which was established in 1982 to provide researchers with well-characterized UK coals (Burchill, 1995). Most of the coals were selected seam samples, and the initial 100 to 200 kg of representative lump coal was thoroughly mixed by CRE and stored in sealed plastic drums. Subsamples of about 0.5 kg each were taken and ground to <0.15 mm prior to chemical and isotopic analyses.

Chemical and Isotopic Analysis
The concentrations of total S and other elements in plant and coal samples were determined with inductively coupled plasma atomic emission spectroscopy (ICP–AES), following a digestion with HNO3 and HClO4 (Zhao et al., 1994). Sulfate in the wheat straw and grain samples from the Rothamsted Broadbalk experiment was extracted with hot water (80°C) and determined with ion chromatography. For S isotope analysis, approximately 10 mg of wheat, lichen, or coal samples was weighed into a tin capsule, and mixed with 20 mg vanadium pentoxide. The S isotope ratio was determined with a continuous flow isotope ratio mass spectrometer (20-20 IRMS; Europa Scientific Ltd., Crewe, UK), which was coupled to an elemental analyzer (ANCA-SL sample converter). A detailed description of the instrumental setup and operation conditions was given by Monaghan et al. (1999). Each sample was analyzed in duplicate. Sulfur isotope results are reported as {delta}34S in per mil. Repeated analyses of the reference materials IAEA S-1, S-2, and S-3 (Ag2S) and NBS 127 (BaSO4) gave mean values of -0.3, 21.8, -32.5, and 20.5{per thousand}, respectively. The results agreed very well with their respective certified values of -0.3, 21.6, -32.1, and 20.3{per thousand}. The overall analytical reproducibility was ±0.25{per thousand}.

To investigate whether there is an isotopic fractionation during S assimilation from sulfate to organic S in wheat, leaf and stem samples were extracted with hot water (80°C) to remove sulfate. After extraction, the plant residues were dried at 80°C, and the {delta}34S values determined. The extracted sulfate in the filtered solution was passed through a column containing an anion exchange resin (Amberlite IRA-420 in Cl- form; Fisher Scientific, Loughborough, UK), and leached with 3 M HCl. The BaCl2 was then added to the leachate to produce the BaSO4 precipitate. The BaSO4 precipitate was collected on a 0.2-µm polycarbonate filter membrane, and washed several times with deionized water. The {delta}34S value of BaSO4 was determined with a mass spectrometer.

Total S in soils was extracted with HNO3 and HClO4 digestion, and precipitated as BaSO4 (containing >1 mg S per sample) (Zhao et al., 1998). The soil sample collected in 1999 was extracted with 0.01 M CaCl2 and the extracted sulfate precipitated as BaSO4. The {delta}34S value of BaSO4 was determined with a mass spectrometer. Since soil cores were bulked in each year, it was not possible to determine the degree of spatial variation within the plot. However, we expect that the topsoil within each plot to be reasonably uniform as a result of annual plowing and cultivation.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Concentration and Isotope Ratio of Soil Sulfur from the Broadbalk Experiment
Changes in soil S concentration and {delta}34S in the control plot of the Broadbalk experiment between 1865 and 1996 were reported previously (Knights et al., 2000). Briefly, the concentrations of total S in the topsoil (0–23 cm) fluctuated little between 1865 and 1996, whereas KH2PO4–extractable SO2-4–S decreased considerably after 1965. The {delta}34S of soil total S decreased steadily from 8.2{per thousand} in 1865 to 4.5{per thousand} in 1965, and thereafter up to 1996 remained relatively stable at between 3.7 and 4.5{per thousand}. The data for 1999 (Table 1) continued the general trends observed for the previous years (Knights et al., 2000). In particular, the concentration of soil extractable SO2-4–S continued to decrease, whereas both the concentration of total S and its {delta}34S remained relatively stable over the last 34 yr. In 1999, extractable SO2-4–S was found to be more depleted in 34S than soil total S, with a difference in {delta}34S of 3.4{per thousand} (Table 1). Epiphytic lichens are good biomonitors for atmospheric S and their S isotope composition has been shown to resemble that of the atmospheric S (Krouse, 1977; Wadleigh and Blake, 1999; Wiseman and Wadleigh, 2002). Lichen samples collected in 1999 from within a 200-m distance of the Broadbalk experiment had a mean {delta}34S value of 2.4{per thousand}, which represents the isotopic signature of atmospheric S at the experimental site in recent years. The fact that the {delta}34S of soil extractable SO2-4–S was lower than that of both soil total S and lichen S (Table 1) suggests that mineralization of organic S to SO2-4 is associated with isotopic fractionation. Several studies have shown a depletion of 34S in SO2-4 compared with soil organic S (Gebauer et al., 1994; Mayer et al., 1995; Novák et al., 2000; Alewell and Novak, 2001).


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  		Table 1. Sulfur concentrations and {delta}34S in the soil from the control plot of the Broadbalk experiment collected in 1865 and 1999, and in lichens collected nearby in 1999.

 
Long-term Trend in the Sulfur Concentration and Isotope Ratio of Wheat from the Broadbalk Experiment
The concentrations of total S in wheat grain and straw samples from the control plot of the Broadbalk experiment fluctuated between 1.1 and 2.0 g kg-1 and 0.6 and 3.2 g kg-1, respectively (Fig. 1a) . Despite large changes in the SO2 emissions in the UK over the last 150 yr (Fig. 1b), there was no clear trend in the concentration of total S in either grain or straw from this site. The concentration of grain S fluctuated within a narrower range than that of straw S, probably because the nutrient concentrations in plant reproductive tissues are more tightly controlled than those in the vegetative tissues. The straw samples contained a much higher percentage of the total S as sulfate (47.0–75.4%, mean 61.3%) than the grain samples (3.3–12.3%, mean 9.6%). Sulfate can be leached from mature plant tissues depending on rainfall (Gregory et al., 1979), thus contributing to the large fluctuation of the total S concentration in straw. Other studies have shown a clear response in the concentrations of S in different types of vegetation, including pasture (Zhao et al., 1998), spruce trees, and mosses (Novák et al., 2001a), to the changing inputs of atmospheric deposition. The lack of a response in wheat (Fig. 1) may be because the crop grown on the control plot was always limited by nitrogen.



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  		Fig. 1. Concentrations of (a) S and (b) {delta}34S in wheat straw and grain from the control plot of the Broadbalk experiment. The data for UK SO2 emissions (b) are from United Kingdom Review Group on Acid Rain (1983) and National Expert Group on Transboundary Air Pollution (2001).

 
The stable S isotope ratio ({delta}34S) in wheat grain and straw decreased rapidly from 6 to 7{per thousand} in 1845 to between -2 and -5{per thousand} in the early 1970s, and since then had increased to 0.5 to 2{per thousand} in the late 1990s (Fig. 1b). In 1999, when the {delta}34S for soil extractable SO2-4 was available (Table 1), both straw {delta}34S (1.7{per thousand}) and grain {delta}34S (0.8{per thousand}) were close to the value for soil extractable SO42- (1.0{per thousand}). This suggests that soil SO2-4 was the dominant S source for plant uptake and that uptake per se was associated with little isotopic fractionation. This is in agreement with Monaghan et al. (1999), who found that the {delta}34S values in wheat were similar to the values of the S sources supplied in a hydroponic culture.

The pattern of changing {delta}34S in wheat straw and grain mirrored the pattern of the SO2 emissions in the UK (Fig. 1b), indicating that the soil indigenous S (derived from the parent material) and the S from anthropogenic sources had very different isotopic signatures at the experimental site. In a previous study (Zhao et al., 1998), we showed a similar pattern in herbage {delta}34S from a long-term (>130 yr) grassland experiment, which is located near the Broadbalk Continuous Wheat Experiment at Rothamsted.

Regression analyses showed a strong and negative correlation (P < 0.001) between grain or straw {delta}34S and annual SO2 emissions in the UK over the last 155 yr (Fig. 2) . The data sets before and including 1970 (the peak of SO2 emissions) and after 1970 were used separately in the regression analyses, because it is evident that they did not follow the same regression line. For both grain and straw, the intercept and slope for the post-1970 data set were smaller that those for the pre-1970 data set. This can be explained by a "memory effect" of anthropogenic S (with a low {delta}34S) in the soil–plant system, as a result of cycling of S between inorganic and organic pools in the soil, which caused a delay in the "recovery" of plant {delta}34S. Even if UK SO2 emissions were to decrease to zero in the future, plant {delta}34S would not return to the high values recorded at the beginning of the experiment. This is because a proportion of the soil S has been derived from anthropogenic sources, which will remain in the soil–crop system for a long time.



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  		Fig. 2. Relationships between UK SO2 emissions and (a) wheat grain {delta}34S or (b) straw {delta}34S in the Broadbalk experiment.

 
The strong relationships between plant {delta}34S at the experimental site and national SO2 emissions over the last 155 yr are striking, and imply that the changes in the inputs of anthropogenic S at the site were parallel to the changes in the national SO2 emissions. This is not unreasonable considering the proximity of the experimental site to London. Extrapolation of the regression lines for the pre-1970 data sets to zero emissions of SO2 gave 7.2 and 7.8{per thousand} for grain and straw {delta}34S, respectively, with a mean of 7.5{per thousand} for the whole plant. This value can be considered as an estimate of {delta}34S in plants growing at the site before any anthropogenic influence. Assuming that S uptake by wheat is not associated with isotopic fractionation, but that mineralization of organic S to sulfate in soil leads to 2 to 3{per thousand} depletion (see above), the total S in the soil of the experimental site before any anthropogenic influence would have a {delta}34S of about 10{per thousand}. This value represents an estimate of the mean isotopic signature for the S derived from soil parent material at the site and from other nonanthropogenic sources, for example, S from marine aerosols and biogenic and volcanic emissions. Marine aerosol–derived S was less than 1 kg ha-1 yr-1 in the region where the experimental site is located (United Kingdom Review Group on Acid Rain, 1983; National Expert Group on Transboundary Air Pollution, 2001), and thus constituted only a small proportion of the total atmospheric deposition recorded at the site, which varied from 10 to 70 kg ha-1 yr-1 over the last 150 yr (Sverdrup et al., 1995).

The linear relationship between plant {delta}34S and national SO2 emissions, particularly in the period before 1970 (Fig. 2), suggests that the anthropogenic S deposited at the experimental site had a relatively constant isotope ratio. Mixing models described by Krouse (1980) could not be used to estimate the isotope ratio of anthropogenic S for the pre-1970 period, because plant S concentration was independent of S emissions (Fig. 1). However, anthropogenic S deposited at the experimental site in the pre-1970 period must have had a negative {delta}34S to explain the negative values of plant {delta}34S during the most polluted phase from 1950 to 1970. In the post-1970 data set, there was a tendency that the data for recent years, when national SO2 emissions fell below 1.5 Tg S yr-1, deviated from the linear trend, and the plant {delta}34S approached a stable level of between 1 and 2{per thousand}. This is consistent with a {delta}34S value of about 2{per thousand} for the atmospheric S recorded in recent lichen samples (Table 1). It appears that atmospheric {delta}34S changed from a negative value in the early 1970s to a current value of about 2{per thousand}.

Sulfur Isotope Ratio of Sulfate and Organic Sulfur in Wheat
Figure 1 shows that all but one of the straw samples had a higher {delta}34S value than the corresponding grain samples. On average, straw {delta}34S was 1.3{per thousand} higher than grain {delta}34S. Because wheat straw contained proportionally more sulfate (approximately 60%) than wheat grain (approximately 10%), the difference in {delta}34S may be attributed to an isotopic fractionation during S assimilation. To test this possibility, sulfate was separated from organic S in eight samples of wheat leaves and straw collected from the control plot of the Broadbalk experiment in 1999. Sulfate had a {delta}34S of 2.7 ± 0.2{per thousand}, whereas organic S in the same samples had a {delta}34S of -0.6 ± 0.7{per thousand}. The difference in {delta}34S between sulfate and organic S (3.3 ± 0.7{per thousand}) was significant (p < 0.001), indicating that assimilation of sulfate to organic S was associated with a depletion of 34S by about 3{per thousand}. Using the average {delta}34S values for sulfate and organic S and their relative proportions in straw and grain, it can be estimated that straw {delta}34S on average would be 1.7{per thousand} higher than grain {delta}34S. This estimate is close to the average difference between straw and grain {delta}34S (1.3{per thousand}) observed in the samples from the Broadbalk control plot. Our results are consistent with the conclusion drawn by Trust and Fry (1992) and Novák et al. (2001a) that organic S is isotopically lighter than sulfate on average by 2{per thousand}.

Sulfur Isotope Ratio of United Kingdom Coal
On average, combustion of solid fuel (coal) contributed to 70% of the total SO2 emissions in the UK between 1970 and 1999 (National Expert Group on Transboundary Air Pollution, 2001). It is therefore useful to determine S isotope ratio in UK coals to help identify sources of S entering an ecosystem. Burchill (1995) presented data for the proximate, ultimate, maceral, and ash analyses for the coal samples used in this study. Table 2 shows the location, the concentrations of organic C, organic S, and pyritic S (data from Burchill 1995), as well as total Fe concentration and {delta}34S determined in this study. There were wide variations in the concentrations of total S (3.5–22.1 g kg-1), organic S (3.3–13.7 g kg-1), and pyritic S (0.2–13.4 g kg-1) in the UK coals. Pyritic S and organic S were the two main forms of S in the UK coals, whereas sulfate S was negligible (Spears et al., 1999). The concentration of pyritic S correlated significantly (r = 0.84, n = 26) with total Fe in the UK coals. Variation in total S was due mainly to the variation of pyritic S, because the two correlated closely and can be described by the regression equation:


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  		Table 2. Concentrations of organic C, organic S, pyritic S, total Fe, and {delta}34S in British coals.

