Uptake and use patterns of nitrogen from urea,oxamide, and isobutylidene diurea by rice plants

Summary Two¹⁵N-labelled slow-release nitrogen (N) sources, oxamide and isobutylidene diurea (IBDU), each at two particle sizes, and¹⁵N-labelled urea were compared at two rates as sources of N for rice (Oryza sativa) under two watering regimes which simulated a transplant (continuous flood, CF) and a direct-seeded (A/F) system of paddy rice culture. Highest grain yields were obtained from −8+10-mesh oxamide particles applied at the rate of 2,000 mg of N/5 kg of soil, CF series; this yield was slightly higher than that obtained from −3+4-mesh oxamide, A/F series. Incubating the N fertilizers in moist (22% water) soil for 21 days immediately before flooding and transplanting rice greatly reduced N supply because of nitrification during the preflood period, followed by denitrification after flooding. This resulted in less plant uptake of N and less grain yield from urea, fine oxamide and IBDU, A/F series. For coarse oxamide, N release during the preflood period resulted in higher N uptake and grain yield in the A/F rather than in the corresponding CF series. The pattern of fertilizer N uptake by rice plants was affected by kind of fertilizer, particle size of oxamide and IBDU, and watering regime. Uptake of fertilizer N generally paralleled uptake of soil N throughout the growth period. Plant tops continued to accumulate some N during the period of grain filling, but much of the N in plant tops was translocated to the grain after heading. There was a large decrease in dry weight, N content, and¹⁵N content of tops after heading. Root weight and N content increased rapidly at first, and then at a diminishing rate until maturity. Unexplained N deficits occurred in the CF series (14–23% of the N applied, depending on N rate and source), and in the A/F series for IBDU (37–43% of the N applied).