Agriculture's Nitrogen Legacy

Growing corn pollutes the water because of farmers use more nitrogen fertilizer than the plants need. New research shows the pollution may take years to show up.

Photo courtesy of Elizabeth Swanner.

The air is about 80% nitrogen, an element that promotes plant growth. You might think plants could absorb nitrogen from the air and grow like crazy. However, atmospheric nitrogen needs to be converted to a different form before plants can use it, and most plants can’t do the conversion.

More than 100 years ago, German chemist Fritz Haber discovered how to do the conversion by reacting nitrogen with hydrogen under high heat and pressure. The company BASF bought Haber’s technology and assigned Carl Bosch to figure out how to scale it up, creating synthetic fertilizer. Both Haber and Bosch won Nobel Prizes for their work, which transformed agriculture and provided massive benefits to humanity.

Excess nitrogen fertilizer pollutes the water

But, there’s a dark side. Crops don’t use all of the nitrogen fertilizer that farmers spread on their fields. The unused nitrogen becomes a pollutant; it is either converted to a potent greenhouse gas (nitrous oxide) or washed into nearby waterways. Excessive nitrogen in water causes rapid algae growth, which reduces recreation value and can be toxic. When the algae die and decompose, they consume all the oxygen in the water, leaving nothing for fish.

Nitrogen pollution is a major global problem, especially in areas with intensive farming like the Mississippi River Basin. In the United States, only 32% of streams and rivers are in good condition for nitrogen. Near the mouth of the Mississippi River, a hypoxic “dead zone” the size of Connecticut has so little oxygen that almost nothing can live there.

The Dead Zone. Source: NOAA

In a recently published paper, my esteemed co-author Kostas Metaxoglou and I studied the data connecting fertilizer-intensive crops to water pollution. The US government measures nitrogen concentration in streams and rivers at thousands of locations in the corn belt. We linked those measurements to the crops that were grown upstream. Most previous studies rely on simulation models of plant growth and water flow to estimate this relationship, but we use actual observed data.

Our study finds 50 times less water pollution than commonly-used models

Here’s a map of the water monitors in waterways east of the 100th meridian, i.e., in places where farmers grow corn mostly without irrigation.

Water quality has not improved in the last 30 years (see below). Since 1990, nitrogen concentration has been about 2.4mg/L in the average river or stream. Despite a lot of attention, the problem isn’t getting better.

Average Nitrogen Concentration in Cor Belt Rivers and Streams

From each monitor, we follow all rivers and streams upstream for 50 miles. For example, the map below shows a monitor on the Mississippi River along the Iowa-Illinois border. The yellow triangle is the monitor. The little blue lines are all rivers and streams within 50 miles that flow towards the yellow triangle.

Our econometric model measures the additional nitrogen at the yellow triangle when another acre of corn is planted in the counties containing the little blue lines. We looked at other crops, but corn was the only one that showed a significant connection to water quality.

An example of counties 50 miles upstream from a USGS monitoring site.

We find that the nitrogen load in small rivers and streams increases by about 0.4 lb per year for each additional acre of corn planted within 50 miles upstream. That number is 50 times smaller than what commonly-used mathematical simulation models say.

So, did we prove the models wrong? Is growing corn causing much less pollution than what we thought?

Sadly, the answer appears to be no. The models are good at calculating how much nitrogen a corn plant uses based on its growth, so they have a good idea of how much unused nitrogen is on corn fields. Our study shows that this excess does not enter rivers and streams within the same year.

So, what happens to the excess nitrogen?

A substantial portion gets stored in the subsurface soil and groundwater and will eventually find its way into rivers and streams. This is known as legacy nitrogen, and there are millions of tons of it (as illustrated below).

Source: Basu et al., “Managing nitrogen legacies to accelerate water quality improvement” Nat. Geosci. 15 (2), 97–105, 2022.

Kim Van Meter and co-authors looked at soil samples in Iowa and Illinois and found that 22-62lb of nitrogen per acre per year had accumulated below ground in recent decades. This number matches the amount of excess nitrogen on cornfields computed by the mathematical models. It also supports our finding that only a small fraction of that excess nitrogen reaches nearby rivers and streams in the same year. Most of it remains stored in the soil.

Potential options to deal with the nitrogen pollution problem include:

1. take land out of corn production

2. get farmers to use less fertilizer on corn

3. expand wetlands

We argue in our paper that option 1 is not a good idea for several reasons. The lost production value would exceed the value of avoided pollution damages, taking one corn field out of production would cause people to plant corn somewhere else thereby blunting the impact, and it does nothing to deal with the legacy nitrogen.

Option 2 sounds great in theory, but it’s hard in practice. Farmers don’t have the technology to apply nitrogen exactly where and when it’s needed and exactly in the right quantities. They spread it as precisely as they can and hope the plants find it and the rain doesn’t wash it away. I am optimistic that AI and robotics will help mitigate this challenge, as may new varieties of corn that convert their own nitrogen.

Economists sometimes suggest taxing fertilizer as a way to reduce use, but that’s difficult because we don’t want to tax all fertilizer (most of it is good), just the excess (which we don’t know). And, using less fertilizer does nothing to deal with the legacy nitrogen.

Option 3 has some promise. Establishing wetlands in streams and rivers can help mitigate both legacy and new surplus nitrogen by catching the nitrogen in waterways and using it for plant growth. In a recent study, Amy Hansen and co-authors find that “fluvial wetland restoration was most effective for achieving long-standing policy goals for sediment and nitrate reduction.”

In ongoing work, Kostas and I are using satellite data on land use to get more granular estimates of how agricultural activity affects nitrogen concentration. We hope this will lead to more precise policy and practice recommendations.

In a different study to the one I cited above, Van Meter and co-authors estimate that, even if surplus nitrogen on cropland were to go to zero and remain there, it would take about 30 years for nitrogen loads in the Mississippi River Basin to reach their 1970 levels. Millions of tons of nitrogen are waiting to pollute waterways, a legacy of corn belt agriculture.

Citation to our paper: “Agriculture’s Nitrogen Legacy” Metaxogolou, K. and Smith, A. Journal of Environmental Economics and Management, 130:103132. 2025.