Summary of risks from cadmium in agricultural soils
Chapter 3: Summary of current information on soil cadmium levels, inputs, and uptake by plants and animals
Cycling of cadmium in agricultural systems: from pasture to plate
There has been a steady increase in the amount of phosphate fertiliser used in New Zealand to a high of over two million tonnes in 2002/03 (or 220,900 tonnes expressed as the elemental phosphorus content). Over the last five year period (2001-2005), approximately 30 tonnes per annum of cadmium were added to New Zealand’s agricultural soils through phosphate fertiliser use.
Historically, New Zealand has sourced its phosphate rock from Nauru, which was very high in cadmium relative to other phosphate rock sources. In 1995, the superphosphate manufacturers embarked on a cadmium reduction programme which resulted in the phasing out of the Nauru supply. A voluntary industry limit for cadmium content in phosphate fertiliser of 280 mg cadmium/ kg P was imposed. The limit has been consistently bettered, over recent years. Since 2001 the weighted average content of cadmium in phosphate fertiliser was about 180 mg Cd/kg P.
There is currently no cost-effective or practical method of removing cadmium from phosphate rock. Low-cadmium containing phosphate rock is either unavailable or difficult and more expensive to source
The cycling of cadmium through agricultural systems is complex, and influenced by many factors. The amount of cadmium present and soil conditions including acidity (pH), organic matter, nutrients, temperature and soil aeration, can increase the amount of cadmium taken up by plants. The availability of cadmium is increased by soil acidity and decreased by the presence of organic matter in soils.
Plant-related factors that influence the uptake of cadmium include: the crop species and cultivar; the types of plant tissue; leaf age and metal interactions. Generally, cadmium is stored in leaves more than in roots, seeds and fruit.
Animals can take up cadmium from ingesting fertiliser directly, through soil uptake during grazing or as a result of eating pasture plant containing cadmium. Of these, the intake of cadmium via pasture is the most significant on average. Cadmium accumulates in the kidneys and livers of grazing animals over time, and so increases in these organs as animal’s age.
Cadmium levels in NZ soils
Based on an analysis of conservation estate and other non agricultural soil samples from various studies, New Zealand has a national average baseline (i.e. the ‘natural’ background level in soils) value for cadmium of 0.16 mg/kg, consistent across all regions and soil types. The current national average concentration for cadmium was 0.35 with a range of 0-2.52 mg/kg.
The cadmium content of agricultural soils will vary from region to region depending on history of phosphate fertiliser use, dominant land use, soil type, climate, sampling depth and bulk density.
Land-use was a key driver of topsoil cadmium concentrations. Cropping, pasture and horticulture land-uses all had higher concentrations of cadmium in soil than background, ‘natural’ land. The reason for this is almost certainly the application of phosphate fertiliser in most agricultural and horticultural land use.
Land used for dairying has the highest national average for cadmium concentration (0.73 mg/kg). Kiwifruit (0.71 mg/kg), berries (0.68 mg/kg), orchards (0.66 mg/kg), market gardening (0.46 mg/kg), drystock pasture (0.40 mg/kg) were also above the national average. Cropped soils appear to be mostly below the national average of 0.35 mg/kg for cadmium; however, these soils are tilled to a greater depth (200 mm) than other land-uses, and dilution decreases the cadmium concentration. Soils where tobacco was grown in the past were more elevated in cadmium (0.34 mg/kg) than other cropping soils. Sites receiving little or no fertiliser had the lowest cadmium concentrations (unfertilised 0.19 mg/kg, plantation forestry 0.14 mg/kg, native forest 0.10 mg/kg).
Results from the analysis of national data were broken down to regional council regions. The region with the highest average cadmium concentration was Taranaki (0.66 mg/kg). Other regions with similar cadmium concentrations include Waikato (0.60 mg/kg) and Bay of Plenty (0.52 mg/kg). Dairy farming with a historically higher use of phosphate fertiliser is traditional in these areas and the soils of these regions have a high propensity to accumulate cadmium according to the FMRA cadmium model. The regions with the lowest cadmium average concentrations were Canterbury (0.17 mg/kg), Gisborne (0.20 mg/kg), Manawatu-Wanganui (0.20 mg/kg), Nelson-Marlborough (0.23 mg/kg), Otago (0.20 mg/kg) and Southland (0.21), all historic sheep farming areas.
Projections of future soil cadmium levels
An initial estimation of future topsoil cadmium concentrations was carried out using the Fertiliser Manufacturers’ Research Association CadBal model and the national data summarised above. Results showed Brown Grey Clay Loams, Yellow Brown Loams and Yellow Brown Podzols soils accumulated more cadmium than the other soil types while alluvial, Yellow Brown Earths and Yellow Grey Earths soils accumulated the least cadmium. Differences in soil type cadmium accumulation appear due to differences in leaching losses and soil bulk densities input to the model.
In the model, sampling depth was related to cadmium concentrations. For example, increasing the sampling depth from 0–7.5 to 0–10 to 0–20 cm was shown to reduce the cadmium concentration from 0.43 mg/kg to 0.37 mg/kg to 0.26 mg/kg for a Yellow Brown Earth under dairy farming receiving 30 kg P ha-1y-1 .
The model also showed pastoral farming resulted in increased soil cadmium content in all regions and nationally. The peat soils of the Waikato region showed the highest potential for cadmium accumulation – although this could in part be due to the low bulk density of these soils not being taken in account in the model. The regions with the highest present-day soil cadmium content also have the highest potential to accumulate cadmium in the future. Sheep/beef farming led to more accumulation of cadmium than dairy when both are under the same fertiliser regime although, in practice dairy farming requires more fertiliser for optimal production than beef and sheep farming. The difference in potential accumulation was due to the difference in sedimentation losses (900 kg ha-1 y-1 for dairy farming and 500 kg ha-1 y-1 for sheep and beef). However, sedimentation losses are due to a range of factors including topography, soil type, leaching class and climate, not just farm type, and this result is questionable.
Cadmium levels in soils under dairy farms were shown to decrease in cadmium with time once soil cadmium exceeded about 1.3 mg kg-1 due to removal in sediment, erosion products and leaching. This result, if validated by empirical observation, may have important implications for farm sustainability and its accuracy should be further investigated.
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