Laura Christianson1 & John Tyndall2
1Department of Agricultural and Biosystems Engineering, Iowa State University, 3145 NSRIC, Ames, IA 50011 USA (email: Laurac@iastate.edu)
2Department of Natural Resource Ecology & Management, Iowa State University, 339 Science II, Ames, IA 50011 USA (email: email@example.com)
Authors' Personal Statement: This Community Essay aims to start a dialogue on the role of targeted environmental technologies in “sustainable agriculture.” Using the new water-quality technology of denitrification bioreactors as a specific example, we focus on the question: are edge-of-field technologies such as bioreactors simply band-aid approaches to sustainable agriculture? Or can they be part of a comprehensive paradigm shift? Denitrification bioreactors are a novel approach for reducing the amount of nitrate in on-farm agricultural drainage, a pollutant that has caused water-quality concerns at both local and national scales. We first address whether or not denitrification bioreactors might qualify singularly as a "sustainable technology" within the conceptual continuum of weak to strong sustainability. Then we introduce a broader perspective on the potential role that targeted technologies might play in multifunctional agricultural landscapes. We suggest that denitrification bioreactors are one technology that can, in a small way, mediate a shift in agrarian paradigms. A transition toward sustainability is a long and gradual process requiring the incorporation of a wide range of approaches including targeted technologies and multifunctional landscapes. While the issues presented here are hardly exhaustive, it is our hope that this commentary spurs broader dialogue within the sustainable agriculture community about the role of technology in the future of agriculture. We are seeking to encourage broader philosophical reflection on work being done in the name of a sustainable agriculture.
Keyword: sustainable agriculture, bioreactor, agricultural technology, mitigation, water treatment, denitrification, farms
Citation: Christianson L. & Tyndall J. 2011. Seeking a dialogue: a targeted technology for sustainable agricultural systems in the American Corn Belt . Sustainability: Science, Practice, & Policy 7(2):70-77. Published online Sep 02, 2011. http:///archives/vol7iss2/communityessay.christianson.html
The current Corn Belt landscape of the United States developed over several generations due to a multitude of factors including technological advances, demographic trends, regional shifts in production goals, economic and policy stimuli, and cultural heritage (Hurt, 2002). Today’s resulting Midwestern landscape is characterized as being a highly complex, socially constructed mosaic of intensive land uses constrained by ecological, technological, and economic capacity (Nassauer et al. 2002). It is clear that the current intensive model of agriculture, while highly productive in terms of economically important crop commodities, is also known to have pervasive impacts on patterns and processes essential to ecosystem function and therefore agricultural productivity and environmental quality (Tilman et al. 2002; Robertson & Swinton, 2005; Dale & Polasky, 2007). Further, many consequences of ecosystem impairment are subsequently passed on to society as costly negative externalities that are increasingly being experienced at multiple spatial and temporal scales (Tegtmeier & Duffy, 2004).
This state of affairs will likely push twenty-first century American agricultural policy toward the promotion of multifunctional landscapes (Ruhl et al. 2007), that is, economically viable landscapes that jointly produce increased quantities of ecosystem goods (e.g., food, fiber, and fuel) and broader arrays of environmental services that control negative externalities, enhance productive capacity, and provide numerous ecosystem benefits (Boody et al. 2005; Jordan & Warner, 2010). Generating, enhancing, and/or maintaining ecosystem services across landscapes is integral to any sustainable agriculture paradigm and will increasingly be viewed as a primary component of operationalized sustainability (Selman, 2008; Taylor-Lovell & Johnston, 2009). Nevertheless, farmers are significantly challenged to find a balance that fulfills their shorter-term production goals and longer-term stewardship interests (Chouinard et al. 2008). In short, the future of agriculture is a very complex socioecological issue with much at stake for farmers, consumers, and the environment.
