Rejuvenating the Gulf's dead zone?
As the oxygen-depleted dead zone in the Gulf of Mexico expands to an expected record size in July, we wonder how to reduce the flow of fertilizer that the Mississippi River carries to the Gulf. One option may be to treat the symptom, as China is doing with the over-fertilized Yellow Sea, where sailors are snagging masses of algal crud as they prepare for the Olympics. Rather than treat the raw sewage that feeds the plant growth, China recruited 20,000 volunteers to scoop out the algae by hand... and install a 30-mile floating, algae-proof fence.
Modified from original map by World Resources Institute
Cutting the amount of fertilizer that reaches the Gulf of Mexico is the logical solution to the dead zone, but achieving that simple-sounding goal has been politically difficult. The federal government has just updated its "action plan" for the dead zone, which is intended to "reduce, mitigate, and control hypoxia in the Northern Gulf of Mexico and improve water quality in the Mississippi River Basin." Unfortunately, the prospect of reducing over-fertilization -- to treat the ultimate cause of the dead zone -- seems no more proximate here than in China, and during the past five years, the average dead zone has been almost triple the "target size" in the fed's action plan.
Real solutions all seem to require pain on the part of farmers, who fertilize to attain maximum yields. Reducing fertilizer use, and/or corn acreage, could cut profits and even cause more hunger in a year when food supplies are extremely pressed, and the idea is not exactly popular among corn farmers, who are finally expecting a profitable year. Furthermore, most large farmers already have a "nutrient plan" that tries to optimize fertilizer use and reduce waste. After all, fertilizer costs money.
But plant nutrients continue to spill into the Gulf, and a lot of them come from corn fields.
But when we dug, we unearthed some bright ideas that might tame the dead zone without causing massive pain to farmers. For one thing, fertilizer that ends up in the Gulf represents a waste of money, and the fast-rising price of nitrogen fertilizer gives farmers more cause to conserve. For another, agriculture researchers are finding ways to grow crops that offer simultaneous environmental and economic benefits. Finally, scientists and policy wonks are belatedly realizing that nature figured out long ago how to eliminate over-fertilization and protect the environment.
Modified from original by the Dead zone Action Plan
Train the drains
Beneath an estimated 70 million acres in the Mississippi Basin is a system of drains that pipe water from the deep soil to a nearby stream (see #3 in the bibliography). These drains, often called "tiles," allow farmers to drive tractors across fields that would otherwise be soggy, but are now recognized as the primary route for removing nitrates from many fields and sending the fertilizing compounds toward the Gulf of Mexico.
Drains are a double-edged sword: they are highly effective at removing nitrate (-NO3) from high-yielding fields. "Drainland is your best land in terms of production, but it also could give your best reduction in nitrate," says Daniel Petrolia, an assistant professor of agricultural economics at Mississippi State University. " Simply reducing application rates can cut fertilizer runoff, "but if you do it too much, you essentially are not going to continue growing," as inadequate fertilizer will depress yields and profits. Yet ceasing to farm drained land is equally unappealing. "Do you want to remove the most productive agricultural land, [because that is where you can] get the most nitrate reduction?"
Nonetheless, we've heard of two methods for reducing the downside of drains, while retaining their benefit on boggy fields. The first technique uses bacteria-rich trenches that "denitrify" nitrogen compounds, converting them into molecular nitrogen, which is released harmlessly into the atmosphere, which is already almost four-fifths nitrogen.
Photo: Dan Jaynes, National Soil Tilth Lab
Denitrifying bacteria are a primary natural method for eliminating nitrogen from soil and water, and to encourage them, Dan Jaynes of the National Soil Tilth Lab in Ames, Iowa, has been experimenting with wood-filled trenches laid parallel to tile drains in cornfields. The trenches are filled with wood chips between six and two feet belowground. The top two feet contains soil, allowing tractors to drive across the trenches.
The denitrifying process is usually limited by the supply of carbon, "so by adding the woodchips, we are adding a carbon source," Jaynes says. "In our design, the shallow groundwater has to move through a trench to reach the tile drain. We are seeing essentially complete denitrification in a trench that's 2 feet wide," placed 10 feet to either side of the drain. Water leaving the experimental drains carries only one-third of the nitrate found in a conventional drain used for comparison (see #4 in the bibliography). However, if the trenches were closer to the tile, Jaynes expects the removal would be complete.
A second tactic is to put valves on the drains, where water normally flows unobstructed. When the field must be dried out so machines can drive across it, the valves are opened. Otherwise, the valve is closed, leaving more water for the crop while reducing the removal of nitrate.
In a test in Illinois, controlling drains reduced nitrate movement by 40 percent in corn fields.
Drain management is not a panacea, however. For example, unless the field is perfectly flat, many control structures may be needed. Nevertheless, Jaynes says, installing valves can make sense. "In farming, we manage all the inputs. We buy hybrid seed, put it on at a certain population, then put on fertilizer, pesticides, but the one thing we don't manage is drainage. We put in an open pipe at four feet deep, and any time water is there, we drain. Why not put in control structure at the end of the tiles, so we can also manage the drainage and not have a dumb pipe?"
