Nutrient enriched agricultural runoff can be severely detrimental to surface water quality, aquatic life and human health. Localized surface water will eventually reach and discharge into larger water bodies, meaning changes in water quality in a stream will affect the watershed which will eventually affect other watersheds. For instance; nitrates from cultivating soybean and corn crops in the midwest can end up in Gulf of Mexico where they will cause oxygen-depleting algal blooms which can cause dead-zones in marine life, fish kills affecting the ecosystem as well as the economy. Planting and maintaining strips of native plants and grasses in between nutrient contributing landscapes and water bodies can protect the watershed from the adverse effects of excess nutrient inputs.
Plants will uptake nutrients from agricultural runoff effectively reducing the amount that gets inadvertently added to water bodies and watersheds. These areas are termed “perennial filter strips” and function to filter nitrates before they can travel to water in 3 ways: 1)nitrates may get sequestered in soils, 2)they may be uptaken by the plants or 3)released into the atmosphere through a process called “denitrification”: reduction of nitrates or nitrites commonly by bacteria (as in soil) that usually results in the escape of nitrogen into the air. A recent study led by David Mitchell of Iowa State University and published in the Journal of Environmental Quality sought to find out which of these processes was most effective and prevalent in filter strips. Mr. Mitchell and his team of scientists compared the relative contributions of denitrification, organic matter and plants towards removing nitrates from agricultural runoff.
The difference in efficacy among these processes is important to our understanding of how and where to plant filter strips and their maintenance requirements. As Mitchell explains, “If plant biomass and soil organic matter are the major nitrate sinks, filter strips may decrease watershed nitrate losses only in the short-term [as these reservoirs would become saturated and unable to continue absorption]. In contrast, if denitrification is the major nitrate sink, filter strips are expected to decrease watershed nitrate losses indefinitely.” Mitchell’s team used land in the Neal Smith National Wildlife Refuge in Iowa as a research site which is located in between a farmed area and the drainage which ultimately flows into a stream.
They conducted a tracer test of sorts, introducing a known amount of nitrates (15N-nitrates: a rare, yet stable, and traceable isotope) into soils of the filter strip. After 137 days they were able to determine whether the 15N-nitrates were denitrified, had been assimilated into plant biomass or is still present in soils. The results indicated that between 3 and 10% of nitrates were recovered from soil organic matter. From 4 to 20% of the 15N-nitrates were found in the plants growing in the filter strip, researchers could not account for 70 to 90% of the nitrates. The assumed method of nitrate removal is denitrification, this releases nitrogen gas or nitrous oxide which is very hard to quantify in the field. Since this process is basically untraceable Mitchell’s research team performed denitrification enzyme assays which measure the potential for the process in soil. The assays were also performed on soils from row crops on agricultural land for comparison.
The results from assays revealed that filter strip soils had higher potential for denitrification to occur as compared to cultivated soils. Apparently filter strips have higher concentrations of dissolved organic carbon which can lead to the increased microbial activity that facilitates denitrification. And while nitrous oxide is an important greenhouse gas, filter strips have greater potential to produce nitrogen gas from nitrates as compared to cultivated soils. With denitrification established as the primary mechanism for nitrate removal in filter strips an infinite nitrate sink is rooted (no pun intended) therein.
Of course more research needs to be conducted across a variety of sites and differing conditions, however, filter strips are an exciting, effective and sustainable solution (or part thereof) of protecting water quality locally and worldwide.
