Weather whiplash

Weather whiplash

cover crops near Nevada
Nice looking rye cover crop near Nevada, IA
November 2020

A big thank you to farmers who planted cover crops after this challenging year. Cover crops will hold soil and nutrients in place through the winter and early spring. That could be especially important this year.

After a drought, nitrogen that might normally have been taken up by a high-yielding crop or flushed away by rainfall remains in the soil. That leftover nitrogen could be available for next year’s crop, but only if:

A) we have a dry spring, or

B) farmers have made use of practices like cover crops or nitrification inhibitors that prevent nitrogen losses during the fallow season.

A wet year in 2013 following a dry year in 2012 (dark blue) caused nitrate concentrations in many central iowa streams to jump as much as 10 mg/L higher than usual (red). Figure from Van Metre et al. 2008

A drought in 2012 following by a wet spring in 2013 led to nitrate concentrations in excess of 20 mg/L in many rivers in Central Iowa. If we have a wet spring in 2021, we could see this happen again. As one scientific paper put it, “weather whiplash drives deterioration of water quality.”

“Weather whiplash” can also help explain the long-term trends I’ve been seeing in the South Skunk River and its tributaries: a decline in nitrate concentrations from 2005-2012, a big jump in 2013, and another decline over the past 7 years. I’ll walk you through my analysis.

Explaining nitrate concentrations in the South Skunk River

Technical details, feel free to skip: This data was collected by the City of Ames just upstream of wastewater treatment plant. The City has monitored the South Skunk River above and below its wastewater treatment plant almost every week since 2003! Flow is measured continuously at a few miles upstream USGS gage near S. 16th St. I’ve summarized nitrate concentrations and streamflow by season (Jan-Mar, Apr-Jun, Jul-Sep, Oct-Dec). At each step of the way, I apply a linear regression equation and graph the model residuals. Taken together, these three factors explain 59% of the variation. The effects of “weather whiplash” may extend beyond one year, since nitrate from some parts of the field may travel more slowly to streams via groundwater.

In a given quarter, nitrate concentrations in the South Skunk can be up to 10 mg/L higher than the long-term average, or as much as 10 mg/L lower. The following graphs show how much variation is left to explain after correcting for current weather, last year’s weather, and season.

Nitrate in the South Skunk is again declining after a sudden increase in 2013.
Low-flow conditions in 2011-2013 explain unusually low nitrate concentrations.
Wet springs following dry years explain unusually high nitrate concentrations.
Seasonal patterns explain some of the remaining variation.
  1. The lowest nitrate concentrations can be explained by streamflow: when the weather is dry and tiles aren’t flowing, nitrate levels in rivers taper off to the background levels seen in groundwater.
  2. The highest nitrate concentrations can be explained by weather over the previous 12 months: a wet period following a dry period will flush out nitrate that’s accumulated in the soil.
  3. After that, there’s still a seasonal pattern independent of rainfall: nitrogen is most susceptible to loss in spring when soils are bare and microbial activity picks up (April-June) and least susceptible when the maturing crop is using up the available nitrogen (July-Sept).
  4. Can some of the remaining pattern be explained by greater adoption of conservation practices in the watershed in the past 5 years? We hope so, but let’s see what happens next spring!

Low hanging fruit?

Nitrogen rate management (MRTN) is the low-hanging fruit of the Iowa Nutrient Reduction Strategy, a win-win for profitability and the environment.  On closer inspection, that fruit is even juicier than we thought; but harder to reach.


Here’s the paradox of nutrient management that the general public fails to grasp.  We don’t know with any certainty at application time how much nitrogen the corn crop will need or how much nitrogen will be left in the soil come July when the crop starts maturing.  Corn stalk nitrogen tests and split applications can improve the accuracy of the guess, but farmers still have to guess.  If they guess too low, they lose income.  So most farmers err on the high side, which means that (all else being equal) more nitrogen will end up in our streams.

Figure by John Sawyer at ISU.  The economically optimum nitrogen rate varies by year, even on the same field.

We may not know what’s the right amount of nitrogen to apply this year, but we can pinpoint a range that makes the most economic sense across sites and years.  In 2005, Iowa State University researchers crunched the numbers and developed an online calculator.  A farmer can enter current prices for corn and fertilizer to get the Maximum Return To Nitrogen (MRTN).  Above that range, they might get a bit higher yield, but the revenue from those extra bushels doesn’t offset the cost of the extra nitrogen.  Applying nitrogen at the MRTN is a rare win-win for profitability and the environment.

Along with cover crops, wetlands, and bioreactors, MRTN was one of the more promising practices for nitrogen outlined in the Iowa Nutrient Reduction Strategy.  Still, the authors cautioned that it wouldn’t get us very far by itself.  An average field would cut nitrate losses by 10 percent.  Since some parts of the state apply less nitrogen than others, universal adoption of this practice would get us a 9 percent reduction in nitrate concentrations.

That modest reduction assumes Iowa farmers currently apply 150 lbs of N/acre on a corn-bean rotation and 190 lbs/acre on continuous corn, a figure the authors admitted were “possibly outdated.”  A 2017 survey by ag retailers found that Iowa farmers apply 169 lbs/acre of nitrogen on corn after beans and apply 210 lbs N/acre on continuous corn.  I updated Dr. Helmers graph with those numbers and an MRTN based on current corn and nitrogen prices to determine the potential water quality benefit.  This low-hanging fruit is juicier than we thought!

And that’s just the average!  Inspired by a recent essay by Chris Jones, I read a manure management plan for a field in one of our watersheds.  It receives 190 lbs/acre of nitrogen and 146 lbs/acre of phosphorus in the form of liquid swine manure. If that manure could be spread over more acres at a rate of 140 lbs/acre, that could reduce nitrate losses by 31%.  If it replaces rather than supplements commercial fertilizer, we could get another 4% cut in nitrate.  Manure is a slow-release fertilizer and less susceptible to leaching.

Is that economical?  Hard to say.  Daniel Anderson reports that liquid swine manure can be moved seven miles and still be cheaper than synthetic fertilizer.  In Story and Boone County that might work.  Hamilton County has 218 manure management plans, so I’m not sure how far you’d need to travel to find a field not already being treated.  Are there changes in processing or additional cost share that would help make it more feasible?  I don’t know.  No-one is talking about it.  But cover crops work equally well.  Winter rye grows thicker with fall-applied manure and can scavenge nitrogen that would otherwise be lost.

Unfortunately, neither of those options (MRTN or cover crops) are even suggested as part of manure management plans and the loudest voices in the room are saying livestock producers don’t need to be doing anything differently.  Until that situation changes, the widespread adoption of rate management that’s assumed in statewide and watershed-based scenarios won’t happen and we will fail to meet our nutrient reduction goals.