Soil N supply and plant N uptake by irrigated rice in Tamil Nadu
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This study was undertaken to assess the N supply capacity of some irrigated rice soils of India and its relationship with N uptake of crops with and without fertilizer N supply and to simulate effects of different soil-N supply patterns on optimizing fertilizer N application using the MANAGE-N model. Soil samples were collected from N-0 plots (no fertilizer N applied) of a multilocation field experiment with rice (Oryza sativa L.) conducted in Tamil Nadu, India, during the southwest monsoon season (July-October 1994) with cultivar IR64. The N supply capacities of soil samples were assessed by static soil test methods and anaerobic incubation with and without K-saturated cation exchange resin. The experiments had a common set of treatments of different N application strategies. Plant samples were collected at different growth stages and N uptake was measured from the biomass and N content, MANAGE-N was used to optimize N application with different soil N supply regimes, The results revealed that soil-N supply capacities assessed by both static and anaerobic incubation methods were related to plant N uptake up to first flowering (r(2) = 0.64 to 0.85) but they failed to correlate with grain yield. Actual N uptake rates of the N-0 crops during different growth periods ranged from 0.28 to 1.17 kg ha(-1) day(-1). Some of the soils could supply N equal to that of a sufficiently fertilized crop in the initial period. Based on the soil-N supply capacities, the soils could be classified into those not requiring basal N and those requiring basal N. Simulation results revealed that if the soil N supply regime observed at Ambasamudram were available at Paiyur, the grain yield of N, crop would increase by 38% and there would be 7 to 11% increased yield for 100 and 200 kg N applications and less fertilizer N would be required for different yield levels obtained with the Paiyur soil-N supply regime. MANAGE-N generated fertilizer N recommendation curves that identified different optimal timing of N application for different soil N supply regimes.
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