Published January, 1999
Sustainable Crop Production: Definition and Methodological Approach for Assessing and Implementing Sustainability I. Lewandowski,* M. Hardtlein, and M. Kaltschmitt ABSTRACT A method for assessing and implementing sustainable crop production is needed to give practical relevance to the frequently used term "sustainable agriculture". The objective of this paper is to present such a theoretical procedure. Therefore the terms "sustainability" and "sustainable crop production" are discussed and defined. On the basis of the definitions, an eight-step procedure for assessing and implementing sustainable crop production is outlined. The steps are (1) identify emissions and other releases linked to different crop production practices, (2) trace each different release from its source (the crop management practice) to its sinks (i.e., agroecosystems and other ecosystems or components of ecosystems directly or indirectly affected by these releases), (3) select indicators that adequately describe the condition of the ecosystem affected directly or indirectly by crop production practices, (4) determine threshold values for the selected ecosystem indicators (i.e., values which should not be exceeded if irreversible changes in the affected ecosystems are to be avoided), (5) transpose the ecosystem threshold values to the farm level by retracing the impact pathways (from Step 2) backward to crop production itself, (6) derive farm-level indicators that point to separate or combined agronomic practices that could cause irreversible changes in affected ecosystems, (7) determine farm-level threshold values for management-induced releases on the basis of ecosystemlevel threshold values, and (8) identify production schemes that adhere to the framework set by the farm-level thresholds. From these production schemes the fanner can select those most in line with his available resources and objectives.
I
N RECENT YEARS, the term "sustainability" has become a catch word in a number of discussions on social, economic, and ecological issues, particularly with regard to long-term global development. The expression "sustainable agriculture" has also entered the debate, especially in the context of analyzing the negative effects of certain crop production methods. Although there is general consensus on the main aspects of sustainability, there is no generally acknowledged definition of sustainable agriculture. Also, a satisI. Lewandowski, Institute for Crop Production and Grassland Research (340), Univ. of Hohenheim, Fruwirthstr. 23, D - 70599 Stuttgart, Germany; M. Hardtlein and M. Kaltschmitt, Institute for Energy Economics and the Rational Use of Energy (IER), Univ. of Stuttgart, Hefibruhlstr. 49a, D - 70565 Stuttgart, Germany. Received 6 Nov. 1997. Corresponding author (
[email protected]). Published in Crop Sci. 39:184-193 (1999).
factory methodological approach for assessing and implementing sustainable agricultural practices does not exist (Farshad and Zinck, 1993). This makes it difficult to agree on what practical results constitute sustainability. To prevent the expression "sustainable agriculture" from becoming a catch word without practical significance, it is necessary to develop a framework within which agriculture must operate to be sustainable. Our objective is to develop and discuss a methodological approach for assessing sustainable crop production and implementing it at the farm level. This entails defining sustainable crop production because elaborating a methodological approach requires a practicable terminology. To this end, a general overview of the development of the term "sustainability" and its prevalent issues is given and a definition of sustainable crop production is derived. The emphasis of the presented methodological approach is on crop production, taking the animal husbandry side of agricultural production into account only in so far as fertilization with animal manure or slurry is one option among many possible crop production practices. However, with some modification, the approach outlined below can be applied to animal husbandry and other farm activities as well.
