Sustainable management of heavy metals in agro-ecosystems

In 1993, the Netherlands Organization for Scientific Research (NWO) launched a priority research program on 'Sustainability and Environmental Quality'. Within this program, the METALS subprogram focusses on the accumulation of metals in economy (e.g., zinc in gutters) and the environment (e.g., soil), the mechanisms behind these processes, the associated risks, the possibilities for a sustainable management of metal flows, and their consequences for society and environment. This Ph.D. thesis has resulted from the research on sustainable management of heavy metals (cadmium, copper, lead, and zinc)in agro-ecosystems in the Netherlands and some other European countries.Accumulation of heavy metals in agricultural soil may cause problems if certain levels are exceeded. The productivity of soil and quality of produce should be protected but at the same time the ecological functioning of the soil should not be damaged, nor should emissions from the soil adversely affect other environmental compartments (e.g., groundwater). Heavy-metal cycles and input and output flows in agroecosystems have to be analyzed to identify the most important sources and processes that lead to accumulation. In order to determine the options for a sustainable heavy-metal management in agriculture, heavy-metal balance sheets are used as a means to quantify input (e.g., fertilizers, deposition) and output (e.g., leaching, crop offtake) flows in agro-ecosystems and to calculate resulting accumulation in soil. Heavy-metal balances of agro-ecosystems can be studied within the context of substance flow analysis (SFA), which is based on the law of conservation of mass. SFA consists of an integrated examination of all flows of a substance or a group of substances within a defined (geographic) system (Chapter 2).Several European studies on heavy-metal balances on different spatial scales and in different agro-ecosystems are reviewed and implications for an effective heavy-metal management of agro-ecosystems are disussed. Heavy-metal balances on a national scale study the agricultural sector as a whole. Although their meaning is quite limited with regard to environmental and specific on-site management analyses, they give valuable information for economic analyses. The studies on heavy-metal balances in Denmark and Finland show that generic measures may be very fruitful. Generic measures at the (inter)national level are required to enable proper procedures and measures with regard to product labeling, company certification, industry convenants, and import and trade with regard to heavy metals. In order to be able to discover relevant options for an effective management of heavy metals in agro-ecosystems, heavy-metal balances on the farm and field scales should be used in addition. These heavy-metal balances could be incorporated in an environmental management system of individual companies (Chapter 3).Quantification of the topsoil (or plough layer) balance can be carried out by different approaches i.e., the static balance (SB), the dynamic balance (DB), and the dynamic soil composition balance (DSCB) approaches. In a SB approach, the output flows are assumed not to be related to the (total) metal content in the soil. The change in heavy-metal content in the plough layer is therefore the result of the net difference between input rate and (constant) output rates. Because a SB does not regard the dependency between soil content and output flows, it cannot realistically simulate the development of the heavy-metal soil content in time. For a simulation of the long-term behavior in time, a DB may be calculated in which the relationship between soil content and output flows in time are explicitly included. Information which is needed to calculate heavy-metal balance sheets on the field scale is shown and the (im)possibilities to aggregate results on the field scale to higher levels of analysis are discussed (Chapter 4).Chapter 5 provides a detailed picture of the 'dynamic soil composition balance' (DSCB) approach which takes into account the composition of both soil amendments and soil when calculating heavy-metal accumulation in soil. The DSCB approach can be used to distinguish the effect of the matrix of soil amendments on the resulting accumulation in the plough layer. This new approach distinguishes heavy-metal input flows that are 'free' (e.g., deposition, mineral fertilizers) and input flows that are 'bound' to a soil matrix (i.e., associated with soil particles in amendments like compost). Also, a distinction is made between output flows that are 'free' (e.g., leaching and plant uptake) and that are 'bound' (e.g., soil adhering to crops and erosion). Cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) flows of arable, dairy and mixed farming systems in the Netherlands are studied. The crop rotation and the choice of fertilizers clearly influence the heavy-metal balance of arable farming systems. In dairy farming systems, the role of feed management is very important, but the effects on the heavy-metal balance are not always straightforward. Mixed farming systems compare favorably with specialized (arable or dairy) farming systems with regard to heavy-metal accumulation. Due to the internal cycling of feedstuff and manure, less inputs are required and thus the import of heavy-metal containing raw materials and products is minimized. Mixed farming need not be restricted to the farm level, because optimization of energy and material use and minimization of waste production may also be enhanced by 'mixed farming' at the regional level. Experimental farms are valuable objects for studying heavy-metal flows by carrying out measurements in agro-ecosystem's compartments. Integral monitoring of nutrient, pesticide, energy, and heavy-metal flows on the farmgate scale is recommended to define sustainable management options for agro-ecosystems (Chapter 6).Calculations based on data gathered at an Italian experimental farm reveal that the permitted annual applicatio Calculations based on data gathered at an Italian experimental farm reveal that the permitted annual application rates of sewage sludge and Bordeaux mixture in Italy pose problems for Cd, Cu, and Zn. Severe Cu pollution of integrated and especially organic vineyards is unavoidable with the currently allowed application rates of Bordeaux mixture (a mixture of copper sulphate and lime used against mildew). The results suggest that the current Italian soil protection policy as well as the policy of the European Union (EU) are inconsistent and not conducive of a sustainable management of heavy metals in agriculture (Chapter 7).By incorporating a well defined speciation model into a dynamic Cu balance of soil, it is possible to show the relative changes in availability and mobility due to increasing soil organic matter content and decreasing pH after changing land use from arable farming to forestry. Interspecies effects are shown to be very important (Chapter 8).Sustainability indicators are introduced to assess the sustainability of current metal cycles in agro-ecosystems based on dynamic heavy metal balances for the plough layer. These characteristic numbers can be used as indicators for potentially adverse effects of current agricultural practices, since they account for quality standards for soil, produce, and groundwater. They can also be used to assess the effects of different management options that aim at preventing quality standards from being exceeded as they provide insight in the dynamics governing input-output relation-ships. These indicators serve for screening and comparing different agro-ecosystems without having to know all processes in detail and thus allow for a proper assessment on a relative basis. Such an assessment may reveal:which heavy metal may cause the largest violation of standards;which environmental compartment is threatened most and for which compartment problems may be expected first;which experimental data assessment should have priority in view of decreasing uncertainty and optimizing predictions;which approaches to avoid violation of standards (shortterm or long-term strategies) are feasible and most effective.The sustainability indicators indicate some points of action for heavy-metaln policy and management, like setting up monitoring programs to carefully examine a country's practices and determining the rate of input, output and accumulation regarding agricultural soils. In this way, regional strategies for heavymetal manage-ment could be developed (Chapter 9).Literature and measurements with regard to long-term field experiments are interpreted and the options for calculating realistic dynamic heavy-metal balances of soil are discussed. Temporal variability of input and output rate parameters should be taken into account in modeling of long-term heavy-metal behavior in the soil-crop system to enable proper interpretation of total accumulation versus effects. Only by long-term monitoring is it possible to measure the magnitude and the direction of changes in soil properties which may have consequences for heavy-metal availability and mobility. Key variables could be gathered by systematic, sequential sampling over extended time periods using adequate monitoring networks and representative agro-ecosystems (Chapter 10).To enhance sustainable management of heavy metals in agro-ecosystems, further development is recommended with respect to scale aspects, environmental management systems, economic and environmental indicators, dynamic modeling, and monitoring. Furthermore, a coherent EU policy and the development of an ethical foundation are needed to advance sustainable agriculture (Chapter 11).