Cadmium and lead in British wheat and barley

Abstract

An investigation of cadmium and lead uptake into British wheat and barley is reported. The results of cadmium and lead analyses performed on samples from the 1998 Cereals Quality Survey, and representative paired soil and grain samples collected from the 1998-2000 harvests are reported. Concentrations of cadmium and lead in the vast majority of samples were below the newly-introduced European Commission Regulation specifying the maximum permissible contaminant levels in foodstuffs. In general, wheat had higher grain concentrations of cadmium than barley, and both species had low concentrations of lead. In the paired soil and grain samples, total soil cadmium and soil pH were found to be the significant factors influencing grain cadmium concentrations. Significant varietal differences in cadmium uptake were observed for both wheat and barley, with certain cultivars having higher concentrations in both the field and pot experiment samples.

The effects of previous sludge applications on wheat and barley grain cadmium concentrations were investigated at two sites with different soil textures. Grain cadmium concentrations approached the maximum permissible contaminant levels at different total soil cadmium concentrations at the two sites, indicating that caution is required in using soil total metal concentration data in isolation for evaluating the potential for grain to exceed the European Commission levels. However, soil pH and total soil cadmium concentration when used together explained a large amount of the variation occurring in grain cadmium concentrations at the two sites.

The effects of fertiliser rate and application forms, and the addition of various soil amendments on cadmium and lead uptake in wheat and barley, were also investigated using pot experiments under controlled environment conditions. The applications of lime and an oxide-rich soil amendment were shown to reduce the concentration of cadmium in grain grown on a non-calcareous soil. Samples from a field experimental site also showed that higher application rates of ammonium nitrate fertiliser could lead to higher grain cadmium concentrations.

Practical implications and recommendations based on these results are discussed. In addition, a short comment on analytical quality control issues with regard to submission of samples for cadmium and/or lead analysis to commercial laboratories is provided.

INTRODUCTION

In recent years, there has been increasing awareness and concern over heavy metal contamination of soils and the potential effects this may be having on the food chain. High concentrations of heavy metals in agricultural soils can occur naturally, or via the application of metal-contaminated sewage sludges, animal manures, fertilisers and atmospheric deposition (Ryan et al., 1982; Alloway and Steinnes, 1999). With regard to human health, the cadmium (Cd) and lead (Pb) concentrations of agricultural produce are of particular importance, as the consumption of agricultural foodstuffs is thought to contribute significantly to the dietary intake of these metals.

The entry of cadmium into the food chain is of concern as it can cause chronic health problems in humans such as bone disease, lung oedema, renal dysfunction, liver damage, anaemia, hypertension and has recently been associated with brittle bones (Nordberg, 1974; Nath et al., 1984; Staessen et al., 1999). Due to this, cadmium is one of a very small group of metals for which the FAO/WHO (1978) have set a limit for the provisional daily intake by humans (70 µg cadmium/day). For example, in the 1980's the US adult population was reported to receive about 20% of the FAO/WHO (1978) allowable daily intake of cadmium from the consumption of grain and cereal products (Wagner et al., 1984). In contrast, during the same period in the early 1980's, grain and cereal products accounted for about 30 to 40% of the daily allowable cadmium intake in the European Community (Hutton, 1982). Similarly, lead is a known physiological and neurological toxin that, together with cadmium, bioaccumulates within the body. Lead is particularly hazardous for children, where it can induce reductions in cognitive development and intellectual performance. In adults, excessive exposure to lead may cause a variety of conditions including renal dysfunction, increased blood pressure, cardiovascular disease and the inducement of a number of trace element deficiencies (Ferguson, 1990).

Agricultural management practices that directly affect cadmium and lead concentrations in the soil and soil solution may influence cadmium and lead accumulation by crops. Specifically, the addition of sludge or fertiliser having high cadmium and lead concentrations to agricultural land may cause significant increases in the uptake of these metals by crops (Grant et al., 1999). Recent research suggests that inputs of ca. 40 tonnes of cadmium and 767 tonnes of lead annually enter agricultural soils in England and Wales (Alloway et al., 2000). In that work, approximately 52% of cadmium and 76% of lead was from atmospheric deposition, with a further 5% and 15% of cadmium and lead coming from sewage sludge applications. However, the total amounts of atmospheric deposition for the whole country can be misleading, as they are based on small amounts per unit area multiplied by a very large land area. McGrath (2000) has shown that the effect of concentrated sources such as sewage sludge and industrial wastes applied to specific areas of land are by far the most important inputs for those particular areas. Those areas that receive applications either currently or in the past, therefore present the greatest risk at the field level of exceeding food regulation limits now or in the future.