DEVELOPMENTS IN DISCUSSIONS ON "SUSTAINABILITY" The term sustainability has its origin in forestry and dates back to the 18th century, where it described the utilization of wood at the rate it regrows. This ensured that the demand for timber and fire wood was met in the long run (Speidel, 1984). Today, sustainability is no longer linked solely to the management of individual natural resources. The term has also become a synonym for sound and acceptable economic, social, and ecological development of society. A frequently cited definition of the term "sustainable development" is the following from the Brundtland-Report, presented by the World Commission on Environment and Development under the chairperson Gro Harlem Brundtland (WCED, 1987): "Sustainable development is development that meets
LEWANDOWSKI ET AL.:
METHODOLOGICAL APPROACH FOR ASSESSING
the needs of the present without compromisingthe ability of future generations to meet their ownneeds." Sustainable development was declared the allembracingobjective of national and international development and environment policy at the United Nations Conference on Environment and Development (UNCED) in Rio de Janeiro in June 1992. Agenda 21 was passed as the global directive for sustainable development (UNCED,1992). As defined, sustainable development pertains equally to economic, social, and ecological issues. However, the disciplines involved have a different understanding of what sustainability is. This often results in very different objectives for assessing "sustainable development". Furthermore, there are conflicts between short-term and long-term goals, e.g., between exploiting resources to meet present needs vs. saving them for future generations. All of this has hindered the practical implementation of sustainability. A comprehensiveconcept, taking all necessary aspects into consideration, is far from being developed (L616, 1991; Haber, 1994b; Rehbinder, 1996). Given today’s interpretation of the term sustainability, any definition of sustainable agriculture should in principle include ecological as well as economic and social aspects (Allen et al., 1991; Neher, 1992; Yunlong and Smit, 1994, USDA1997). In spite of basic agreement on the essential componentsof sustainable agriculture, the existing attempts at defining it differ in the degree of precision and in what is emphasized (Dunlap et al., 1992; Farshad and Zinck, 1993; Christen, 1996). Therefore no widely accepted, uniform, or specific definition of sustainable agriculture exists. Unlike manyother economicsectors, agricultural production depends primarily on natural resources. Crop production is integrated into natural systems. Manyresearchers working on sustainable agriculture focus on an analysis of natural resources and the preservation and functioning of crop production within natural systems. Thus, they are placing their emphasis mainly on ecological aspects of sustainability (Edwardset al., 1993; Haber, 1994a; Werner, 1995). From the conference of the European environmental ministers at Helsinki in August 1993, Eckert and Breitschuh (1994) derived a comprehensive definition of sustainable agriculture (our translation): "Sustainable agriculture is the managementand utilization of the agricultural ecosystem in a way that maintains its biological diversity, productivity, regeneration capacity, vitality and ability to function, so that it can fulfill--today and in future--significant ecological, economic, and social functions at the local, national, and global level, and that does not harm other ecosystems." In view of the conflicts existing between ecological, economic, and social goals, it makes sense first to develop a definition and an implementable methodological approach for sustainable agriculture. On the basis of this, an approach encompassingecological, economic, and social aspects can then be developed in cooperation amongthe disciplines involved. Because of the special dependencyof agriculture on natural resources, it makes sense to start a detailed elaboration of sustainable agri-
AND IMPLEMENTING
SUSTAINABLE
CROP PRODUCTION 185
culture from an ecological point of view. Once this has been satisfactorily achieved, economic and social aspects can be elaborated and integrated. METHODOLOGICAL
APPROACH
The following explanations of our methodological approach start off with an ecologically oriented understanding and thus a restricted interpretation of sustainability. Against this background, a definition of a sustainable crop production is proposed. On the basis of this definition, the fundamental considerations and methodological steps for the assessment and implementation of sustainable crop production practices are presented and discussed. Definition Sustainable crop production, from an ecological perspective, must consider the agricultural ecosystem and other ecosystems directly or indirectly affected by agricultural production practices. The numerous interactions and interdependencies within and amongdifferent ecosystems and ecosystem components must also be considered. Furthermore, statements about sustainability are only possible in relation to a specific location, differentiated over space and time, and for each of the ecosystems involved. As a basis for subsequent elaboration, the following definition is specified. Agricultural crop production is (ecologically) sustainable if the productivity as well as the ability to function (among other things the regenerative power and the buffering capacity) of the open system within which plants are cultivated, are permanently maintained to the full extent. Neither the agricultural ecosystem as a whole, nor its components(principally water, soil, airclimate, flora, and fauna), nor other ecosystems which are directly or indirectly affected by crop production, nor the interactions amongthese ecosystems and their componentsare altered irreversibly over the long run. Basic Considerations In accordance with the above definition, our approach for the assessment and implementation of sustainable crop production focuses on the objects to be protected (i.e., ecosystems--including agroecosystems--and their componentssuch as water, soil, air-climate, flora, and fauna). Agricultural production uses natural resources, which can to someextent lead to their degradation (e.g., soil) and depletion (e.g., groundwater). In turn, this result in reduced productivity of agricultural ecosystems. Moreover, agroecosystems as well as other ecosystems (e.g., forest ecosystems) can sometimesbe irreversibly affected by various emissions and releases resulting from agricultural crop production. In this context, fossil fuels (such as diesel fuel) and other non-renewable resources are seen exclusively as sources of emissions and releases resulting from their utilization; they are not discussed under the aspect of resource depletion and resource use efficiency. On the basis of these considerations, maintaining the
186
CROP SCIENCE, VOL. 39, JANUARY-FEBRUARY 1999
FARM LEVEL
ECOSYSTEM LEVEL
impact pathwayassessment
depositions ambient concentrations
emissions
Indicators
Indicators
releases
loads
impact pathwayassessment Fig. l. Mainaspects of the elaboration of the methodological approachshowing that the farm level and the ecosystem level have to be linked by an impact pathway assessment.