Nitrogen is an essential nutrient, but a slippery one -- if it's applied too long before the crop needs it, it can leach or erode into streams -- heading down the Big Muddy and feeding the dead zone. Now, faced with environmental and economic troubles (down-the-river nitrogen, after all, does nothing for the crop it was bought to help), agricultural researchers are looking at something called "precision farming."
The term describes a "process of going about farming, where you are tailoring your inputs to meet the needs, on a spatial basis, but also on a temporal basis," says Newell Kitchen, of the Cropping Systems and Water Quality Research Unit of USDA's Agricultural Research Service, located at the University of Missouri. "It's all about targeting, optimization, what is going to be best crop in the best place, to optimize in terms of productivity, and environmental suitability, to conserve and in the end maintain soil and water quality on the landscape."
To reduce overuse of nitrogen, Kitchen and his colleagues have been testing tractor-mounted sensors that illuminate corn plants, and then analyze the reflected light to detect nitrogen shortages. "You compare the relative reflectance from one area to another, and in many cases, it helps you understand the nitrogen health status of a plant," Kitchen says. "The sensors would be on the front of the fertilizer applicator, so as you drive down the field, looking at the corn crop, they assess its condition, and an onboard computer has an algorithm that says, we need this much. It's a one-pass application, you sense and apply fertilizer at the same time."
Photo: Courtesy Kenneth Albrecht, University of Wisconsin-Madison
The potential benefits of using enough nitrogen but not too much are environmental -- a smaller dead zone -- and economic, less money wasted for unneeded fertilizer. The rising price of nitrogen "is making people think about this," says Kitchen, especially among farmers who already have a suitable fertilizer applicator and are "savvy about how to deal with sensors, and the idea of site-specific management."
So far, the system has shown an average benefit of $8 to $12 per acre in reduced fertilizer cost, although that calculation excludes the sensor cost, Kitchen says. Having a convenient method to quickly determine the nitrogen status of a crop may help farmers think more carefully about the details of fertilizer applications, he adds. "It makes farmers more aware of the timing. Whenever you can synchronize the nitrogen application to the time when the crop is going to use it, typically you come up with higher nitrogen use efficiency." In other words, equal crop with less inputs and thus less waste...
Cornfields are notorious for soil erosion and for releasing nitrate because the soil is bare for six or seven months per year when the plant is removed as silage for cows or the residue is removed for animal bedding. Could an experimental technique slash soil erosion on bare cornfields, cut farmer costs, and restrict the release of nitrates into rivers?
Corn is virtually never grown with a companion crop, but Ken Albrecht, a professor of agronomy at the University of Wisconsin-Madison, has been experimenting with growing in a cover crop of kura clover. This legume spreads to fill open spaces and transfers nitrogen from the air to the soil -- reducing or eliminating the need for nitrogen fertilizer, Albrecht says.
Courtesy Ken Albrecht, University of Wisconsin-Madison
Once the kura clover is started, it remains on the field indefinitely, but is not harvested, and its leaves, rhizomes (underground stems) and roots all protect against soil erosion. Before planting corn, farmers kill a strip of the clover, and then emplace the corn seed with a standard no-till planter. A low dose of herbicide sets the remaining clover back slightly, so the corn can dominate the field, while allowing the clover to supply essential nitrogen.
In a year with normal weather, the clover-corn combo yields as well as a conventional corn crop, but in dry years, the yield may lag by 10 to 15 percent, Albrecht says.
And what about nitrate leaching and runoff? Albrecht says these questions remain, since it's taken 10 years to work the kinks out of this radical corn-growing system, and researchers are "just starting to look at the environmental effects." Yet sketchy, preliminary data show a 10-fold reduction in soil erosion after heavy rains, and a two- to five-fold reduction in nitrate leaching below the root zone, whence nitrate can enter groundwater and/or surface water.
"It's kind of a neat system for the environment," says Albrecht. "On conventional corn, if we have a big rainfall right after applying nitrogen fertilizer, like we did this spring, a large amount of agricultural chemicals ends up in the Mississippi River and the Gulf. With kura clover, there is a slow release of nitrogen that apparently matches the demand by the growing corn, so we don't have the big spikes in nitrate concentration" in the soil that are prone to move downriver.
In future tests, Albrecht plans to measure how the corn-clover combination reduces soil erosion and fertilizer movement.
The clover system is not suited to cornfields where only the grain is harvested, because the corn residue would smother the clover. However, if the whole plant is removed as silage to feed cows, or if the residue is removed for animal bedding or biofuel after the grain harvest, the clover will cover the soil and help keep soil and nutrients where they belong.
Although only a few farmers have tried the corn-clover combo on their farms, the soaring price of nitrogen fertilizer, whose production gobbles massive volumes of costly natural gas, is likely to bring more adherents, since clover can supply the necessary nitrogen at almost no cost.
Can nature bring life to the dead zone?
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive