Heavy-metal balances can be used to determine the options for a sustainable heavy-metal management in agriculture. These balances are a means to quantify heavy-metal (input and output) flows and the resulting accumulation. Heavy-metal flows in agrosystems can be studied within the broader context of substance flow analysis and industrial ecology on a variety of scales and for different systems.
Heavy-metal balances on a national scale provide valuable information for economic analyses. Generic balance studies indicate that legislation limiting metal emissions by industry and using fertilizers with low metal content may be very important. Whereas policies based on direct economic instruments or generic regulations often ignore farm characteristics and individual management options, field-scale and farm-gate balances give farmers specific feedback on effective options for better heavy-metal management. Farmgate balances show the total contaminating potential, whereas field-scale balances enable a direct link with criteria for the protection of soil and other environmental criteria.
Dynamic heavy-metal balances on the field scale taking into account accumulation, leaching and uptake can identify 'hot spots' regarding specific crops and applications for different metals among and within various systems. The dynamic balance approach thus proves to be a useful tool with which to compare the heavy-metal management of agrosystems.
Although data from long-term field experiments are needed to study the long-term environmental consequences of applying fertilizers and soil conditioners, they may not give sufficient information because variation in data collection may hamper the reliability of data and it may be difficult to maintain relevance for current agricultural practices owing to changing practices, technologies, cultivars and natural variation. Therefore, in addition to monitoring programs, projection models are needed to assess environmental consequences of different management practices. Dynamic heavy-metal balances are useful for projection purposes.
Perspectives for sustainable heavy-metal management in agrosystems are as follows:
• monitoring strategies using representative agrosystems (experimental farms) and adequate monitoring networks;
• quantification of uncertainties;
• dynamization and incorporation of speciation and environmental fate and effect modeling in balance studies;
• scale aspects (compatibility of SFA on different scales: 'up-scaling' and 'down-scaling');
• sustainability indicators (socioeconomic, ethical, environmental, agronomic aspects and integrated approaches);
• mixed farming systems (using the potential of a region);
• coherent international policies (covenants, trade barriers and so on).
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