A Few Heavy Storms Cause a Big Chunk of Nitrogen Pollution From Midwest Farms

T. C. Knight / Getty Images
By Chaoqun Lu
Some effects of extreme weather are visible – like half a million acres of flattened corn in Iowa left behind after a derecho that hit the Midwestern United States on Aug. 10.
Other effects are harder to measure, but can be just as harmful. One example is agricultural nitrogen runoff from farmlands in the Mississippi River Basin. It mainly comes from fertilizer that farmers apply to millions of acres of crops.
Plants can't use all of the nitrogen in fertilizer because fertilizers are usually applied in excess. This excess can wash off farm fields into local rivers and lakes, degrading water quality and stimulating algae blooms. Traveling down the Mississippi River, it contributes to the yearly formation of a dead zone in the northern Gulf of Mexico, covering several thousand square miles; oxygen levels there are so low that fish and shellfish cannot survive.
The Mississippi River Basin covers over 1.245 million square miles across 32 U.S. states and two Canadian provinces. Shannon1 / Wikipedia, CC BY-SA 4.0
Excess nitrogen in drinking water also threatens public health. Ingesting high levels of nitrate, a nitrogen compound, can reduce red blood cells' ability to transport oxygen, a condition that is especially dangerous for infants.
My work as a quantitative ecologist examines how ecosystems respond to external factors such as adding nitrogen. In a recently published study, I worked with colleagues to quantify nitrogen runoff from land into rivers and streams. We found that infrequent but heavy rainfall events account for one-third of annual total runoff and nitrogen leaching from soils across the Mississippi Basin. This tells us that managing nitrogen is likely to be more challenging if climate change continues to make heavy rains more frequent.
Too Much of a Good Thing
Plants can't grow without nitrogen, but using too much or applying it improperly can cause problems. In the U.S. Midwest, one of the most intensively farmed areas in the world, farmers have added large amounts of synthetic nitrogen fertilizer to the land to boost crop yields.
Long-term monitoring data from river gauges shows large year-to-year variations in the quantity of nitrogen that flows down from the Mississippi River Basin into the Gulf of Mexico. Yearly changes in farmers' fertilizer use are not large enough to explain these fluctuations.
Studies show that annual total precipitation is a significant factor in these changes. But we know less about the role of daily rainfall – particularly heavy rains – in mobilizing and transporting nitrogen.
Stream gauge measurements show that the amount of dissolved inorganic nitrogen (DIN) moving from Mississippi River Basin states to the Gulf of Mexico fluctuates dramatically from year to year. Heavy rainfalls can produce higher nitrogen levels. Modified from Lu et al., 2020, CC BY-ND
Heavy Rains Have an Outsized Impact
My collaborators and I wanted to assess the impacts of extreme rainfall events in the Midwest. In this region, many cropped fields are laced with buried networks of drainage channels, known locally as tile drainage. These pipelines are designed to move excess moisture out of fields. But they can also channel large surges of water and nutrients into rivers and streams after heavy rainfalls.
It is challenging to determine how individual rainfall events affect nitrogen leaching and movement within a drainage basin. Rain happens here and there, so it's hard to distinguish a single storm's impact from river gauge monitoring data. Rainfall events also vary a lot by season and intensity.
Our study used a well-tested model to quantify how much nitrogen is washed out by each rainfall event, as well as total nitrogen delivered to the Gulf of Mexico. We looked closely at heavy rainfall events, which we defined as the top 10% of historical daily precipitation amounts for any location in a given month.
Climate records show that over the past 20 years, a growing share of annual precipitation has come in heavy rainfall events across two-thirds of the Mississippi River Basin's land area. The region that receives a total of more than 15.7 inches (400 millimeters) of heavy rain per year has expanded from areas in Louisiana and Arkansas northward to Corn Belt states like Illinois and Indiana, where nitrogen fertilizer is heavily used.
Percentage increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events) from 1958 to 2012. Globalchange.gov
We found that one-third of annual total runoff and nitrogen leaching loss come from heavy rainfall events, which happen on only about nine days per year on average across the basin. Nearly half to three-quarters of heavy rainfall in the basin occurs in spring and summer, with a monthly peak in May.
This timing coincides with the planting and seed germinating stages of corn, when the plants are using minimal amounts of nitrogen. We wondered whether changing when and how farmers apply fertilizer could reduce nitrogen runoff.
When to Fertilize
When during the year to apply fertilizer is a long-standing question in both precision agriculture and environmental science. Midwest farmers apply over 90% of nitrogen fertilizer before crops germinate in springtime and after harvesting. This means that a fair amount of available nitrogen accumulates in the soil before crops start taking it up. When heavy rainfalls occur, it is likely to be washed out.
Nitrogen fertilizer application restrictions on crop ground went into effect in certain areas of Minnesota on Sept.… https://t.co/2xW8POSKyy— DTN/The Progressive Farmer (@DTN/The Progressive Farmer)1603121761.0
We set up modeling experiments to test whether postponing fertilizer application could make the water running off of farmlands cleaner. In our alternative fertilizer management scenario, we assumed fertilizer was applied only twice, after crops developed. We expected this would reduce the amount of unused nitrogen accumulating in soils.
Our results predicted that this modification could reduce nitrogen loading to the Gulf of Mexico by up to 16%. This would be a significant step toward goals set by the U.S. Environmental Protection Agency, which is working with states to reduce nutrient loads entering the Gulf of Mexico by 20% by 2025 and 45% by 2035.
However, even under the postponed fertilizer application scenario, we still found the frequent heavy rains in the recent decade could enhance nitrogen loss during summer and early fall. Scientists predict that if climate change continues at its current rate, it will cause more extreme rainfall events in the Midwest, which, we think, would reduce environmental benefits from alternative nitrogen management practices.
