Perspectives For Sustainable Heavymetal Management In Agrosystems

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Different strategies for heavy-metal management will have different consequences for the resulting steady state. Short-term strategies may aim at increasing the soil's buffer capacity. For soils with a low sorption (or buffering) capacity, metal concentrations tend to increase in groundwater and crops. This may result in groundwater and crop quality standards being exceeded, but without significant accumulation in the soil. Good management practice might attempt to lower the crop uptake rate, for example by raising the organic matter content (increasing retention), favoring competition by applying calcium and magnesium and competing heavy metals. Changing tillage practices may influence the stratification of pH, organic matter and metals. Moreover, cultivars may be changed and acid soils may be limed to increase the pH (see, for example, McLaughlin et al., 1994, 1995).

Some practices that aim at lowering crop uptake (such as stimulating competition) may at the same time lead to higher leaching rates or vice versa, thus resulting in a trade-off between leaching and uptake. Moreover, minimizing output rates by management practices will result in the steady-state content being reached later at a higher level. Selecting cultivated crops with pronounced heavy-metal removal (within critical limits) can be very sensible for farming systems with low input. A higher crop uptake rate results in a lower steady-state soil content due to less accumulation.

Long-term strategies focus on reducing inputs to soils. This results in the steady state being reached with lower total accumulation and lower output rates. Input reduction can be achieved by reducing the amount of heavy metals in source material (quality) and by reducing the amount of fertilizer or manure added to the soil (quantity). This kind of input reduction could be aimed at by decreasing application (for example, by educating farmers on how to use nutrient and heavy-metal balances) or by changing the production system (for example, to a mixed farming system).

Industrial ecology suggests that economic actors (including farms as well as firms) should optimize the use of energy and material, minimize waste production and use the effluents of one process as input for another process (Frosch and Gallopoulos 1989). Encouraging symbiosis, or relationships of mutual benefit, is evidently one key to industrial ecology. There is evidence that mixed farming systems compare favorably with specialized (for example, grain or dairy) farming systems with regard to heavy-metal accumulation. Owing to the internal cycling of forage and manure, fewer external inputs are required and thus imports of heavy metal-containing raw materials and products are minimized (Moolenaar and Lexmond 1998).

Mixed farming need not be restricted to the farm level. Optimization of energy and material use and minimization of waste production may be enhanced by exchanging intermediate outputs between two or more specialized farms. To test the hypothesis that such a mixed farming system might improve sustainability of agriculture, Bos and van de Ven (1999) carried out an interesting study in the Dutch province of Flevoland. They quantified nutrient balances, labor requirements and labor income for a specialized grain farm, a specialized dairy farm and both combined into a mixed farming system, all exchanging land, labor and machinery. It was concluded that in a mixed farming system it is possible to realize a higher income and to reach higher production levels without increasing environmental pollution. Prospects for mixed farming systems at the regional level depend on the future balance between integration (to enhance sustainability) and specialization.

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