A high-tech look into how nitrogen interacts with organic matter and
decomposing crop residues in submerged soils may have confirmed the
cause of a potentially serious problem facing Asia's rice farmers.
Growers of lowland rice (Oryza sativa) have described significant
declines in yield after several years of intensive cropping. Opinions
vary, but researchers have eyed a decrease in availability of soil organic
nitrogena key crop nutrientas a cause.
Annual cultivation of two or three rice crops in flooded lowland soils,
a technique used for about one-fourth of global rice production, has
been used in Asia's rice industry for more than three decades. "It's
a practice that will expand as Asian populations continue to increase,"
says soil scientist Dan Olk. He works in the Soil and Water Quality
Research Unit of ARS's National
Soil Tilth Laboratory in Ames, Iowa.
Olk and Iowa State University scientists examined the nitrogen-availability
theory through nuclear magnetic resonance (NMR) spectroscopy, which
allows detailed examinations of complex molecules made up of different
elements, such as nitrogen, carbon, and oxygen. This technology has
made possible, for the first time, direct viewing of soil nitrogen's
They found that in oxygen-free environments such as flooded rice paddies,
nitrogen bonds strongly with soil organic matterthe dark-colored,
tough material remaining after crop straw and roots and manure added
to soil have finished initial decomposition. The result is unusually
stable forms of nitrogen that aren't readily available to the growing
Nitrogen Bonds to Chemical Structures
Tests conducted on a continually submerged, triple-cropped soil revealed
thatinstead of being taken up by rice plantssignificant
amounts of organic nitrogen form bonds, through shared electrons, to
chemical structures known as aromatic rings.
NMR spectroscopy, which provides atomic resolution images of biological
molecules such as proteins and nucleic acids, showed these aromatic
rings were made of lignin derivatives that are abundant in rice soils.
Lignin forms plants' woody tissues, giving them strength.
Olk says the research "provides unique and strong evidence supporting
the role of nitrogen deprivation in yield decline."
Olk and his university collaborators, staff scientist Jingdong Mao
and professor Klaus Schmidt-Rohr, examined how this knowledge can be
applied toward identifying and solving the rice-yield decline.
"One solution would be to aerate soil while crop residues are
decomposing," says Olk. "With tropical rice fields, this process
lasts about 6 weeks after plowing or tilling." He says this step
can alter the chemical reactions involved in the conversion of crop
residues into new soil organic matter, allowing more extended nitrogen
release. "This would maintain nitrogen availability to the crop
throughout the growing season."
Making more soil nitrogen available to crops would lessen the need
to apply it as fertilizer, which would also help the environment.By
Agricultural Research Service Information Staff.
This research is part of Soil Resource Management, an ARS National
Program (#202) described on the World Wide Web at www.nps.ars.usda.gov.
Daniel C. Olk is with the USDA-ARS
National Soil Tilth Research Laboratory,
2150 Pammel Dr., Ames, IA 50011; phone (515) 294-8412, fax (515) 294-8125.
"Rice Yield Mystery Solved? Key clues revealed about declines
in Asia" was published in the September
2004 issue of Agricultural Research magazine.