Environ. Sci. Technol. 1904, 28, 2170-2175
Effect of Phosphate Fertilizers and Atmospheric Deposition on Long-Term Changes in the Cadmium Content of Soils and Crops Fiona A. Nicholson' and Kevin C. Jones Institute of Environmental and Biological Sciences, Lancaster University, Lancaster LA 1 4YQ, United Kingdom
A. E. Johnston Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Hertfordshire AL5 2JQ, United Kingdom
Trends are reported in the Cd content of herbage collected and stored from the Park Grass Experiment at Rothamsted Experimental Station, a semirural location in the U.K. Samples from 1861to 1992 were bulked for 5-year periods from two phosphate fertilizer-treated plots, now with soil pH 4.9 (unlimed) and 6.5 (limed), respectively. Analysis of Cd was by Zeeman correction GFAAS following a nitric acid digestion. The data are compared with those from control plots which have not received superphosphate. On the unlimed plots, levels of Cd in the herbage from the phosphate-treated plot were very similar to those on the control for the years to 1930; whereas from 1940 to the present, the Cd concentrations in herbage were considerably greater on the phosphate-treated plot. On the limed plots, Cd concentrations in the herbage from both the phosphate-treated and control plots have remained similar to each other, although both plots exhibit a small increase of about 1pg of Cd k g l year-l since the beginning of the liming treatment in 1903.
Introduction
Quantification of the extent to which soils have accumulated Cd and the subsequent effect on plant uptake has important implications for human Cd exposure, since dietary intake of plant-based foodstuffs, especiallycereals, forms the major route of Cd exposure for the general population (1). In addition, Cd may enter the human food chain via the consumption of meat and milk from livestock that have received contaminated feedstuffs. Both atmospheric deposition and the use of phosphate fertilizers are major sources of Cd to agricultural soils (2, 3). Atmospheric emissions of Cd from anthropogenic activities have increased substantially worldwide over the last century ( 4 ) , and significant further increases are predicted by the year 2000 ( I ) ; at the same time, the application of phosphate fertilizers to agricultural soils has become a widespread practice. The rate of Cd input to soils from both of these sources is greater than the losses that occur through crop uptake, leaching, etc., and mass balance calculations suggest that the current Cd burden of contemporary soils is at a level considerably above the historical background and could increase further (5). Consequently, the potential Cd input to crops, both as a result of uptake from the soil and from direct deposition to the crop surface, has also changed over time. Previous studies have established trends in soil and crop Cd levels using samples from the long-term experimental plots at Rothamsted Experimental Station in the U.K. Analysis of soils from untreated plots over periods of up to 130 years by Rothbaum et al. (6) and Jones et al. (7) indicates that atmospheric deposition has been a significant source of Cd to these soils, increasing the Cd content 2170
Environ. Sci. Technol., Voi. 28, NO. 12, 1994
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of the top 23 cm by between 27 % and 56 % Data for soils receiving superphosphate additions showed some interesting differences. Where superphosphate has been added to soil low in organic matter, of neutral pH, and growing arable crops, there has been no extra accumulation of Cd above that observed on the control plots, even though at least 2 g of Cdlha was added each year in the superphosphate (6, 7). The results for acid grassland soils with 5% organic matter are very different. On these plots, there has been a much larger increase in Cd on the phosphatetreated soils than on the untreated soils, indicating that the use of phosphate fertilizers is increasing the Cd content of such soils over long periods of time. Long-term increases in soil Cd content could have major consequences in terms of increased food crop uptake of Cd; however, a brief review of the literature suggests that there is still some uncertainty in this area, in particular concerning phosphate fertilizers as a source of plant Cd. Some authors have reported a positive relationship between soil and plant Cd concentrations (8-10) especially on acid soils (5,11),although Singh (12)showed there was no simple relationship between the two. Results from several studies using phosphate fertilizers indicate that the Cd from this source is at least partially available for plant uptake (13);the Cd concentration in the fertilizer, the amount applied, soil properties, and crop species all being important variables. Mortvedt e t al. ( 1 4 )found that Cd concentrations in wheat products were not significantly changed by the use of phosphate fertilizers, except where large amounts of Cd were added to acid soils. Later, the same author analyzed crop samples from long-term (>50 years) fertility experiments in the United States and concluded that application of fertilizers has not resulted in increased Cd concentrations in major crops (15). Similarly, Rothbaum e t al. (6) analyzed plants grown on soils from long-term phosphate trials in New Zealand and found that very little of the Cd applied in superphosphate was taken up in grasslclover herbage. Analysis of selected samples from the Rothamsted archives suggests that there has been no consistent trend in the Cd concentration of winter wheat or spring barley grain grown on superphosphate- or farmyard manure (FYM)-treated plots over the last 100 years (16, 17), although concentrations were higher in winter wheat grown on P-treated plots (71 pg of Cdlkg) compared to between 30 and 40 pg of Cdlkg in both barley and wheat grain grown on other P- and FYM-treated soils. In contrast, Jones e t al. (18)reported increased Cd concentrations since 1861 in grass/legume/weed herbage grown on acid soil where the Cd had increaseddue to atmospheric deposition. Herbage samples from the limed section of this plot had consistently lower Cd levels than those from the unlimed section. From the analysis of a limited number of samples, there was some evidence that herbage from the P-treated Park 0013-936X/94/0928-2170$04.50/0
0 1994 American Chemical Society
Table 1. Cadmium Content of Superphosphates Applied to Rothamstead Experimental Plotsa
Table 2. Temperature Program for Cadmium Determination by Graphite Furnace
year Cd year Cd Rothamsted received content Rothamsted received content sample code at station (mg/kg) sample code at station (mg/kg)
step no.
