WaterSubject

The soil is a long-term sink for heavy metals, including zinc (Zn), copper (Cu), nickel (Ni), lead (Pb), chromium (Cr), cadmium (Cd), arsenic (As) and mercury (Hg). Although these elements have different mobility and bioavailability in soils, leaching losses and plant uptake are usually relatively small compared with the total quantities entering the soil from different sources. As a result, they slowly accumulate in topsoils, with long-term implications for agricultural soil quality and function. Reducing heavy metal inputs to soils is therefore a strategic aim of UK and European Union (EU) soil protection policies; however, information on the extent of soil heavy metal contamination from different sources is required for actions to be effectively targeted. Thus, a quantitative inventory of heavy metal inputs to agricultural soils was developed to determine the scale and relative importance of the different metal sources.

A study from 1983 identified atmospheric deposition as the principal source of heavy metals to agricultural soils at a national level, with inorganic fertilisers and pesticides responsible for relatively small rates of addition. Biosolids (sewage sludge) and livestock manures were also identified as significant sources of metals, albeit to limited land areas. However, a number of important changes have taken place since 1983, including the introduction of limits on metal supplementation in pig feeds, reduced metal concentrations in biosolids, the banning of pesticides containing Hg and As, the sourcing of low-Cd rock phosphates and improved industrial emissions controls.

The agricultural use of biosolids is well established in the UK and is managed, regulated and monitored to minimise environmental problems. In particular, the controls specify maximum permissible heavy metal concentrations in soils and maximum annual rates of addition to protect soil fertility, crop yields and quality, and human and animal health These values can also provide a benchmark to assess and compare heavy metal accumulation rates from other sources such as livestock manures, composts and industrial ‘wastes’. The regulations on agricultural use of biosolids currently represent the only statutory-based mechanism for controlling heavy metal inputs to soils in the UK. However, a Publicly Available Specification for Compost (BSI PAS 100) has recently been introduced specifying limits on heavy metal concentrations in composted materials.

In this study, we present an inventory of heavy metal inputs to agricultural land in England and Wales from major sources including atmospheric deposition, biosolids, livestock manures, inorganic fertilisers and lime, industrial by-product ‘wastes’ and composts, based on published data on the heavy metal contents of these materials and estimated quantities applied to agricultural land. The relative importance of the different metal sources at a national and individual field level was estimated from the total quantities of metals deposited on agricultural land, as well as the annual metal input rates per hectare of farmland. We also assessed the implications of these input rates in terms of the time required to reach the soil metal limit values where biosolids are recycled to agricultural land.

Inventory of heavy metal inputs

Annual heavy metal inputs to agricultural land in England and Wales (year 2000) from all the major sources considered are summarised in Figure. For Zn and Cu, 37–40% of the total annual inputs to agricultural land were derived from livestock manures, 39–49% from atmospheric deposition and 8–17% from biosolids. In contrast, 55–77% of Ni, Pb and As inputs were from atmospheric deposition and only 6–26% from livestock manures. For Cd, 53% of inputs were from atmospheric deposition, 30% from inorganic fertilisers (mainly phosphate fertilisers) and lime, and 11% from livestock manures. The major sources of Cr inputs to agricultural land were phosphate fertilisers, biosolids and atmospheric deposition. Approximately 85% of Hg inputs were from atmospheric deposition.

Although atmospheric deposition was an important source of heavy metal inputs to agricultural land on a national scale, input rates on an individual field basis were small compared with other sources. The highest input rates of all metals were from biosolids (applied at 250 kg total N/ha/year). However, biosolids generally represented <25% of total metal inputs and are applied to <1% of agricultural land. Zinc and Cu input rates from pig manures applied at the same N rate (250 kg total N/ha/year) were equivalent to 46–52% of biosolids inputs, and Zn inputs from layer manure were equivalent to 60% of biosolids inputs. Metal input rates from cattle manures were generally low in comparison with biosolids and pig or layer manures, except for Zn (because of supplementation of dairy cattle diets to maintain fertility) and As (probably as a result of contaminated mineral feed supplements).

Implications for soil quality

The heavy metal input rates were used to estimate the time (number of years) required to raise topsoil metal concentrations from background values (median concentrations in England and Wales) to the maximum permissible limits for heavy metals stipulated in the Code of Practice for Sludge Use in Agriculture, assuming all fields received inputs from atmospheric deposition and there were no metal losses (e.g. via crop offtake or leaching). Soil Zn would be raised to the limit value (200 mg Zn/kg dry soil) after approximately 80 years of biosolids additions, compared with 130–164 years if pig or laying hen manures were applied annually at a rate of 250 kg/ha total N. However, these times would decrease if soil Zn concentrations were already elevated above background values. In comparison, it would take >1700 years for atmospheric deposition alone to raise topsoil Zn to the limit concentrations. Similar estimates for other metals are summarised in Table.

This inventory has been based on total heavy metal inputs. However, it is widely recognised that metal bioavailability and plant uptake will vary considerably, depending on the form of the metal entering the soil, soil physicochemical conditions and plant genotype. Hence, the impact of soil metals will vary between sites, particularly where biosolids and livestock manure applications input relatively large amounts in the long term, compared with the relatively small amounts added per unit area by atmospheric deposition. Some workers have already attempted to develop risk assessment methods that use soil physicochemical characteristics to estimate soil sensitivity to biosolids heavy metal inputs at both a site scale and a national scale. There is now considerable scope to expand this approach to develop a policy tool that could predict soil metal concentrations on a spatial and temporal basis based on changes in legislation (e.g. reduced livestock feed trace element concentrations) or other factors (e.g. falling animal numbers due to foot and mouth). This could be extended by overlaying the data with information from soil maps, which would allow us to identify soils that are particularly vulnerable to additional metal inputs either because they already have high soil metal concentrations or because they support sensitive ecosystems.