Understanding the fate of nitrate in perennial filter strips

Runoff from agricultural fields contains nutrients that can adversely affect surface water quality, human health, and aquatic life. Nitrates from corn and soybean croplands across the Midwestern United States, for example, can end up in the Gulf of Mexico. There, nitrates can cause algal blooms which deplete oxygen from Gulf waters. Currently, such blooms cause a “dead zone” devoid of marine life across an area as large as Connecticut.

But perennial filter strips – areas of native plants or grasses – can help. Filter strips can reduce the amount of nutrients, including nitrates, entering water sources and ultimately ending up in the Gulf. There are three ways filter strips can remove nitrates from agricultural runoff. The nitrates may be sequestered in soil organic matter or taken up by plants. Nitrates may also be released into the atmosphere as nitrogen by microbes through a process called denitrification.

landscape view of agricultural fields with perennial grass

Which of these processes is most prevalent in filter strips was a question that David Mitchell, lead author of a new study, wanted to answer. He, and fellow researchers at Iowa State University, compared the relative contributions of denitrification, organic matter, and plants towards removing nitrates from agricultural runoff. Their study was recently published online in the Journal of Environmental Quality.

 It is important to understand how filter strips reduce the amount of nitrates reaching streams. “If plant biomass and soil organic matter are the major nitrate sinks, filter strips may decrease watershed nitrate losses only in the short term,” Mitchell explains. Eventually these reservoirs would fill up, and filter strips would no longer remove nitrates effectively. “In contrast, if denitrification is the major nitrate sink, filter strips are expected to decrease watershed nitrate losses indefinitely.”

Mitchell and colleagues conducted their study in the Neal Smith National Wildlife Refuge in Iowa. At the study site the filter strip is located between a farmed area and the drainage from the watershed. The drainage ultimately flows into a stream.

The researchers introduced a known amount of nitrates into filter strip soils. They used a rare, stable isotope of nitrogen called 15N so they could trace it. After 137 days they determined whether the 15N-nitrates had ended up in plant biomass or organic matter or were removed from the soil, presumably by denitrification.

Between 3 and 10% of the 15N-nitrates were recovered in soil organic matter. Between 4 and 20% in plants growing within the filter strip. That left 70 to 92% of the 15N-nitrates unaccounted for.

Because denitrification results in loss of nitrates from the soil as either nitrous oxide or nitrogen gas, it’s not easy to trace. Instead, Mitchell and colleagues carried out denitrification enzyme assays. These assays measure the potential for denitrification in filter strip or row crop soils.

They found that filter strip soils had a significantly higher potential for denitrification compared to cultivated soils. “This difference supports our idea that there is more denitrification happening in filter strip soils compared to row crop soils,” says Mitchell.

It turns out filter strip soils contain increased levels of dissolved organic carbon. This is likely because “in general prairie soils have increased below-ground productivity compared to row-crop soils,” says Mitchell. Higher quantities of dissolved organic carbon – with other factors like temperature and moisture being equal – can lead to increased activity of the microbes that facilitate denitrification.

One concern with denitrification is the release of nitrous oxide, which is a potent greenhouse gas. However, recent research indicates filter strip soils have a significantly greater potential to convert nitrates into nitrogen instead of nitrous oxide when compared to cultivated soils.

Mitchell and colleagues were able to establish denitrification as the leading process by which filter strips remove nitrates from agricultural runoff at their experimental site. Unlike soil organic matter or plants, denitrification is an indefinite nitrate sink. Nitrates will continue being removed through this process as long as the amount of nitrates entering the filter strip is not more than its denitrification potential. Filter strips could, therefore, be expected to remove nitrates from cropland runoff over extended time spans.

According to Michael Castellano, an assistant professor at Iowa State University and an author on the study, there are several ideas for future work. For example, researchers want to evaluate the effectiveness of the filter strips over a longer time period and in croplands across Iowa.

 In addition to decreasing nitrate losses into streams filter strips can provide a number of other benefits. These include reducing the amount of erosion sediments that reach surface waters and providing wildlife habitat. “While there are a lot of factors in play, it does look like filter strips could be a promising management practice,” says Mitchell.