Nitrate levels in central Iowa’s Raccoon River trended lower during the 15 years ending in 2014, despite increased corn plantings and higher fertilizer applications in the river’s watershed during that period, according to a newly released case study.

The study, by the Iowa Soybean Association (ISA), also highlights the fact that weather variations, not fertilizer applications, are the primary factor causing fluctuations of nitrate levels in the river.

The scientific study, released in early December during Iowa State University’s 2015 Integrated Crop Management Conference, shows that unpredictable weather and natural levels of nitrogen in Iowa’s rich soils act as the primary drivers of nitrate levels in water in the Raccoon River, said Roger Wolf, ISA’s director of environmental programs. "This study gives us data we need to help farmers address the issue of nutrient loss through the Iowa Nutrient Reduction Strategy," he said.

Dave Miller, Iowa Farm Bur­eau Federation (IFBF) director of research and commodity services, said the new ISA study builds on the findings of earlier studies that also showed nitrate levels are trending lower in the Raccoon River despite additional acres of corn in the watershed. The new study also backs earlier research showing that weather variables are the greatest determinants in the variation in nitrate levels in the Raccoon River, he said.

Complex system

"This study further confirms the complex nature of crop landscapes and the interactions with cropping systems, nutrient management and nitrates in river water," Miller said.

The ISA study also points to the value of the Iowa Nutrient Reduction Strategy, which provides farmers with research-backed information on a range of practices designed to reduce nutrient loss from fields and improve water quality, both Miller and Wolf said.

The ISA study used data from water samples collected from several sites in the Raccoon watershed over the 15 years from 1999 through 2014. It contrasts sharply with comments voiced by officials of the Des Moines Water Works (DMWW), which is suing three counties in northwest Iowa over the nitrate levels in the Raccoon River, its primary water source.

Bill Stowe, CEO of the water supplier, and other DMWW leaders have argued that farming practices, not weather, are the primary factor in nitrate levels. Stowe has criticized fertilizer rates used by farmers, as well as the use of drainage and other agronomic practices.

Extensive data

The ISA study measured more than 3,000 water samples that were collected in sites around the watershed through Agriculture’s Clean Water Alliance, a group of central Iowa ag suppliers working to monitor and improve water quality. In addition, some water samples near the mouth of the river were collected and analyzed by DMWW. The ISA study collected weather data from four weather stations and agronomic information, such as tillage, fertilizer and manure rates, from ISA’s On-Farm Network.

The researchers, Wolf said, launched the case study with the hypothesis that higher corn acreage in the Raccoon Valley, and increased fertilizer applications, would increase nitrate levels in the river water. "But the nitrate levels actually trended lower, so more corn did not translate into more nitrate in the water," he said.

Specifically, the ISA study showed decreasing nitrate trends from 39 of the 41 collection sites, despite one year of exceptionally high concentrations in 2013, when heavy spring rains followed a severe drought in 2012. The study also highlighted how nitrate levels are heavily dependent on weather, because nitrate levels were the lowest in the drought year of 2012 and the highest in 2013, when unusually heavy rains flushed unused fertilizer from the soil.

"It shows you could spend a lot of time trying to adjust fertilizer levels and end up pretty frustrated," Wolf said. "It’s a very complex system that depends a lot on the weather."

Farming practices helped

In addition, the ISA study showed improved farming practices may be helping to reduce nitrate levels in the Raccoon River by increasing crop efficiency in using nitrogen to produce grain. "Management has become increasingly sophisticated in recent years, and practices related to cover crops, tillage, weed and pest control, drainage and integrated nutrient use may be increasing nitrogen use efficiency and improving water quality," the study’s authors said.

While farmers have always incrementally improved manage­ment, the Iowa Nutrient Reduction Strategy has helped to put a strong focus on practices to improve water quality, Miller noted. "The Iowa Nutrient Reduction Strategy is designed to utilize a wide range of management approaches as farmers take steps to reduce the environmental impacts of crop production on the landscape and is consistent with the data that shows that just reducing corn acres or nitrogen fertilizer applications are not apt to address the issue," he said.

Value of Iowa strategy

Wolf said the results of the new ISA study point to the value of many key practices recommended by the state’s nutrient reduction plan, developed by the Iowa Department of Agriculture and Land Stewardship and the Iowa Department of Natural Resources with technical assistance from Iowa State University (ISU).

The study points to cover crops as a key solution because they are growing in the early spring and late fall, when fields would otherwise be bare, Wolf said. "We need them to manage the shoulders of the growing season when the crops are not there," he said.

ISU research has shown that nitrate losses typically occur in the early spring and late fall, when microbes help to unlock a portion of that natural nitrogen but there are no crops on the fields to take it up. That mismatch of timing of when the nutrient is naturally produced and when the crops roots can take it up often leads to the nitrogen loss into the water, according to ISU soil scientist Michael Castellano.

Going forward, nitrate levels will continue to be strongly tied to precipitation and river discharge, the ISA study said. "This weather-driven variability makes assessing progress toward water quality goals difficult," it said.