Nitrogen Efficiency of Urea Amended With Inhibitors and Cations Applied to Rice

Urea, the major source of nitrogen (N), is subjected to extensive gaseous N losses to the atmosphere. Ammonia (NH3)volatilisation and denitrification losses are important mechanisms for N losses from urea and are causes of poor fertilizer use efficiency by lowland rice. The study was undertaken in a series of five experiments to evaluate means of increasing the efficiency of urea-N under flooded rice soil conditions. A laboratory incubation experiment of different rice soils was conducted to study the kinetics and transformations of urea in flooded soils. A greenhouse experiment was carried out on marine and riverine alluvial soils to evaluate urea-N efficiency in direct-seeded (DS) rice and transplanted (TP) rice under similar N management practices using lSN-Iabelled urea. Two greenhouse and one field studies were conducted to evaluate the effectiveness of inhibitors and cations on marine and riverine alluvial rice soils on the efficiency of urea-N using 15N recovery techniques. Urea hydrolysis followed the first-order kinetics and rate constants ranged from -0.032 to -0.076 ha-1 A lag phase existed in flooded soil conditions and it varied from 6 to 15 hours. The half-life of urea ranged from 12 to 26 hours. Urea conversion to NH4 -tN was initially rapid with about three-fourths being converted within 48 hours of incubation and total conversion completed within 108 hours.

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