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Humans have been making some pretty frightening messes for a long time. I dealing with those messes, nature has been an integral part of our history of what we now call waste management. There have always been plants (or other organisms) who colonize, and in some cases thrive, in our industrial, commercial and residential debris fields.
Urban brownfields, mining operations, and even the neighborhood dry cleaners have their own set of signature contaminates, and their own set of clean-up challenges. Toxic compounds in the soil can potentially contaminate groundwater, surface water or end up as dust, traveling far beyond the origination site. These contaminates can be taken up by food plants; ending up in our food system, entering our lungs, or wind up fouling our drinking water.
Mitigation can often be almost as destructive as the contamination itself. Some methods either move or contain the problem, without addressing the actual existence of the toxic compound. Removing contaminated soil often involves massive land disturbance and transportation costs, coupled with disposal costs – either incinerators or landfills. Some areas may have limited accessibility due to remoteness or topography. Various containment, encapsulation and stabilization may be effective, but may also limit the potential use of the land. Soil-washing is a viable solution in some instances, but like many other forms of soil cleaning, can be unrealistic on a large scale.
In many cases, bioremediation – the conscious effort to get plants, bacteria or fungi to remove or alter the threat – without them becoming a mess in their own right, is a viable strategy.
There has been a lot of fascinating and impactful innovations and discoveries in bioremediation – microbes are used to clean up oil spills, a yeast has been discovered that absorbs metal pollutants.
In agriculture, one of the most promising is the use cover crops
Some of us may not think of soil nutrients as toxic contaminants, but in the wrong place at the wrong levels , they can be. While NPK – nitrogen, phosphorus and potassium as well as other nutrients are necessary for plant growth, they can be problematic when the leach off farm and concentrate elsewhere. Cover crops can do a great job capturing these nutrients, keeping them on the farm, and out of waterways.
Plants can be a great tool for contamination clean-up. While their activity is limited to the root zone (hydrocarbon loving plants won’t address leaking underground fuel storage tanks at old gas stations), they are relatively inexpensive, benign, and efficient. While farmers have been using cover crops to scavenge excess fertilizer nutrients to increase efficiency and prevent them from becoming pollutants, there are more ambitious applications for cover crops in the works.
There are a few basic mechanisms a plant can use to clean up toxic compounds. They can stabilize contaminants within their root systems or within the rhizosphere – the microbial community associated with the root system. Plants can take up contaminants and convert them to a gas, releasing them to the atmosphere. Plants can take up contaminants and store them in their leaves, which can then be clipped and removed. Or, in a process known as phytodegradation, a plant can take up a contaminant into its root system; actually metabolizing the contaminant. In this case, scientists are left with the challenge of developing a plant with the agronomic properties to make it viable, while introducing the metabolic process for cleanup. Researchers at the University of Washington have done just that, by incorporating genes to metabolize RDX – a toxic compound found in munitions sites;- into Switchgrass and Creeping Bentgrass, plants viewed favorably by both graziers and wildlife managers.
UW engineers introduced two genes from bacteria that learned to eat RDX and break it down into harmless components in two perennial grass species: switchgrass (Panicum virgatum) and creeping bentgrass (Agrostis stolonifera). The best-performing strains removed all the RDX from a simulated soil in which they were grown within less than two weeks, and they retained none of the toxic chemical in their leaves or stems.
While not a perfect solution, the use of genetically engineered native plants, in conjunction with other bioremediation techniques, can provide a low-cost, low risk, low impact approach to a common and challenging waste-management problem. Being able to transform a contaminated area cheaply and easily into one that can be profitably grazed could be a great step in the right direction.
Reference sites
http://onlinelibrary.wiley.com/doi/10.1111/pbi.12661/full – University of Washington study
http://www.academia.edu/211973/Management_Practices_and_Phytoremediation_by_Native_Grasses
http://infohouse.p2ric.org/ref/07/06189/ – soil-washing
http://www.nature.com/news/2004/040129/full/news040126-10.html – land mine biodetector
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