Management of rice production systems to increase productivity

(1) Higher mineralization rate in the rhizosphere of upland rice
compared with other crops. Upland rice may enhance nitrogen
mineralization in soils by the secretion of some enzymes such as
protease and/or materials promoting microorganism activity.
(2) Upland rice has greater ability to take up organic nitrogen directly.
If a crop is capable of taking up organic nitrogen in the form of amino
acids, peptides and protein, the crop could absorb nitrogen with less
competition from microorganisms, as compared with crops which
absorbs nitrogen mainly as nitrate (Yamagata and Noriharu, 1999).
Nitrogen, usually found as ammonium in anaerobic lowland soils, is
found as nitrate in aerobic upland soils. Nitrate is sufficiently mobile in the
soil that root morphology and uptake properties do not limit uptake until soil
N levels are very low (Drew, 1990). When ammonium nitrate is applied, the
plant absorbs the ammonium faster than the nitrate. The optimum pH is
lower for nitrate uptake than ammonia. At optimum pH, nitrate is as effective
as ammonia as a source of nitrogen for rice (Tanaka et al., 1984), but nitrate
leaches from an upland field faster than in lowland culture (Yoshida, 1975).
Increases in the cost of inorganic fertilizers make it important to
examine the relationships between grain yield and nitrogen utilization. Parr
(1973) defines fertilizer N use efficiency as the percentage recovery of fertilizer
N by the crop. This may be estimated as the difference in uptake by the
aboveground portions of fertilized and unfertilized plants and expressed as
percent of the N applied. This method is often used by researchers to
interpret results obtained from fertilizer N experiments. This conventional
method of expressing the fertilizer N utilization includes the uptake of both
applied fertilizer and native soil N (Reddy and Patrick, 1980).
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