productivity and the ability to function (i.e., the regenerative power and buffering capacity) of the various ecosystems should be the measuring stick for assessing the effects of agricultural crop production. Wesuggest using the term "ecosystem" to mean either complex systems or components thereof which are directly or indirectly affected by crop production practices. Agroecosystemsare a thus subset of all ecosystems to be taken into account. From the definition above, ecosystems should not be changed irreversibly by any crop management practice. To attain this goal, the farmer needs a farm level framework to help guide the management decisions which are undertaken. This framework should provide practical guidelines on how to produce crops without causing irreversible changes in the affected ecosystems. For the practical implementation of sustainable crop production, indicators play an important role. Indicators can be defined as measurable parameters, which characterize a system by reduction of complexity and integration of information. Thus, at the farm level, indicators are required to describe those agricultural management practices, which mayinfluence the affected ecosystems. Moreover, at the ecosystem level, indicators are needed to describe the condition of the ecosystems affected directly or indirectly by crop production practices. The link between farm and ecosystem level can be modeled by studying the impact pathways of emissions and releases. This means that emissions to the atmosphere and releases to the pedosphere originating from crop
production practices are traced to the respective affected ecosystems; here they arrive as ambient concentrations and depositions (resulting from emissions) and loads (resulting from releases) (Fig. Methodological
Approach
A frameworkfor the farmer that will ensure the sustainability of crop production--according to the definition outlined above---can be derived by following the eight-step procedure outlined in Fig. 2. 1. Identification of actual emissions and releases resuiting from crop production. 2. Determination of ambient concentrations, depositions and loads by assessing the corresponding impact pathways to ecosystem(s) and their components. 3. Selection of ecosystem indicators to describe the condition of the ecosystem and its components affected directly or indirectly by crop production practices. 4. Determination of threshold values for these indicators. 5. Determination of maximumtolerable emissions and releases based on the threshold values identified above by tracing the impact pathways back to the farm level. 6. Derivation of farm-level indicators to describe those agricultural managementpractices relevant in the context of sustainable crop production.
LEWANDOWSKIET AL,:
METHODOLOGICAL APPROACHFOR ASSESSING AND IMPLEMENTING SUSTAINABLE CROP PRODUCTION 187
FARM LEVEL
ECOSYSTEM LEVEL
Crop ProductionMeasures
Identificationof actual emissionsand releases Impact pathway spreadingprocesses (with or withouttransformation)
Determination of ambient concentr.,depositions, loads
II
Selectionof suitable ecosystem-indicators
I
Identificationof maximally tolerable values
l
Tracing]the impact pathwaybackwards
I
tolerable emissions and (~)1 Determination of maximally re eases
farm-level-indicators
usesof farminputs
Identificationof the most promising crop production measures that satisfy the criteria of sustainable crop production Fig. 2. Flow diagramof the methodological steps for setting the frameworkfor sustainable crop production.
