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Key Clues to Cause of Rice Yield Dilemma

By Luis Pons
September 1, 2004

An Agricultural Research Service scientist and colleagues may have confirmed the cause of a potentially serious problem facing Asia's rice farmers.

Decreased availability of soil organic nitrogen, a key crop nutrient, has been eyed as a possible explanation for significant yield declines for some growers of lowland rice (Oryza sativa) in Asia. The declines come after several years of intensive cropping in regions where two to three rice crops per year are common.

While studying nitrogen's interactions with decomposing crop residues in submerged soils, the researchers--soil scientist Dan Olk of the ARS Soil and Water Quality Research Unit in Ames, Iowa, and staff scientist J. D. Mao and professor Klaus Schmidt-Rohr of Iowa State University--found strong evidence supporting the role of nitrogen deprivation in the yield decline.

The team examined nitrogen availability to rice plants through nuclear magnetic resonance (NMR) spectroscopy, which allows detailed examinations of complex organic molecules made up of different elements such as nitrogen, carbon and oxygen. It has made possible, for the first time, direct viewing of soil nitrogen's chemical forms.

They found that in oxygen-free environments such as flooded rice paddies, nitrogen bonds strongly with soil organic matter, the dark-colored material remaining after crop straw and roots added to the soil have finished their initial decomposition. The result is unusually stable nitrogen forms not readily available to growing plants.

Tests conducted on a submerged, triple-cropped soil revealed that instead of being taken up by rice plants, significant amounts of organic nitrogen formed bonds, through shared electrons, to chemical structures known as aromatic rings.

Based on their findings, the scientists say farmers might be able to combat rice-yield declines by aerating the soil while crop residues are decomposing. This would make more soil nitrogen available to crops, thus decreasing the need for fertilizer application.

Olk's unit is part of the ARS National Soil Tilth Laboratory in Ames. ARS is the U.S. Department of Agriculture's chief scientific research agency.

Read more about this research in the September issue of Agricultural Research magazine.