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Measuring nitrates during high water times

By Staff | Apr 29, 2015

-Farm News photo by Larry Kershner EMILY ZIMMERMAN, an Iowa State University graduate student, inserts a new lysimeter into one of the 36 units of the nitrate plot. The lysimeters are used to test the nitrates remaining in soil after heavy rain events.



AMES – A former pollinator study plot is being repurposed to determine which plants in a buffer strip are best in retaining nitrates following heavy rain events.

Emily Zimmerman, an Iowa State University graduate student who is leading the study, said with the increased attention to nitrate loads that reach Iowa’s surface waters from farm fields, the plot was necessary to determine the plants best suited to retain the most nitrate amounts in the soil.

Nitrate, which occurs naturally in soil, is essential for plant survival.

Emily Zimmerman

Farmers add nitrate-ladened fertilizer to replace what crops use. But the nitrogen they add, which is not harmful to humans, is transformed into nitrates as it sits in soil.

Unlike another crop nutrient- phosphorus -nitrate is not locked in the soil. So during heavy rains, nitrates not used by plants or present before plants can get to it, leach from the soil and into surface water.

In the human body, nitrate is converted into nitrite, which is toxic in the digestive systems of infants and livestock.

Zimmerman said the ISU plot, divided into 36 2-by-2-meter units, are planted with nine treatments – willow trees, alfalfa, switchgrass, corn and a variety of native prairie grasses.

Among the prairie grasses are mixtures of varieties, from two to as many as 14 grasses and forbes.

These include spotted geranium, pale purple coneflower, blackeyed Susan, smooth oxeye, purple prairie clover, Culver’s root, Canada wildrye, big and little bluestem, Indian grass, rough dropseed, prairie sage, meadow aziza, swamp milkweed and stiff goldenrod.

All of these plots are typical of the vegetation used in field buffer strips, Zimmerman said, and each treatment is replicated four times.

Buffer strips are planted between a field edge and a body of water. They are designed to catch runoff water and prevent erosion into surface waters.

Zimmerman said the plan is to simulate a 1-inch rain event, followed immediately with a runoff event, during the fourth week of each month. These will be done monthly from April through October.

Following the water events, water samples will be taken three times between rain events using lysimeters. Normally associated with measuring plants’ evapotranspiration rates, these will be sampled to determine how much nitrate remains in the units, Zimmerman said.

The amount of runoff is based on the amount of precipitation received between simulations, Zimmerman said. However, she said, “The 1-inch simulations will remain consistent.”

Nitrate will be added to the water simulations at a rate of 7 miligrams per liter.

“No other fertilizer will be added to the plots,” she said.

Plant diversity will also be recorded throughout the experiment, and each plot will be hand weeded to ensure that the diversity in each plot is maintained.

“We’ll also do water readings to determine plant use,” Zimmerman said, “and to see if it makes a difference in nitrates retained, or lost depending on how you look at it.”

The plot was used by ISU researcher Kelly Gill from 2010 to 2012 as a pollinator and beneficial insect study.

The nitrate plot will be conducted during the 2015 and 2016 growing seasons, Zimmerman said.

The nitrate plot is being supervised by Dr. Lisa Schulte Moore, an ISU associate professor in natural resource ecology and management.

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