7. Determination of critical uses of farm inputs as the threshold values for these indicators. 8. Identification of production schemes that adhere to the frameworkestablished by the threshold values for farm inputs, thus satisfying the criteria for sustainable crop production. These steps are described in more detail below. As an example, the effects of nitrogen (N) fertilization will be used to illustrate each point. The first step identifies emissions and releases caused by crop production practices (e.g., application of N-fer-
tilizer). These releases mayeither be material or nonmaterial, airborne or non-airborne. An example for a material airborne release would be ammonia, whereas nitrate is a material but non-airborne release. The type and magnitude of these emissions and releases depend uponthe location and its properties (e.g., light or heavy soil, climate), crop type, and management practices carried out at the site under investigation. For N fertilization, the identification of emissions and releases is complicated by the fact that N compoundsare subject to various transformation processes within the soil. There can be gaseous emissions such as NOx, leaching of
188
CROP SCIENCE,
VOL. 39,
NO~-into groundwater or surface runoff. The magnitude of these emissions and releases depends on the type and amount of N fertilizer applied, the timing of application, and howthese processes relate to N uptake by the crop over time. To be comprehensive, it is important to consider not only the emissions and releases arising directly from crop production but also those arising from the manufacture and transport of farm inputs themselves. For example, during production of N fertilizer, NOxand NH3 are released. Similarly, emissions and releases linked to the processing and distribution of farm products to other industries or to households need to be considered. The second step traces the impact pathway of each type of emission and release from its origin or source (the crop managementpractice including all previous, as well as following processes) to the receptor or sink (ecosystemsdirectly or indirectly affected by these emissions and releases). This so called impact pathway assessment forms the link between the farm level and the ecosystem level. It involves the consideration of the relevant pathway (i.e., the atmosphere or pedosphere pathway), of how a release is spread (e.g., by water by wind), and the distance traveled. Releases can be spread without alteration or undergo physical and/or chemical transformation processes (NOx, for example, is spread along an atmospheric path and can be transformed to HNO3). Impact pathways can be described by using appropriate models. Impact pathway assessment can help identify the ecosystems (e.g., the forest ecosystem, the aquatic ecosystem, the agroecosystem) affected by each emission or release. Also, the ambient concentrations, depositions, loads and other effects "arriving" at the identified ecosystems can be differentiated over space and time, and characterized with respect to their specific source (e.g., N-depositions into a forest ecosystem, resulting from certain agricultural managementpractices). Thus, to identify and describe potential changes in the affected ecosystems, relationships between deposition rates and resulting potential changes must be known. For example, changes in forest soil acidity depend--amongother things--on the deposition rate of acids, which in turn depends, in part, on the NO~depositions. The third step identifies indicators describing the affected ecosystems. These indicators describe potential changes caused by emissions and releases from crop production, including all pre-processing and subsequent steps. Such indicators maybe simple parameters, including the nitrate concentration in a lake. They mayalso be aggregate parameters, such as the degree of soil acidification, which is influenced by several depositions. Ecosystem indicators must be simple, reliable, and easily measured. Ecosystem indicators are therefore defined as follows: Measurable or determinable parameters which describe the condition of the object to be protected (i.e., an ecosystem or ecosystem component). Ecosystems can usually tolerate a certain ambient concentration, deposition, or loading without undergoing irreversible changes that lead to a reduction in pro-
JANUARY-FEBRUARY1999
ductivity or ability to function. However,irreversible changes will occur when ambient concentrations, depositions, or loads exceed an ecosystem-specific threshold value. The fourth step involves determination of threshold values for the ecosystemindicators. These are the values which should not be exceeded if irreversible changes within the ecosystems are to be avoided (according to the definition of sustainable crop production outlined above). These values can only be determined on the basis of an extensive ecosystem analysis, taking into account the interactions and interdependencies within and among different ecosystems and ecosystem components. An example of threshold values would be the nitrate and phosphate concentrations beyond which the ecosystem of a lake may"turn over" (e.g., the existing fish population cannot exist any more). Another example is the soil pH in a forest ecosystem at which roots of the trees become subject to toxic levels of heavy metals. Such threshold values for the identified ecosystem indicators will vary. The values will depend on many factors including the specific ecosystems which are affected and their location. Ecosystemindicators and their corresponding threshold values put the objective "sustainability" into tangible terms by laying downcomprehensible environmental quality standards at the ecosystem level. This implies that the values are derived on the basis of scientific methodsand not on social consensus. According to the underlying definition, the production of crops is sustainable whenemissions and releases resulting from various crop production practices do not exceed threshold values set for affected ecosystems. Therefore, in a sustainable crop production scheme, the amountand type of N fertilization over a certain period of time is considered tolerable when it does not lead to the surpassing of threshold values for tolerable N-depositions or loads in any of the affected ecosystems (such as a forest ecosystem affected by soil acidification or fish populations affected by nitrate loads). In the fifth step, these threshold values are transposed from the ecosystem level to the farm level by tracing the already identified impact pathways (Step 2) back the crop production practices. This identifies maximum tolerable emissions and releases relevant to crop production. However, crop production is only one source of emissions or releases within an economy. Other activities, such as traffic, industrial production, or householdoperations also lead to emissions ending as depositions in affected ecosystems. To account for these activities in regard to crop production as a part of the overall economy, threshold values must be allocated to all the sources of emissions and releases (e.g., traffic, industry, households, agriculture). Then, only the fraction apportioned to crop production should then be taken as the baseline for the fifth step. From Step 1, emissions and releases resulting from various production practices are already known. Now after completing Step 5, those crop production practices that are pertinent and those that are not can be identified. Only those production practices, whose emissions
LEWANDOWSKIET AL.:
METHODOLOGICAL APPROACHFOR ASSESSING AND IMPLEMENTING SUSTAINABLE CROP PRODUCTION 189
or releasesare likely to causeundesired,significant(i.e., threatening to exceedthe threshold values) effects at the ecosystemlevel, need to be further examined.If, for example,the NO~-concentrationin a lake is found to exceedthe threshold value, then the N fertilization schemesor any other identified agricultural practices (e.g., the growingof leguminouscrops) needto be examined as possible causes. As part of the sixth step, the relevant crop managementpractices are described by farm-level indicators. These are defined as: Indicators whichidentify and describe separate or combinedagronomicpractices which could cause irreversible changesin agricultural and other affected ecosystems. These indicators should be simple, reliably determinedby the farmer, and relatively easy to verify. Farmlevel indicators maydescribe individual practices, such as N fertilization per hectare per year, or several practices, such as the input of energy per hectare per year (i.e., total annual consumptionof diesel fuel through plowing, harvesting, and other operations). In developingthese farm-levelindicators, the interactions within a crop production system must be taken into account. If, for example,the threshold value for NH3has been exceeded,not only the amountof Napplied is relevant, but also all practices whichinfluence Nuse efficiency. This would include the amountof N available in the soil at a given point in time, crop uptake, and accompanyingsoil cultivation practices. The maximum tolerable releases and emissions from Step 5 must then be translated to threshold values for
these farm-levelindicators. This is the task of the seventh step. Thefarm-level thresholds set a quantitative limit on the use of the farm inputs involved in crop production. Thesethreshold values are therefore called "critical uses of farm inputs". Theytake into account the production site and the crop being grown(i.e., the permissibleamountof N fertilizer dependson soil properties and crop uptake over time). Thresholdsfor these indicators mustallow for flexibility in planning(i.e., giving a permissible time period instead of setting a specific date for fertilizer application).Finally, for crop production to be sustainable, it must operate within the framework established by the threshold values (i.e., critical uses of farminputs) for the variousfarm-levelindicators. Onthe basis of the acceptable management practices, a range of production schemescan be designed, each of which remains within this framework.For example, there are different ways to minimize N losses--and thereby adhere to the thresholds values for ecosystems. This mayinclude reducing the overall amount of N applied, splitting the application, or using a moreappropriate, slow-releasetype of Nfertilizer. Among this set of optional production schemes, the most promising ones (whichare likely to be accepted and used by farmers) have to be identified. Therefore, in Step 8, each potential option for producingcrops should undergoan evaluationprocess. This wouldallow the farmerto select the option most compatible with labor, capital, and other restrictions within his farmingoperations(Fig. 3). Hemayfor example, prefer the labor intensive option of split applicationto the cost intensive option of slow-
Framework for sustainablecrop production
/
Option
Option 2
Optionn
variousoptionsof production schemes within the framework
Economic evaluation
Practical recommendations for a crop productionI that is sustainable
I
Fig. 3. Derivation of suitable production schemes that remain within the frameworkof sustainable crop production.
190
CROP SCIENCE,
VOL. 39,
release fertilizers. Other aspects, subsumedunder "social considerations", mayalso be taken into consideration during this evaluation process. Thus, this methodology which so far has focused on an ecological oriented sustainability, can be extended to include economicand social aspects as well. This approach thereby fulfills the criteria of a more comprehensiveinterpretation of sustainability. DISCUSSION The methodological approach discussed is intended to help make the term "sustainable agriculture" more practical and relevant. Because of the somewhat conflicting objectives of ecological, economic, or social aspects to be taken into consideration, the proposed approach aims at assessing sustainability and deriving practical recommendationsfor sustainable crop production using ecologically based thresholds. ~ Other approaches for assessing and implementing sustainable crop production, in which objectives and methodological steps are laid downsimilarly, are rare within the knownliterature. This makes it difficult to compare comprehensively and discuss the present approach with other methods. However, some elements (especially indicators and their threshold values) of our approach are already being used for various purposes. Therefore, the following critical review will first concentrate on the elements which form the basis for our approach. Additionally, as far as available, existing methods which also deal with the sustainability of crop production (but use definitions other than the one proposed here) will be discussed to show how our approach compares with the others. Indicators
and Threshold Values
An important element of our approach is the identification of ecosystem indicators which describe the condition of affected ecosystems and farm-level indicators which describe the relevant agricultural management practices. By using ecosystem indicators and their threshold values, direct and indirect effects of agricultural crop production on ecosystems can be determined. These indicators focus directly on the object (i.e., the ecosystem) to be protected. Farm-level indicators and their thresholds pertain to the source of ecological loading and are thus linked only indirectly to the object that is to be protected. They are, however, subject to the farmer’s influence, and therefore the hinge at which improvements can be initiated. Nieberg and Isermeyer (1994) differentiate similarly between direct indicators (comparable to the ecosystem indicators) and indirect indicators (comparable to the farm-level indicators) for discussing and assessing effects of agricultural production on the environment. Direct indicators (e.g., nitrate content in the ground water) are used to measure the condition of the object that is to be protected. Such direct indicators are of greater interest to scientists than to farmers, administrators, or politicians. They are used to monitor the environment and to indicate undesired changes therein. However,
JANUARY-FEBRUARY 1999
because of the divergence in time between cause and effect for a given practice, direct indicators are only useful to a limited extent in controlling the activities of farming enterprises or formulating agricultural and environmental policies. Indirect indicators are based on farm-level, regional, or other parameters (e.g., amount of fertilizer per hectare, sum of moneyspent on pesticides). As such, indirect indicators have very little to do with the actual effects on the environment (Nieberg and Isermeyer, 1994). In our approach, (indirect) farmlevel indicators are derived from (direct) ecosystem indicators. Wesuggest this demonstrates a major methodological advance by linking the ecosystem and farm levels through the impact pathways (Fig. 1). The significance of providing a link between these levels is clarified by examiningthe "driving force - state response model" elaborated by the OECD(1997). recent years, this model has become more and more widespread (UBA,1997); it uses driving force indicators to describe factors that cause (undesired) effects the environment (e.g., traffic, resource consumption). Driving forces are usually associated with humanactivities. State indicators describe the condition of the natural resources or the composition, structure, and functioning of the ecosystems in question. The measures and strategies undertaken to counteract the effect on the environment are subsumedin the category identified as response indicators. The OECDapproach deals with various aspects of an economy and thus covers a much broader range of environmental indicators. However, it does not give a detailed description of what sustainable crop production may be, as has been the intention of our methodological approach. Nonetheless, comparisons can be drawn between the indicator categories. Crop management practices are analogous to driving forces; ecosystem indicators are comparable to state indicators. In spite of apparent differences between the indicators used in the present approach and those in other indicator systems, this exampleshows that it has become commonpractice to use indicators to describe the condition of ecosystems (components) as well as farm level activities in the discussion of environmental effects. Indicators reflecting the condition of the soil play a particularly important role because the soil is a significant mediator between the farm and the ecosystem level. It is a key agroecosystem component. Not only does the productivity of the overall food production system very much depend on the quality of the soil, but the condition of other ecosystems components(e.g., water, air) also depends on the condition of the soil resource (Harris et al., 1996). Harris et al. (1996), whogive an overview of methodological approachesto evaluating soil quality and health, point out the fact that there are a range of different indicators for the description of the complex system "soil" with its various functions. They emphasize that before soil indicators can be used for the elaboration of relevant managementstrategies, there is need for further research on the development of simple, respectively "usable" indicators. Indicators which aggregate
LEWANDOWSKI ET AL.:
METHODOLOGICAL APPROACH FOR ASSESSING
various properties describing soil quality and soil health, such as microbial biomass, organic C, aggregate stability, or respiration, must be elaborated. In their function of setting standards at the ecosystem and at the farm level, indicators themselves are of little use without their corresponding threshold values. Threshold values show whether the objective aimed at (here "sustainability") has been met or not. "Critical loads" or "critical levels" are threshold values which are well knownfrom the literature. Strickland et al. (1993) describe the use of critical loads and levels as "an approach for estimating the amountsof pollutants that sensitive ecosystemscan absorb on a sustained basis without experiencing measurable degradation". According to UBA(1996), "critical levels" are defined "concentrations of pollutants in the atmosphere, above which direct adverse effects on receptors, such as human beings, plants, ecosystems, or materials, mayoccur according to present knowledge". "Critical loads" are defined as "a quantitative estimate of an exposure to one or more pollutants, below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge" (Nilsson and Grennfelt, 1988). "Critical levels-loads" are discussed within the UN-ECE(United Nations - Economics Commissionfor Europe) with the objective of elaborating abatement strategies for air pollution. The emphasis is on ambient concentrations of SO2, NOx, NH3, and 03, as well as on N and S loads, leading to acidification and eutrophication of natural ecosystems. Thus--within the presented methodological approach--the concept of "critical levels-loads" can be used to determine threshold values for ecosystem indicators. However, the work conducted so far within the UNECE has concentrated on air pollution and only on specific ecosystems (e.g., forest ecosystems, freshwater and marine ecosystems), whereas an application of the present approach will also consider the impact paths of other releases and ecosystems. For example, the maximumtolerable amount of erosion can be defined as the "critical load" for the loss of soil. Thus, the application of indicators and their corresponding threshold values is not substantially new. Indicators have already proved their usefulness in discussing, monitoring, and reducing environmental effects. Existing
Approaches
Farshad and Zinck (1993) provide an overview various starting points and approaches for assessing and evaluating sustainable agriculture. With regard to sustainability, maintenance of productivity plays an important role in most of the approaches. Retrospective approaches analyze former agricultural practices and thus identify valuable practices which have made agriculture sustainable. Coevolutionary approaches aim at monitoring the parallel evolution of socio- and ecosystems from the past to the present to analyze sustainability. Land evaluation approaches help to assess present-day sustainability; below some examples for this will be men-
AND IMPLEMENTING
SUSTAINABLE
CROP PRODUCTION
191
tioned. Someapproaches aim at estimating the productive potential over time by elaborating equations, which contain certain parameters such as for example agronomic productivity, total energy input, some measure for soil degradation or a measure for water quality. Other approaches within this category concentrate on carrying capacity models, in which threshold values (e.g., maximumnumber of population or a maximum yield) are defined. These thresholds should not be exceeded to prevent irreversible changes in the environment especially with regard to natural resources. Other approaches concentrate on agroecosystem analysis and define sustainability as the ability of a system to maintain its productivity whensubject to stress or perturbation. Proposals for ecological managementstrategies (e.g., alternative cropping systems and weed and pest management)often are derived from agroecological approaches. The various approaches provide a broad range of usable instruments for assessing sustainability. However, they currently stress the productivity and functioning of the agroecosystem and do not explicitly discuss other ecosystems affected by crop production. Moreover, the approaches fail to show howsustainable crop production can be implemented at the farm level on the basis of a sound methodology and scientifically based standards. These deficits will be demonstrated by the following examples. The LISA program (Low-Input Sustainable Agriculture), which began in 1988 in the USAand was later renamed SARE(Sustainable Agriculture Research and Education), is designed to makeagricultural production more sustainable particularly by promoting the genetic improvement of crops (see Madden,1994). For example, corn varieties adapted tb low-input production systems satisfying the SARE criteria (e.g., low-Nor no-till systems) are to be selected. Pesticide use is to be reduced by planting pest resistant cultivars. Such recommendations are targeted at improving resource use efficiency or at reducing resource consumption. While the SARE approach is useful to implement crop production systems with less environmental impact, it will not help in identifying sustainability criteria or in elaborating a methodfor assessing sustainability. Stoorvogel et al. (1995) elaborated a methodological framework called USTED(Uso sostenible de Tierras En el Desarollo; Sustainable Land Use in Development). USTEDworks with a linear programming model to determine optimal land use. The objective "optimal land use" was defined as maximizing total regional net farm income as a proxy for the net benefit of economic development. The framework was applied by Jansen et al. (1995) to the Negev settlement in Costa Rica. Two major crop production problems in that area were identified, for whichtwo sets of sustainability indicators were drawn up: nutrient balances in the soil (for N, P and K) and an index of biocide use. As with the method presented in this paper, the USTEDapproach intends to measure sustainability by using scientifically based indicators. However, the indicators selected ("nutrient balances" and "pesticide use") only cover fertilization
192
CROP SCIENCE, VOL. 39, JANUARY-FEBRUARY 1999
and pesticide use, and do not describe effects of other agricultural practices such as soil cultivation. They are strictly farm-level indicators and thus describe only indirectly the effect of agricultural production on agro- or ecosystems. This approach may in principle be extended to include further indicators. However, the linear programming method employed sets a limit on the number of sustainability indicators that can be taken into account. Additionally, the necessity to derive threshold values for these indicators using a sound and acceptable procedure limits the range of "usable" indicators. Biewinga and van der Bijl (1996) propose a methodology for assessing the sustainability of energy crops in different European regions. Economic, social and ecological aspects are taken into account, but the emphasis is placed on the latter. A total of 15 different criteria (indicators) are analyzed for 10 potentially interesting energy crops in four regions. The values calculated for each criterion are transformed into a score from 0 (very bad) to 10 (very good). Those crops with the best overall rating for a given region are considered most sustainable for that region. This means that only a relative rating (i.e., between different options) and not an absolute assessment in relation to the objective given (i.e., the protection of the ecosystems) is possible. A (quantitative) standard that should be achieved is not set. On the basis of this method, no recommendations can be given to the farmers on how to make individual management practices in energy crop production more sustainable. Moreover, the criteria have been pre-selected on the basis of those issues most commonly discussed with respect to making agricultural production less environmentally damaging. No scientific basis is provided on how criteria should be selected and in what respect they are indicative of sustainability. In summary, to date no methodological approach for assessing and implementing sustainable crop production is known that follows a procedure similar to the one presented in this paper. Those approaches which have been mentioned cover certain aspects of sustainability but pursue objectives and procedures different from those proposed here. However, from existing research, elements can be derived that contribute to an enhanced understanding of our methodological approach.
OUTLOOK The objective of this paper is to develop and discuss a methodological approach for assessing sustainability of agricultural crop production and to show a way to transfer the insights gained into practical farming through the elaboration of farm-level indicators. This is done on the basis of a definition of sustainable agriculture in which the ecosystems affected by crop production are emphasized as the objects to be protected. The scarcity of knowledge on the links and interactions within and among different ecosystems is a fundamental difficulty in elaborating such a practical framework using the methodological approach presented. Therefore, future research activities should concentrate on studying the interdependencies within and among
ecosystems to gain a better understanding of the effects of incoming loads. Further research will have to focus on a systems approach in the determination of indicators at the ecosystem level (see, for example, Karlen et al., 1997). Parallel to these research activities, the present approach has to be worked out in much more detail. In particular, modeling the impact pathways for different emissions and releases between crop production site and the affected ecosystem is quite difficult. However, some examples already exist. In ALS (1996), the pathways of airborne emissions under various meteorological conditions have been described, taking the possible transformations of these emissions into consideration. Similarly, the path of releases from crop production into the soil and other ecosystems have been studied especially for N (see for example Engel et al., 1993; Engel and Priesack, 1993). Such models need to be refined and extended further to contribute to the impact pathway assessments required for our approach. In summary, our methodological approach needs further interdisciplinary research and has to be examined in more concrete terms. Furthermore, some methodological problems have to be solved before the approach can be used comprehensively to implement successfully sustainable crop production in practice. This also pertains to considerations on how to further integrate social and economic demands, although Step 8 already permits this to some degree. However, the proposed approach provides a first approach to assess and implement "sustainability" in crop production using scientifically grounded standards to ensure in the long term a "development that meets the needs of the present without compromising the ability of future generations to meet
their own needs". ACKNOWLEDGMENTS
The authors thank Andrea Bohn for her manifold contributions, especially in editing and improving the structure of
this paper.
LEWANDOWSKI ET AL.: METHODOLOGICAL APPROACH FOR ASSESSING AND IMPLEMENTING SUSTAINABLE CROP PRODUCTION
193
