Nitrate levels in central Iowa’s Raccoon River have trended lower in the past 15 years despite a significant increase in corn acres in the watershed during that period, according to a new study released last week.
The water quality study, conducted by the Iowa Soybean Association (ISA) and presented at the ISA On-Farm Network conference in Ames, looked at some 10,000 water samples captured in the Raccoon River watershed. It also collected information on actual applications of commercial fertilizer and manure on 468 fields gathered by ISA’s On-Farm network. The results showed that, on average, the nitrate levels in the Raccoon River have trended down 0.28 milligrams per liter per year since the data set began in 1999.
Contrast to modeling
The new study stands in contrast to others that have concluded that increased corn acres driven by biofuel demand have led to higher nitrogen delivery to the Raccoon, along with other Iowa rivers and streams, said the study’s leader Chris Jones, an environmental scientist for ISA.
Because farmers applied more nitrogen to raise corn, many studies used models to conclude that more of the nutrient would automatically end up in the river. The ISA study, in contrast, used actual data on nitrogen use, not simply models or sales data.
"We think that the studies using models need to be taken with a grain of salt because they don’t have the bedrock of data to support their analysis," Jones said. "Good data is really the key."
The nitrate levels in the Raccoon River have been a flash point over the years because the river is a main water source for Des Moines and surrounding communities. In addition, a spike in nitrate levels in 2013 was caused by an extraordinarily wet spring following drought conditions in 2012. That spike led Des Moines Water Works officials and others to point to the increased corn acreage in the Raccoon watershed in calling for mandatory controls on applications of nitrogen and other nutrients.
Unusual 2013 conditions
The unusual conditions in 2013 created an impression that nitrogen levels in the Raccoon are rising, but the new data actually show a long-term decline even with added corn acres, Jones said.
There are several reasons that increased corn acreage in the watershed may actually be responsible for the downward trend of nitrate levels in the river, he noted.
First, as farmers plant more acres to corn and fewer to soybeans, fewer acres of soybean stubble are being tilled into the soil during the fall. That tilling tends to lead to a rapid mineralization of organic nitrogen in the soil, which releases nitrate that is susceptible to loss, Jones said.
A second reason is that farmers using a corn-on-corn rotation often have a better idea of optimal nitrogen targets each season, Jones said.
In contrast, farmers using a corn-soybean rotation find it more difficult to target nitrogen rates because the amount fixed by soybeans can vary widely and is hard to predict.
A third reason, Jones said, is that more water typically flows in tiles under soybean fields than under corn fields, which have a higher evaporative transfer.
"We know that tile flow is the primary delivery mechanism for the nitrate, so it stands to reason that if we decrease tile flow we are going to decrease the export of nitrogen," he said.
Going forward, ISA hopes to recruit more farmers into its study of nitrate levels in the Raccoon River watershed to further improve the data, Jones said. It also plans to do more small-scale studies to evaluate practices, such as cover crops, strip tillage and bioreactors, which can help farmers trim nitrogen loss and further reduce nitrate levels in the Raccoon and other Iowa rivers, he said.
"We still have a lot of questions that are important to answer as we continue to develop more effective methods to improve water quality and reduce nutrient concentration in the river even further," Jones said.
A separate study published in the Journal of Environmental Quality in 2012 also found that there is no statistically significant, observed increasing trend in nitrogen concentrations in the Raccoon River from 1992 to 2008.
However, it showed there are substantial significant intra-annual variations in nitrogen concentrations and the existence of a very strong seasonal pattern.
Overall, the data in the study support the conclusion that this is a very stable biological system over multiple decades. Variations in rainfall and temperature contribute more to the monthly variation in nitrogen concentration than do the changes in nitrogen application rates.
The results indicate that timing of nitrogen fertilizer application has a significant explanatory role in determining monthly levels of nitrogen concentration in the river, but that rainfall and temperature patterns are even more significant determinants of month-to-month variability.