Arguments and concerns about a sustainable future for Corn Belt agriculture in the United States have been framed from a number of overlapping, systemic perspectives (e.g., Bell et al. 2004; Flora & Flora, 2007; Nassauer et al. 2007). To channel the discussion and start a dialogue, this essay purposely chooses one angle on which we, as an agricultural engineer and a natural resource economist, have unique, pointed perspectives: agro-environmental quality and the role of environmental technology in a “sustainable agriculture.”
In the agricultural domain, it is easier to recognize what is clearly not a sustainable condition than to identify what is; as such, a key point in closing a sustainability gap is to eliminate or mitigate the offending state of affairs (Boron & Murray, 2004). In the American Midwest, a key environmental indicator of declining Corn Belt sustainability is the acute deterioration of water quality due to nutrient and sediment loading, which has created cascading negative effects across multiple scales (Helmers et al. 2007). Expensive local drinking-water treatment, combined with national concerns about hypoxia in the Gulf of Mexico, means that the prime source for this nutrient pollution, Corn Belt agricultural drainage, needs to be a major starting point for addressing environmental sustainability (Goolsby & Battaglin, 2000; McMullen, 2001). Moreover, the timing is critical for addressing these agro-environmental issues as the 2008 Gulf Hypoxic Zone was the second largest on record and the 2009 zone was unusually severe in certain locations (USEPA, 2011). This hypoxia is one of the United States’ largest water-quality concerns; the resulting death of aquatic organisms represents a severe disruption of ecosystem function as well as lost economic opportunity for the Gulf’s associated aquatic industries.
The Role of Technology
In response, agricultural scientists have amplified their calls for the increased role of technology in sustaining agriculture and the environment (e.g., Aldy et al. 1998; Tilman et al. 2002; Secchi et al. 2008). Recent research regarding on-farm options for water-quality improvement has led to new ideas such as denitrification bioreactors for nitrate removal from agricultural drainage (Jaynes et al. 2008; Christianson et al. 2009; Woli et al. 2010).
Denitrification bioreactors are an innovative technology that maximizes the natural process of denitrification, a conversion of problematic nitrate to comparatively benign nitrogen gas by native soil bacteria. Denitrification technologies were originally used to treat nitrate pollution in groundwater in the 1990s (Schipper & Vojvodic-Vukovic, 1998), and this idea has now proven promising to treat agricultural drainage waters (van Driel et al. 2006; Schipper et al. 2010). In the case of agricultural drainage, a pipe that receives drainage water from between 8–20 hectares (ha) (20–50 acres) is intersected with a trench filled with woodchips (e.g., 3–6 meters wide and 30 meters long or approximately 10–20 feet wide x 100 feet long). Beneficial bacteria colonize the woodchips and use them as their carbon source (i.e., food) to provide energy for nitrate conversion as the nitrate-laden drainage waters flow by. This nitrate-mitigation strategy is promising for the American Corn Belt, reducing nitrate concentrations by over half and even as high as 99% depending upon a number of environmental factors such as flow rate and temperature as well as bioreactor design (Jaynes et al. 2008; Woli et al. 2010). In terms of nitrate-load reduction (i.e., considering volume of water treated in addition to nitrate-concentration changes), bioreactors may remove 33–55% of the total nitrate amount that would otherwise have gone downstream (Jaynes et al. 2008; Woli et al. 2010). After the upfront installation cost, to which governmental cost-sharing may apply, comparatively little maintenance cost or time is required over a life of at least ten years (Schipper et al. 2010).1
Though denitrification bioreactors for agricultural drainage are still a new idea, a handful are operational in Iowa, Illinois, Minnesota, and Canada (van Driel et al. 2006; Christianson et al. 2009; Willette, 2010; Woli et al. 2010). To date, most installations have been via private groups (e.g., watershed associations, commodity groups) or research organizations (e.g., universities, United States Department of Agriculture’s Agricultural Research Service), though researchers think that eventually individual landowners will instigate installations. With “field-scale” treatment areas, bioreactors do not treat wide swaths of land but are ideal for individual Midwestern drainage systems. Bioreactors can be incorporated into existing conservation practices such as grassed buffers (Christianson et al. 2009), and their “edge-of-field” treatment means that they are minimally affected by variable in-field practices (e.g., no-till, fertilizer management, increased cropping due to demand for biofuel feedstock). Additionally, once installed, this technology has very low external energy requirements; as drainage water flows through the woodchips, the bacteria do the work. However, beyond the standard applied research questions regarding pollution-removal effectiveness and cost, critics have brought up a broader, equally important question: are edge-of-field technologies such as bioreactors simply physical and metaphorical band-aid approaches treating problematic symptoms rather than addressing the sustainability of agriculture as a whole?
This is not a new question, as Allen et al. (1991) expressed concern that many technological approaches to agricultural sustainability seem to accept the current industrial evolutionary path of crop production as given. The concern that certain technologies seek “sustainable” approaches to “conventional” production paradigms suggests that such strategies may simply delay the inevitable collapse of an inherently unsustainable model. More recently, an article in the online journal Grist challenged bioreactors specifically by pondering why Midwestern researchers were pursuing denitrification-bioreactor research, as the technology “fall(s) flat when you realize it’s just a technical fix for the status quo of over-fertilized conventional commodity crops” (Hoffner, 2009).
To give our discussion credence in the sustainability realm, we first address whether or not denitrification bioreactors might qualify singularly as a “sustainable technology” within the conceptual continuum of weak to strong sustainability (Turner, 1993). We then introduce a wider perspective on the role that bioreactors might play in agricultural landscapes. While the issues presented here are hardly exhaustive, it is our hope that this commentary spurs broader dialogue within the sustainable agriculture community about technology’s role (in general, as well as specific to bioreactors) and to encourage broader philosophical reflection on various approaches to sustainable agriculture.
The notion of weak or strong sustainability seems a reasonable place to start in philosophically evaluating a particular technological approach to mitigating agricultural externalities. To a large extent, the distinction between weak and strong sustainability is the degree to which human-made capital can substitute for natural capital (in this case natural process). Table 1 articulates philosophical distinctions as described by Turner (1993) on a progressive continuum of “very weak” to “very strong” sustainability across different perspectives on resource use, substitutability, and economic growth. Agricultural land use, technology, and externalities all have significant overlapping economic qualities, suggesting that this continuum of sustainability is an appropriate exploratory framework (Stoneham et al. 2003).
Table 1 Spectrum of Overlapping Sustainability Positions (Adapted from Turner, 1993) with rationalizations of bioreactor “weak” to “strong” sustainability.
A Role for Denitrification Bioreactors in a Multifunctional Landscape?
Despite our interpretation of bioreactors as being appropriate technology within a sustainable agriculture, we ultimately contend that such a perspective is far too narrow in contextual scope—that is, nontechnical, philosophical critiques positioned strictly at the technology level disregard what a sustainable agricultureprobably is; one that is purposefully multifunctional and highly productive with minimal (and perhaps correctable) tradeoffs (e.g., Tilman et al. 2002; Wilson, 2007). In this case, the argument may well be contextualized by assessing the role a particular technology might play in a multifunctional landscape. Multifunctional agriculture will require, in varying degrees, a targeted approach to land use. Targeted agricultural land use is founded on the premise that strategically-placed conservation practices can produce more than proportionate ecosystem benefits relative to their total spatial extent (Secchi et al. 2008; Taylor-Lovell & Johnston, 2009). The determination of targeted land uses may be thought of as a two-stage process. The first stage involves identifying key locations at a watershed-scale where land-use modification or technological remediation could have significant impact. The second stage involves targeting land-use change at the field/farm-scale by examining localized variable source areas and soils and accounting for equipment requirements and other management considerations. Ultimately, such a targeted approach can significantly improve ecosystem-service delivery, minimize land-use tradeoffs, and efficiently use monies already allocated to conservation via various government programs (Secchi et al. 2008).
