Nanomaterials In Soil: Our Future Food Chain?
To feed a growing population under increasing natural resource constraints, the World Bank, the United Nations Food and Agriculture Organization (FAO) and other international organizations are promoting “sustainable intensification” as the future of agricultural production.1 The application of nanotechnology techniques to agricultural crop inputs is one of the proposed tools for “sustainable intensification.”2 These applications include reducing the volume of pesticide use through adding nano-silver particles to pesticides to make them more effective in targeting pests with a smaller pesticide volume; adding nano–metal oxides to target soil pathogens, e.g., those resulting from fertilizing with non-composted manure; adding nano-silicon to increase water uptake efficiency in plants; developing a DNA-based nanobio-sensor in a polymer to coat fertilizers, which would release only as much fertilizer as “demanded” by plant root ionic signals.
Each of these applications presents its own opportunities, risks and knowledge gaps. Thus far, governments are allowing the commercialization of ENMs and nano-enabled products while they deliberate whether and how much to regulate nanotechnologies. One European Commission summary of a decade of tentative steps towards a mixture of regulation and industry “self-regulation” states, “Nanotechnologies-related products/activities are presently regulated essentially by using existing provisions, but given the unique features of nanotechnologies doubts exist about the effectiveness of this approach. The use of specific hard regulation is advocated by some parties, but so far, the strategies from authorities worldwide have been essentially on probing the extendibility of existing regulatory schemes to nanotechnologies and/or to ensure compliance with them. In the last few years, voluntary measures have been endorsed by public bodies and industry to build confidence and trust, promote safety or gather data.”4
As a result of the intragovernmental debate over whether to develop nanotechnology-specific regulation, governments have not yet conducted thorough assessments of nano-specific risks, nor have they required pre-market and post-market safety assessments of nano-enabled products. Notwithstanding the lack of such assessments, a FAO/World Health Organization convened expert group report stated, “It is expected that nanotechnology-derived food products will be increasingly available to consumers world-wide in the coming years.”5
More than two decades ago, two eminent toxicologists advised that “it would be prudent to examine and address environmental and human health concerns before the widespread adoption of nanotechnology.”6 With the exception of some medical applications of nanotechnology, governments, corporations and even university-based start-up companies have ignored this advice. As a result, governments have allowed hundreds of—perhaps more than a thousand—consumer products marketed as incorporating ENMs7 to be commercialized without any pre-market safety assessment.
According to Internet advertisements, ENMs are already being used in “nano-fertilizers.”8 Because governments do not regulate ENMs in fertilizers, they do not test these products, nor, of course, their product claims. Due to manufacturer confidentiality claims, determining the volume of ENMs in consumer and industrial products is very difficult, but for the five most widely used of more than 250 ENMs, one academic study estimated up to 40,000 tons a year are produced in the United States alone.9
Nano-sizing, in theory, should make fertilizer nutrients more available to nanoscale plant pores, and therefore result in greater nutrient use efficiency. However, the dosing of fertilizers and “biosolids”—water treatment residues used as fertilizer—with ENMs also chronically exposes soil microbes and microfauna, as well as the plants themselves, to levels of chemical reactivity that may be toxic. Among the factors that are believed to increase toxicity of ENMs over their macro-scale counterparts are “particle size, shape, crystal structure, surface area, surface chemistry and surface charge.”10 Nano-sizing, because of its exponentially greater surface-to-mass ratio, makes toxins more bioavailable and bioaccumulative in tissues that macro-scale materials cannot penetrate.
Here we review a small part of the rapidly growing scientific literature that raises questions about how ENMs might affect soil health and soil biodiversity in field trials and subsequently the commercial and chronic application of ENMs in agricultural soil. The questions concern not only the intentional use of ENMs in fertilizers, but the incidental presence of ENMs in “biosolids,” defined by the U.S. Environmental Protection Agency (EPA) as “treated residuals from wastewater treatment that can be used beneficially.”11 Biosolids are often used to fertilize agricultural fields. As a Purdue University researcher recently noted, “Land application of biosolids is standard procedure now [at least in the United States] . . . If any of that [biosolid] contains nanotubes, that could be a problem.”12
That problem has many dimensions. U.S. regulators are only beginning to propose nano-specific occupational safety rules to protect workers, such as a new draft rule that will cover carbon nanotubes,13 but it is not clear if this rule would protect farmers and farmworkers applying nanotubes in biosolids. The farm workers who apply the biosolids with carbon nanotubes (CNTs), for example, might be, over time, at risk of the afflictions of laboratory rats’ lungs exposed to CNTs: “inflammation, fibrosis, and toxicological changes in the lung. When the [CNTs] are applied to skin cells, biochemicals that indicate cellular damage increase.”14
There is no informed, broad-based constituency to support regulating ENMs in fertilizers and biosolids to protect soil health and soil biodiversity. A first step toward the eventual regulation of ENMs in soil could be a series of participatory technology assessments that would bring together farmers, soil micro-biologists, fertilizer manufacturers, ENM manufacturers, biological engineers and interested civil society representatives. Such technology assessments would allow the layperson, informed by science, to raise questions about ENMs and nano-enabled products that should be asked prior to commercialization, and indeed, prior to technology investment, particularly with public funds. A hybrid of expert and layperson technology assessment could draw on some of the methodology of the Expert and Citizen Assessment of Science and Technology that fed into the Convention on Biological Diversity proceedings.15 However, the relatively smaller topical focus of nano-fertilizers would be conducive to mixing and matching different knowledge bases among participants. This process would also consider the broader natural resource and social context of the use of a technology.
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