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Subterranean fixation.

For years, farmers have known the benefits of planting corn after alfalfa and soybeans. These two leguminous crops are well known for their ability to supply nitrogen from the air for their own growth and to enhance soil fertility for succeeding crops.

Now, an ARS soil scientist in St. Paul, Minnesota, is getting to the bottom of how alfalfa transfers nitrogen to the soil by viewing the underground death and decay of roots.

"Most knowledge of how roots affect plant growth and health has been inferred by studying plants above ground," says Michael P. Russelle of the ARS Plant Science Research Unit. "However, to really understand nutrient uptake and cycling by plants, we need to know more about root growth and death."

An acre of alfalfa is laced with about 13,000 miles of roots in the top 6 inches of soil and many more thousands below that. The development of small video cameras has enabled Russelle and University of Minnesota agronomist Markus Dubach to observe these root mazes through clear plastic tubes buried in the soil.

In the summer of 199 1, they took a miniaturized video camera into the field and monitored how many alfalfa roots died during a growing season. Analysis of time-lapse sequences of those videos shows that 64 percent of the fine roots at 4 inches and 50 percent of the fine roots at 8 inches decomposed as the alfalfa grew.

"Plants lose small roots during growth, just like they lose leaves, and then replace them with new ones," says Russelle. Decomposing roots are a source of nitrogen that can contribute to a more fertile soil. The challenge for Russelle and Dubach is to find out how much nitrogen is contributed by dead alfalfa roots.

But two major problems have stood in their way. The first was that no one knew how much nitrogen these fine roots contained.

So Russelle and Dubach grew alfalfa in root observation boxes to learn the age and condition of many roots. Later, they surgically removed the roots from the soil and measured their nitrogen content.

They calculated that if 2,000 miles of fine roots decomposed, about 1 pound of nitrogen from the air would be transferred to the soil.

The next problem was that no one knew the lifespan of fine alfalfa roots. These fine roots - often less than one sixty-fourth of an inch in diameter - could be seen with the miniaturized camera. The camera magnifies a section of soil one-half by two-thirds of an inch in size and projects an image on a television monitor. In the upper 8 inches of soil, these fine roots have a lifespan of less than 2 months during the summer.

The story of fine roots dying and decaying in a field is just the small picture. The big picture involves nitrogen recycling throughout crop-lands in the Corn Belt.

The nitrogen cycle goes like this: Alfalfa is planted as a forage for animals. While the alfalfa grows, bacteria living in nodules on the roots capture nitrogen from the air. The soil inherits the valuable nitrogen left behind as roots die and after the alfalfa stand is plowed under in preparation for the next crop.

After harvest, the alfalfa is fed to animals as hay or silage. While some of the nitrogen is converted to meat and milk, most - about 70 percent - is excreted as manure. Most of the manure nitrogen can be returned to the soil, thereby completing the nitrogen cycle.

Alfalfa - the most popular dairy forage in the United States - is grown on 26 million acres. More than 10 million acres are grown in the eight states of the eastern part of the Com Belt - Illinois, Indiana, lowa, Michigan, Minnesota, Missouri, Ohio, and Wisconsin.

"This alfalfa acreage adds more than 1 million tons of nitrogen to farms in the region every year through nitrogen fixation," says Russelle. Russelle and Todd A. Peterson, an ARS research associate, have studied alfalfa's impact on the regional nitrogen cycle.

On the average, farmers here could cut fertilizer nitrogen use by 14 percent, without reducing corn yields, and save about $ 100 million a year.

"It's important for farmers to recognize this nitrogen source and avoid over-application of fertilizer nitrogen," says Russelle. "By including alfalfa as part of their cropping system, they get a valuable crop and free nitrogen besides."
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Title Annotation:nitrogen transfer by roots
Author:Cooke, Linda
Publication:Agricultural Research
Date:Aug 1, 1993
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