 
The {delta}34S for the total S in the UK coals ranged from -11.4 to 7.8{per thousand} (Table 1). This range is within the range of -30 to +30{per thousand} reported for coals worldwide (Nielsen et al., 1991). Four of the five coal samples from southern Wales were particularly negative in the {delta}34S values (around -10{per thousand}). The only sample from Kent (southeastern England) also had a negative {delta}34S (-6{per thousand}). In contrast, samples from Scotland and northern England (Northumberland, Durham, and Yorkshire) were characterized by positive values (1.4–7.8{per thousand}). Samples from the Midland region of England (Midlands, Nottinghamshire, and Derbyshire) had {delta}34S values ranging from -3.5 to 5.5{per thousand}, with a tendency of lower values in the southern side of the Midland region. These results suggest a general geographical pattern of increasing {delta}34S in the coals from south to north. Environmental conditions for coal deposition and maturation are known to influence the S isotope ratio and pyritic S concentrations in coals (Nielsen et al., 1991). However, there was no significant correlation between {delta}34S and the concentration of either total S, organic S, or pyritic S in this data set.

Because of the large variability in coal {delta}34S, it is generally difficult to relate coal {delta}34S to atmospheric {delta}34S (Alewell et al., 2000). However, it is an unlikely coincidence that both the coals from southeastern England and southern Wales and the anthropogenic S deposited to the Rothamsted Broadbalk experiment prior to 1970 had negative {delta}34S values. We have not been able to find detailed records of the sources of coal used for combustion in Southeast England (including the greater London region) for the last 150 yr. However, anecdotal evidence and costs of transport suggest that, in the period up to the early 1980s, coals produced locally and from South Wales had been used in Southeast England. Since the early 1980s the sources of coal have become diversified, including imports of coal from overseas.

Contribution of Atmospheric Sulfur to the Broadbalk Experiment
Assuming a mean {delta}34S of 10{per thousand} for soil indigenous S derived from the parent materials and S derived from nonanthropogenic sources (see above), and a {delta}34S of between -6 and -10{per thousand} (values for coals from Southeast England and South Wales) for anthropogenic S in the pre-1970 period, it can be estimated that anthropogenic S contributed to 18 to 22% of the plant S in the initial years of the Broadbalk experiment (1845), and this contribution increased to 62 to 78% at the peak of pollution in 1970. Similarly, soil total S in 1865 contained 9 to 11% anthropogenic S, and the proportion increased to 28 to 37% by 1965 (with a soil {delta}34S value of 4.5{per thousand} according to Knights et al., 2000). These are rough calculations only, because of the uncertainty for the {delta}34S of anthropogenic S before 1970. Although combustion of coals contributed to the majority of the total SO2 emissions in the UK (approximately 70% between 1970 and 1999, and possibly >70% before 1970), uses of other fuels (e.g., petroleum and natural gas) also contributed to SO2 emissions; but there is no information available about the S isotope ratios in other fuels combusted in the UK. These calculations cannot be applied to the post-1970 period, because the isotopic ratio of anthropogenic S has probably changed considerably since then due to the long-distance transport of low-sulfur coals.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
We have shown in this study that {delta}34S values of the archived wheat grain and straw samples from the control plot of the Rothamsted Broadbalk Experiment reflected the dramatic changes in the SO2 emissions in the UK over the last 155 yr. Both grain and straw {delta}34S correlated strongly and negatively with UK SO2 emissions, but the relationships were different for the pre- and post-1970 data sets. Soil {delta}34S also decreased considerably, particularly during the period from 1865 to 1965. A negative {delta}34S value was inferred for the anthropogenic S deposited at the experimental site before 1970. Analysis of representative UK coal samples showed a geographical pattern in their {delta}34S values, with the coal samples from southeastern England and southern Wales being depleted in 34S ({delta}34S = -6 to -10{per thousand}). Using the S isotope data, we estimated that anthropogenic S contributed 62 to 78% of the S uptake by wheat at the peak of SO2 emissions in 1970, and accounted for 28 to 37% of the topsoil S in 1965.


   ACKNOWLEDGMENTS
 
We thank Maureen Birdsey for technical assistance in isotopic analysis. Rothamsted Research receives grant-aided support from the Biotechnology and Biological Sciences Research Council of the United Kingdom.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome