chemical nature of organic matter in a rice soil, scientists examine a spectrum
acquired through NMR spectroscopy. Click the image for more information
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Key Clues to Cause of Rice Yield
Dilemma By Luis
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.
about this research in the September issue of Agricultural Research