If a targeted approach to agriculture is truly a legitimate path toward sustainability, where then does a technology such as a bioreactor fit within this concept of multifunctional agriculture? The notion of targeted technologies continues to beg Hoffner’s (2009) implicit question, “Are these only band-aids?” A reasonable analysis of this question may begin by examining bioreactors in the context of the “economy of pragmatism.” Realistic, multifunctional farms in the American Midwest will still require artificial drainage to maximize economic profitability (Comis, 2005). The complexities of soil-water relationships and farm-level nutrient management suggest that runoff and nutrient leachate will remain remedial issues even in multifunctional systems (Coiner et al. 2001; Randall & Goss, 2001). Denitrification bioreactors and similar water-quality technologies can reduce nitrate loads in drainage from both conventional and alternative operations. Therefore, certain technologies will have a place even as ideas and patterns shift. The transition toward sustainability is a long and gradual process involving shifting agrarian paradigms that will invariably require the incorporation of a wide range of technologies, approaches, and philosophies.
Of course, there are other technologies that incorporate or augment natural processes to effectively provide the same denitrification (nitrate removal) process as bioreactors in addition to a host of other ecological functions. Foremost are restored or constructed wetlands that additionally provide services such as biodiversity and habitat, flood protection, groundwater recharge, and carbon sequestration (O’Geen et al. 2010). However, government programs or water-quality professionals do not intend (that we are aware of) to “pit” bioreactors against wetlands or suggest 1:1 fungability with regard to direct and indirect ecological outcomes (Christianson et al. 2009). In fact, in most cases denitrifying bioreactors complement other best management practices and do not preclude a variety of mitigation strategies (Woli et al. 2010). Ultimately, bioreactors are best utilized within a “suite of solutions” for achieving water-quality goals in an agricultural watershed (ISA, 2011). Because a degree of remedial immediacy is involved with local and regional water-quality goals, various technologies and best management practices are collectively required to gain aggregative benefits on a watershed-scale.2
In the context of targeted land use at multiple scales, bioreactors have a number of key technological advantages. They can easily be targeted at the field/farm level to optimize impact while requiring a relatively small surface footprint (approximately 0.1% of the drainage area); this is an important factor when minimizing land-use costs is paramount. Further, bioreactors can be installed in locations where wetlands cannot be built (ISA, 2011) and many farmers may have access to required excavation equipment and be able to use on-farm materials (e.g., wood chips), lowering installation costs. However, as Christianson et al. (2009) note, these two technologies offer options for different scales: bioreactors are primarily intended for farm-scale with a relatively small treatment area of 8 to 20 ha (20 to 50 acres), while wetlands are able to treat far greater drainage areas of 405 to 1,012 ha (1,000 to 2,500 acres); used together multi-scale synergies can result, which can reduce the overall cost of subsurface-drainage treatment in a watershed (ISA, 2011). Ultimately, as noted above, improved water quality is an aggregative function of several management interventions across numerous farms within a watershed. Within this context, we believe it is clear that denitrifying bioreactors can, at least, play an effective incremental role.
A Word or Two Beyond the Physical
Even though bioreactors are a focused technology to improve the environmental sustainability of agriculture as narrowly indicated by nitrate loads, it is thought their use and promotion (e.g., via demonstration projects, participatory research, and cost-share programming) may provide a unique opportunity to open the door for proactive mindsets and a broader discussion about sustainability (e.g., Willette, 2010); this phenomenon of social learning and dialogue is common in the context of agricultural demonstration and on-farm research within the American Corn Belt (Lemke et al. 2010; Petrehn, 2011) as well as international agricultural contexts (e.g., Verstraeten et al. 2003). With on-farm communication about innovative technologies such as bioreactors comes something even more powerful with regard to motivating farmer-management intentions: education about the relationship among land use, environmental quality, and agricultural sustainability (Lemke et al. 2010). The idea here is that bioreactors and other applied technologies can (to some degree) be a technological segue to increased interest in environmentally sustainable agricultural. The novelty of bioreactors, as well as their scaled technological advantages and complementary nature, provides an interesting and complex backdrop for landowner education regarding innovation, environmental quality, and potential roles for farmers as land stewards. This feature could indeed contribute to positive but nuanced social outcomes that enhance and expand the role a “community of farmers” might have in defining regional identity by better protecting social and environmental amenities (Bell et al. 2004). Work is currently underway in Iowa to explore this educational dynamic and to characterize farmer opinions, concerns, and potential intentions regarding the use of bioreactors (Christianson & Helmers, 2009).
It is important that research not lose sight of the reality of conventional agriculture while trying to achieve land use that leans toward the stronger side of sustainability. In agriculture, land use (to a large degree) dictates the suite of “goods” and “bads” associated with that landscape. Water-quality dilemmas in the American Midwest are creating social pressure of remedial immediacy—to deny a well thought-out water-quality management approach seems counterproductive in this context. Technology, such as the denitrification bioreactor, that is effective for nitrate mitigation, scale appropriate, compatible with other technology/management, affordable, and broadly appealing to farmers has an enhanced probability of being embraced. As noted earlier, the adoption of bioreactors reflects a pointed private interest in stewardship with a degree of internalized responsibility (i.e., internalized cost at private expense) toward agro-environmental quality. This may reflect farmer behavior, motivation, and interest that, in the aggregate, helps “pave the pathway” toward a more sustainable agriculture. To the degree that a technology helps initiate broader understanding of the link between land stewardship and sustainability, all the better. This dynamic surrounding bioreactors strikes us as a technological approach to a pragmatically defined issue that not only treats symptoms of socially inefficient land use but also promotes a different agricultural paradigm. The American ecological designer William McDonough has articulated, “Sustainability takes forever—that’s the point.” Some critics argue that bioreactors and related technology cannot play a potential role in agriculture on the assumption that such technology belies the complexity of sustainability (e.g., Hoffner, 2009); we wonder if they are themselves failing their own assumptions by assuming a complex, systemic change will occur all at once.
Ultimately, we freely admit that we struggle philosophically with this issue and are broadly seeking insights. It is our hope that this article initiates the kind of dialogue that will encourage others to be reflective about their research and to contemplate the broader philosophical implications of technology in agriculture.
1 Though no denitrification bioreactors treating drainage waters have yet failed due to woodchip exhaustion, it is thought that utilized woodchips would be removed and the excavation refilled with new chips if treatment was to continue.
2 A potential unintended consequence of the use of mitigation technology is that producers may feel somewhat insulated from potential off-farm effects and be prone to overfertilization or intensifying tillage. Nevertheless, since the usage of mitigation technology is voluntary (often motivated by an active desire to manage environmental risk) we believe that this type of moral hazard is limited.
Aldy, J., Hrubovcak, J., & Vasavada, U. 1998. The role of technology in sustaining agriculture and the environment. Ecological Economics 26(1):81–96.
Allen, P., Van Dusen, D., Lundy, J., & Gliessman, S. 1991. Integrating social, environmental, and economic issues in sustainable agriculture. American Journal of Alternative Agriculture 6(1):34–39.
Ayres, R., van den Bergh J., & Gowdy J. 1998. Viewpoint: Weak versus Strong Sustainability. Discussion Paper IT 98–103/3. Amsterdam: Tinbergen Institute.
Bell, M., Bauer, D., Jarnagin, S., & Peter, G. 2004. Farming for Us All: Practical Agriculture and the Cultivation of Sustainability. University Park, PA: Pennsylvania State University Press.
Boody, G., Vondracek, B., Andow, D., Krinke, M., Westra, J., Zimmerman, J., & Welle, P. 2005. Multifunctional agriculture in the United States. BioScience 55(1):27–38.
Boron, S. & Murray, K. 2004. Bridging the unsustainability gap: a framework for sustainable development. Sustainable Development 12(2):65–73.
Chouinard, H., Paterson, T., Wandschneider, P., & Ohler, A. 2008. Will farmers trade profits for stewardship? Heterogeneous motivations for farm practice selection. Land Economics 8(1):66–82.