furnace temp
P78 1925 ll.lb S6788 1955 5.1 P116 1929 29.0 s7379 1959 3.6 P122 1930 40.0 S7660 1962 4.0 P318 1941 20.gC S7932 1967 5.4 52709 1941 4.6c 58021 1970 4.3 P334 1941 7.9 s7973 1972 5.1 S2905 1942 4.6c S8063 1972 5.3 S3380 1943 8.4c S8lOO 1976 5.3 P575 1947 3.3 S8109 1977 6.9 S5951 1950 4.8 S8116 1978 7.1 P616 1951 17.lC 58172 1984 7.4c P620 1951 8.8c 58188 1986 46.gC P621 1951 19.8c S8194 1988 22.2c P638 1953 8.6c a Data from Jones et al. (7) except where indicated. Mean of GFAAS and XRF analyses. Data produced for this paper.
1
2 3 4 5
110 130 250 1900 2500
Grass plots contained a higher concentration of Cd (299 pglkg) than that from the control plot (203 pglkg) in the years since 1920 (16, 17). These analyses have been extended here, and this paper reports the trends in Cd over the last 130 years in the herbage from unlimed and limed sections of a P-treated Park Grass plot. Materials and Methods
Herbage, Soil, and Superphosphate Samples. The Park Grass experiment was started in 1856 on an area that had already been under permanent grassland for at least 200 years. Full details of the experiment are described elsewhere (19). The samples used in this study were taken from plot 4/1, which has received superphosphate (33 kg of P/ha) annually since 1859. Prior to 1889, the superphosphate was made from bone ash and sulfuric acid, which would have contained CO.1 mg of Cd/kg. Since then, commercial superphosphate has been used. Jones et al. (7) analyzed samples of superphosphates archived at Rothamsted from 1925 to 1978; the Cd concentrations of these and of some additional superphosphate samples analyzed for this study are presented in Table 1. The Cd concentrations between 1942and 1984have been relatively low, averaging 7.1 mg of Cd/kg, so that an average superphosphate application of 0.4 t/ha would have added 2.8 g of Cd/ha to the soil. It should be noted, however, that some superphosphate samples contained considerably higher Cd levels, and it is likely that Cd inputs to the plots have varied widely and may at times have been greater than the current average. In 1903, plot 4/1 was halved; one half received applications of lime (CaC03) every fourth year and, by 1991,had a pH of 6.5; the other half has continued without lime and had reached a pH of 4.9 in 1991. Soil samples, usually from the top 23 cm, have been taken periodically since 1876. The soil on the plots has been undisturbed for at least 200 years, so downward movement of nutrients and metals is due solely to leaching, translocation downwards in the roots, and earthworm activity. The herbage, usually cut in June, was unwashed prior to being chopped into short lengths for dry matter determination and storage in glass, cardboard, or metal containers. Analytical Methods. Stock samples of herbage were recently subsampled, the subsamples being ground in a
("0
time (s) ramp hold
15 5 20 0 1
20 40 35 5 5
internal Ar gas flow (mL/min) 250 250 250 50 250
Glen Creston mill to pass a 1-mmgrating and then bulked for 5-year periods in proportion to yield. Five milliliters of concentrated HN03 (Primar grade) was added to three 0.25-g replicate samples, appropriate reference materials, and blanks in 100-mL conical flasks, which were covered with glass bubbles and left to digest in the cold overnight. The following day the flasks were heated on a hot plate until brown fumes were no longer being produced (about 8 h), after which the glass bubbles were removed, and the acid was allowed to evaporate to near dryness. A sample of 0.05% HN03 (Primar grade) was then added to the digests which were filtered using Whatman No. 542 filter paper and made up to volume in 25-mL volumetric flasks. Following appropriate dilution and using the method of standard additions, the Cd content of each extract was determined by GFAAS on a Perkin-Elmer Model 4100 ZL graphite furnace atomic absorption spectrometer with Zeeman effect background correction to eliminate interference from nonatomic absorption. The temperature program used for the graphite furnace is given in Table 2. The superphosphate samples were ground to a fine powder in a ceramic pestle and mortar prior to being digested in nitric acid as described above. The Cd content was determined on undiluted sample digests using a Perkin-Elmer Model 2280 flame AAS. Results and Discussion
Quality Assurance Data. The limit of detection (LOD) for Cd in the herbage samples, determined using the method described by Adams (201,was 5 pg Cd/kg. Each digested sample was analyzed three times using the method of standard additions. New standards were made up for each run from a stock solution of 1ppm Cd in 1% v/v HN03;each GFAAS run was carried out on a different day. Using this method, reproducibility between analyses was good, with most samples having an RSD
