Monday, June 6, 2016

Iowa’s Raccoon River: nitrate trends and implications for Minnesota farmers



A research paper about nitrate trends in the Raccoon River in west-central Iowa has been getting a lot of attention recently, as the Raccoon River is the subject of the Des Moines Water Works lawsuit that every Midwestern farmer knows about.  A copy of the article was forwarded to me, and I think there is some interesting information on nitrogen management that came out of it.

The researchers looked at nitrate concentrations flowing through the Raccoon River Watershed to Des Moines from 1999 to 2014.  Here’s some of the key information from their study: 

- 75% of the sites they sampled within the watershed had average nitrate concentrations above 10 mg/L (the drinking water standard) during the months of April to July (the months where leaching and erosion are typically highest from farm fields).  However, it doesn’t look like nitrate levels in the river have been increasing at all from 1999 to 2014.

-The biggest factor that affected nitrate concentrations and total amounts of nitrate in the water from year to year was rainfall.  Because of large changes in precipitation from year to year (ranging from 22 to 46 inches), there was a lot of variability in nitrate losses.  On average, less than one pound of nitrogen was lost to the river per acre of farmland in the best year (2000, a dry year), but an average of 28 lbs of nitrogen was lost per acre in the very wet year of 2013.

- Nitrate concentrations were very high in years following major droughts (including after the 2012 drought).  In the case of 2013, there was a very wet spring with a lot of losses, and there had already been a lot of residual nitrogen in the soil since the corn didn’t take much up in 2012 and it didn’t leach through the soil.  Sampling for residual nitrates after a drought year is really important to help prevent over-applying fertilizer.

-In the Raccoon River watershed, average nitrogen rates on corn after soybean were 159 lbs, or 195 lbs if it came from manure; average nitrogen rates on corn following corn were 189 lbs, or 238 lbs from manure.  This average was above the Iowa State-recommended range for corn following soybean, and within the recommended range for corn following corn.

-Average leaching losses of nitrogen were around 10% of the fertilizer nitrogen applied plus nitrogen fixed by soybeans.  This is pretty typical for the Upper Midwest; in Minnesota, leaching losses typically range from 0-20% of applied fertilizer nitrogen, depending on soil type, rate, and weather.


The researchers tried to understand why nitrate wasn’t increasing in the Raccoon River.  Corn is often implicated as being the biggest source of nitrates from farm fields, but corn acres increased 19% over the 15 years they studied, while nitrate levels did not increase at all.  There were two other notable trends that may be important- total cultivated acres decreased by 2.5%, and soybean acres decreased by 24%.  They thought the decline in soybean acreage was preventing any increase in nitrate leaching from increased corn acres.  Here’s their reasoning:

-Carbon-to-nitrogen ratios are much lower for soybean residue than for corn residue.  As a result, corn residue ties up quite a bit of nitrogen as it breaks down, while soybean residue does not (this is why we can grow corn after soybeans on typically 40 lbs less nitrogen than corn after corn without affecting yield potential).  

-They’ve seen less denitrification occurring in or after soybean years than in continuous corn, making leaching a larger potential loss in corn-soybean rotations.

-There is a lot more tile flow under soybeans that under corn, because corn transpires a lot more water than soybeans do.  We’ve seen this happen consistently in Minnesota as well (you can see some results in this publication), although in Minnesota, nitrogen losses from corn-soybean systems were still lower than continuous corn.

Another possibility is that, on average, nitrogen is being applied in excess to corn after soybean.  If nitrogen efficiency under current practices is higher in continuous corn production, the increase in continuous corn at the expense of corn-soybean rotations probably wouldn’t increase Raccoon River nitrates.  Average rates on corn after soybean were quite a bit higher than recommended, while the average rate on continuous corn was well within the recommended rates, which supports this interpretation. Regardless, it’s nice to see some hard data on the state of water in the Raccoon River.

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