Christianson, L., Bhandari, A., & Helmers, M. 2009. Emerging technology: denitrification bioreactors for nitrate reduction in agricultural waters. Journal of Soil and Water Conservation 64(5):139A–141A.
Christianson, L. & Helmers, M. 2009. Producer Education of Nitrate Reduction Strategies and Evaluation of Acceptance. Project Number GNC09–103. College Park, MD: Sustainable Agriculture Research & Education.
Coiner, C., Wu, J., & Polasky, S. 2001. Economic and environmental implications of alternative landscape designs in the Walnut Creek Watershed of Iowa. Ecological Economics 38(1):119–139.
Comis, D. 2005. Underground drainage: a secret of America’s bounty. Agricultural Research Magazine 53(9):4–6. http://www.ars.usda.gov/is/AR/archive/sep05/drainage0905.pdf.
Flora C. & Flora, J. 2007. Rural Communities: Legacy and Change. Boulder, CO: Westview Press.
Goolsby, D. & Battaglin, W. 2000. Nitrogen in the Mississippi Basin:Estimating Sources and Predicting Flux to the Gulf of Mexico. USGS Fact Sheet 135–00. Reston, VA: United States Geological Survey.
Helmers, M., Isenhart, T., Kling, C., Moorman, T., Simpkins, W., & Tomer, M. 2007. Agriculture and water quality in the corn belt: overview of issues and approaches. Choices 22(2):79–85.
Hoffner, E. 2009. Gulf dead zone fix falls flat. http://www.grist.org/article/gulf-dead-zone-fix-falls-flat. August 21, 2011.
Hurt, R. 2002. Problems of Plenty: The American Farmer in the 20th Century. Chicago: Ivan R. Dee Publications.
Iowa Soybean Association (ISA). 2011. Bioreactor Basics. http://www.iasoybeans.com/environment/programs-initiatives/programs/bioreactors/basics. August 21, 2011.
Jaynes, D., Kaspar, T., Moorman, T., & Parkin, T. 2008. In situ bioreactors and deep drain-pipe installation to reduce nitrate losses in artificially drained fields. Journal of Environmental Quality 37(2):429–436.
Jordan, N. & Warner, K. 2010. Enhancing the multifunctionality of US Agriculture. Bioscience 60(1):60–66.
Lemke, A., Lindenbaum, T., Perry, W., Herbert, M., Tear, T., & Herkert, J. 2010. Effects of outreach on the awareness and adoption of conservation practices by farmers in two agricultural watersheds of the Mackinaw River, Illinois. Journal of Soil and Water Conservation 65(5):304–315.
McMullen, L. 2001. Remediation at the water treatment plant. In R. Follett & J. HatfieId (Eds.), Nitrogen in the Environment: Sources, Problems, and Management. pp. 455–460. New York: Elsevier.
Moran, E. 2010. Environmental Social Science: Human-Environment Interactions and Sustainability. Hoboken, NJ: Wiley-Blackwell.
Nassauer, J., Corry, R., & Cruse, R. 2002. The landscape in 2025: alternative future landscape scenarios: a means to consider agricultural policy. Journal of Soil and Water Conservation 57(2):44A–53A.Nassauer, J., Santelmann, M., & Scavia, D. 2007. From the Corn Belt to the Gulf: Societal and Environmental Implications of Alternative Agricultural Futures. Washington, DC: Resources for the Future Press.
O’Geen, A., Budd, R., Gan, J., Maynard, J., Parikh, S., & Dahlgren, R. 2010. Mitigating nonpoint source pollution in agriculture with constructed and restored wetlands. In L. Donald (Ed.), Advances in Agronomy.pp. 1–76. New York: Academic Press.
Petrehn, M. 2011. Mapping the Social Landscape of Grazing Management in the Corn Belt: A Review of Research and Stakeholder Perceptions of the Multifunctionality of Iowa Grazing Systems. Master’s Thesis. Department of Natural Resource Ecology and Management. Iowa State University, Ames, Iowa.
Prokopy, L., Floress, K., Klotthor-Weinkauf, D., & Baumgart-Getz, A. 2008. Determinants of agricultural best management practice adoption: evidence from the literature. Journal of Soil and Water Conservation 63(5):300–311.
Randall, G. & Goss, M. 2001. Nitrate losses to surface water through subsurface, tile drainage. In R. Follett & J. HatfieId (Eds.), Nitrogen in the Environment: Sources, Problems, and Management. pp. 95–122. New York: Elsevier.
Robertson, G. & Swinton, S. 2005. Reconciling agricultural productivity and environmental integrity: a grand challenge for agriculture. Frontiers in Ecology and the Environment 3(1):38–46.
Ruhl, J., Kraft, S., & Lant, C. 2007. The Law and Policy of Ecosystem Services. Washington DC: Island Press.
Schipper, L. & Vojvodic-Vukovic, M. 1998. Nitrate removal from groundwater using a denitrification wall amended with sawdust: field trial. Journal of Environmental Quality 27(3):664–668.
Schipper, L., Robertson, W., Gold, A., Jaynes, D., & Cameron, S. 2010. Denitrifying bioreactors: an approach for reducing nitrate loads to receiving waters. Ecological Engineering 36(11):1532–1543.
Secchi, S., Tyndall, J., Schulte, L., & Asbjornsen, H. 2008. High crop prices and conservation: raising the stakes. Journal of Soil and Water Conservation 63(3):68A–73A.
Selman, P. 2008. What do we mean by sustainable landscapes? Sustainability: Science, Practice, & Policy. 4(2):23–28. http://sspp.proquest.com/archives/vol4iss2/communityessay.selman.html.
Stoneham, G., Eigenraam, M., Ridley, A., & Barr, N. 2003. The application of sustainability concepts to Australian agriculture: an overview. Australian Journal of Experimental Agriculture 43(3):195–203.
Taylor-Lovell, S. & Johnston, D. 2009. Creating multifunctional landscapes: how can the field of ecology inform the design of the landscape? Frontiers in Ecology and the Environment 7(4):212–220.
Tegtmeier, E. & Duffy, M. 2004. External costs of agricultural production in the United States. International Journal of Agricultural Sustainability 2(1):1–20.
Tilman, D., Cassman, K., Matson, P., Naylor, R., & Polasky, S. 2002. Agricultural sustainability and intensive production practices. Nature 418(6898):671–677.
Turner, R. 1993. Sustainability: principles and practice. In R. Turner (Ed.), Sustainable Environmental Economics and Management: Principles and Practice. pp. 3–36. New York: Belhaven Press.
United States Environmental Protection Agency (USEPA). 2011. Hypoxia in the News. http://www.epa.gov/owow_keep/msbasin/gulfnews.htm#zone. August 21, 2011.
van Driel, P., Robertson, W., & Merkley, L. 2006. Denitrification of agricultural drainage using wood-based reactors. Transactions of the American Society of Agricultual and Biological Engineers 49(2):565–573.
Verstraeten, G., Poesen, J., Govers, G., Gillijns, K., Van Rompaey, A., & Van Oost, K. 2003. Integrating science, policy and farmers to reduce soil loss and sediment delivery in Flanders, Belgium. Environmental Science & Policy 6(1):95–103.
Willette, J. 2010. Bioreactor Field Day Proves to Be a Learning Day. http://www.agrinews.com/bioreactor/field/day/proves/to/be/a/learning/day/story-2887.html. August 21, 2010.
Wilson, G. 2007. Multifunctional Agriculture: A Transition Theory. Wallingford, UK: CABI Publishing.
Woli, K., David, M., Cooke, R., McIsaac, G., & Mitchell, C. 2010. Nitrogen balance in and export from agricultural fields associated with controlled drainage systems and denitrifying bioreactors. Ecological Engineering 36(11):1558–1566.