Reducing the amount of nitrogen that escapes from land into water bodies while maintaining food production is a significant challenge. Our study complements the well-known 4R concept for managing nutrients: Using the right fertilizer product, at the right rate, at the right time, and in the right place. To get that timing right, our research shows that along with crop nitrogen demand, farmers should also consider the occurrence of heavy rainfall.
Chaoqun Lu is an assistant professor of Ecology, Evolution and Organismal Biology at Iowa State University.
Disclosure statement: Chaoqun Lu receives funding from the National Science Foundation, Iowa State University, and the Iowa Nutrient Research Center.
Reposted with permission from The Conversation.
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A Game of Jenga
<p>Think of it as a game of Jenga and the planet's climate system as the tower. For generations, we have been slowly removing blocks. But at some point, we will remove a pivotal block, such as the collapse of one of the major global ocean circulation systems, for example the Atlantic Meridional Overturning Circulation (AMOC), that will cause all or part of the global climate system to fall into a planetary emergency.</p><p>But worse still, it could cause runaway damage: Where the tipping points form a domino-like cascade, where breaching one triggers breaches of others, creating an unstoppable shift to a radically and swiftly changing climate.</p><p>One of the most concerning tipping points is mass methane release. Methane can be found in deep freeze storage within permafrost and at the bottom of the deepest oceans in the form of methane hydrates. But rising sea and air temperatures are beginning to thaw these stores of methane.</p><p>This would release a powerful greenhouse gas into the atmosphere, 30-times more potent than carbon dioxide as a global warming agent. This would drastically increase temperatures and rush us towards the breach of other tipping points.</p><p>This could include the acceleration of ice thaw on all three of the globe's large, land-based ice sheets – Greenland, West Antarctica and the Wilkes Basin in East Antarctica. The potential collapse of the West Antarctic ice sheet is seen as a key tipping point, as its loss could eventually <a href="https://science.sciencemag.org/content/324/5929/901" target="_blank">raise global sea levels by 3.3 meters</a> with important regional variations.</p><p>More than that, we would be on the irreversible path to full land-ice melt, causing sea levels to rise by up to 30 meters, roughly at the rate of two meters per century, or maybe faster. Just look at the raised beaches around the world, at the last high stand of global sea level, at the end of the Pleistocene period around 120,0000 years ago, to see the evidence of such a warm world, which was just 2°C warmer than the present day.</p>Cutting Off Circulation
<p>As well as devastating low-lying and coastal areas around the world, melting polar ice could set off another tipping point: a disablement to the AMOC.</p><p>This circulation system drives a northward flow of warm, salty water on the upper layers of the ocean from the tropics to the northeast Atlantic region, and a southward flow of cold water deep in the ocean.</p><p>The ocean conveyor belt has a major effect on the climate, seasonal cycles and temperature in western and northern Europe. It means the region is warmer than other areas of similar latitude.</p><p>But melting ice from the Greenland ice sheet could threaten the AMOC system. It would dilute the salty sea water in the north Atlantic, making the water lighter and less able or unable to sink. This would slow the engine that drives this ocean circulation.</p><p><a href="https://www.carbonbrief.org/atlantic-conveyor-belt-has-slowed-15-per-cent-since-mid-twentieth-century" target="_blank">Recent research</a> suggests the AMOC has already weakened by around 15% since the middle of the 20th century. If this continues, it could have a major impact on the climate of the northern hemisphere, but particularly Europe. It may even lead to the <a href="https://ore.exeter.ac.uk/repository/handle/10871/39731?show=full" target="_blank" rel="noopener noreferrer">cessation of arable farming</a> in the UK, for instance.</p><p>It may also reduce rainfall over the Amazon basin, impact the monsoon systems in Asia and, by bringing warm waters into the Southern Ocean, further destabilize ice in Antarctica and accelerate global sea level rise.</p>The Atlantic Meridional Overturning Circulation has a major effect on the climate. Praetorius (2018)
Is it Time to Declare a Climate Emergency?
<p>At what stage, and at what rise in global temperatures, will these tipping points be reached? No one is entirely sure. It may take centuries, millennia or it could be imminent.</p><p>But as COVID-19 taught us, we need to prepare for the expected. We were aware of the risk of a pandemic. We also knew that we were not sufficiently prepared. But we didn't act in a meaningful manner. Thankfully, we have been able to fast-track the production of vaccines to combat COVID-19. But there is no vaccine for climate change once we have passed these tipping points.</p><p><a href="https://www.weforum.org/reports/the-global-risks-report-2021" target="_blank">We need to act now on our climate</a>. Act like these tipping points are imminent. And stop thinking of climate change as a slow-moving, long-term threat that enables us to kick the problem down the road and let future generations deal with it. We must take immediate action to reduce global warming and fulfill our commitments to the <a href="https://www.ipcc.ch/sr15/" target="_blank" rel="noopener noreferrer">Paris Agreement</a>, and build resilience with these tipping points in mind.</p><p>We need to plan now to mitigate greenhouse gas emissions, but we also need to plan for the impacts, such as the ability to feed everyone on the planet, develop plans to manage flood risk, as well as manage the social and geopolitical impacts of human migrations that will be a consequence of fight or flight decisions.</p><p>Breaching these tipping points would be cataclysmic and potentially far more devastating than COVID-19. Some may not enjoy hearing these messages, or consider them to be in the realm of science fiction. But if it injects a sense of urgency to make us respond to climate change like we have done to the pandemic, then we must talk more about what has happened before and will happen again.</p><p>Otherwise we will continue playing Jenga with our planet. And ultimately, there will only be one loser – us.</p>By John R. Platt
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