The impacts on grain metal concentrations of long-term anthropogenic inputs to soils are still not clear. A number of investigators have shown that the long-term application of cadmium-containing fertilisers may increase cadmium uptake by crops (Andersson and Siman, 1991; Nicholson et al., 1994). In contrast, similar studies have indicated that, often despite an increase in soil cadmium content with P fertiliser application, cadmium concentration in crops has not increased with long term fertiliser use (Mordvedt, 1987; Jones and Johnston, 1989; He and Singh, 1993). In general, it appears that the rate and total loading of contaminant inputs, the soil type and the crop variety are important factors in the relationship between added metal and plant uptake.

The presence of high concentrations of contaminant metals in agricultural produce can have limitations on its sale in the international community, and there are a growing number of examples where this has affected exports. Durum wheat shipped from the US to Finland and Switzerland, peanuts to Australia and flax to Germany have been rejected because of their cadmium concentrations. Sunflower kernels from the US shipped to Germany have come under extreme limitations due to cadmium, which in turn forced the industry to change where the crop was grown to minimise cadmium in the kernels. Previous international grain cadmium and lead limits of 0.1 mg/kg fresh weight have led the US agricultural community to seriously evaluate and quantify the concentrations of cadmium and lead in their agricultural products. However, evidence on cadmium and lead concentrations in cereals has previously been difficult to obtain, as this issue can be a sensitive one. A survey conducted by Rothamsted in 1993 indicated that about 4% of UK wheat grain samples exceeded 0.1 mg/kg cadmium (Chaudri et al., 1995). Little work has been done on grain cadmium and lead in the UK recently, but the Maltsters Association of Great Britain reported values for lead in barley grain of up to 0.9 mg/kg, with as many as 38% of 65 samples taken from grain stores exceeding 0.1 mg/kg (Farmers Weekly, 16 May 1997).

In consultation with the constituent member states, the European Union has recently introduced legislation defining the maximum permissible levels for cadmium and lead concentrations in a range of foodstuffs, including wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) grain (European Commission, 2001a). For cereals excluding wheat grain, bran, germ and rice, the maximum permitted cadmium concentration is 0.1 mg/kg (fresh weight), while the respective limit for the aforementioned exceptions at 0.2 mg/kg (fresh weight). For lead, the corresponding limit for all cereals is 0.2 mg/kg (fresh weight).

An important point from the regulation is that 'Food ingredients used for the production of compound foodstuffs should comply with the maximum levels set in this Regulation prior to addition to the said compound foodstuff in order to avoid dilution'. This would therefore appear to preclude the possibility of dilution of non-conforming grain batches with grain batches having lower metal concentrations. It is also noted that the regulation stipulates the Commission will review the maximum levels of heavy metals every five years, before 5 April 2003 for the first time, with an overall objective of ensuring a high level of consumer health protection. Elsewhere it is stated that these reviews will take account of the advance of scientific and technical knowledge and improvements in manufacturing or agricultural processes, with the objective of steadily decreasing the maximum permitted levels of cadmium and lead in food.

Project Objectives:

As UK soils may potentially be more contaminated with cadmium and lead than those in the US and Canada (McGrath and Loveland, 1992), it was important that good information was obtained on the range of cadmium and lead concentrations in British grain in order to:

anticipate any threats to farming practices and viability of exports,
understand the factors which lead to high cadmium and lead concentrations, and
assess possible strategies for reducing these contaminants to acceptable levels in situations where high crop metal uptake occurs.
This project was initiated in 1998, partially as a response to the concerns of the milling and malting industries that, based on their earlier commissioned research, significant amounts of UK grain may exceed the proposed EU food quality standards circulating in draft form at that time. The specific objectives of the project were:

To assess the geographical distribution of cadmium and lead concentrations in grain samples and relate these to soil properties.
To quantify the effects of variety, nitrogen fertiliser form and rate, and soil amendments on the cadmium and lead uptake into cereal grains.
To measure to what extent soil or dust contamination may be responsible for observed high concentrations of metals in grain samples.
To measure the effects of past sewage sludge applications on grain cadmium and lead, using existing field experiments.