MICROPLOT SIZE AND RETAINER EFFECTS ON RICE GROWTH AND N-15 ACCUMULATION

Nitrogen studies on flooded rice (Oryza sativa L.) often use N-15 as a tracer, which is usually contained within microplots and retainers to restrict root growth and to prevent N-15 movement in floodwater and t he soil solution, Little research has addressed the effects of the mic roplot and retainer on plant growth, N accumulation, and plot microcli mate, The objectives of this field study were to (i) determine the eff ects of microplot size and retainer on plant growth, N accumulation, f loodwater and soil temperature, and floodwater and soil pH and (ii) de termine the size of microplots with and without retainers that most cl osely simulate a field plot. 'Cypress' rice was drill-seeded in a Crow ley silt loam soil (fine, smectitic, thermic Typic Albaqualf) in circu lar microplots (15, 35, and 60 cm in diameter) with retainers, and squ are microplots (75 cm long) with and without retainers. Nitrogen-15-la beled urea was applied preflood at 151 kg N ha(-1), The control was a 2.1- by 7.6-m field plot fertilized with unlabeled urea at 151 kg N ha (-1). All plots were harvested at 90% heading. Plants in 15-, 35-, and 60-cm microplots were 8.5 to 25.4 cm shorter than plants in the field plot in at least one year. Plant dry matter and total N accumulation in microplots did not differ from the field plot, except for total N a ccumulation in the 15-cm microplot and dry matter in the 35-cm micropl ot. Water and soil temperature in microplots with retainers were 1.0 t o 3.5 degrees C lower than in the field plot. Water and soil pH did no t differ between microplots and the field plot, The 75-cm square micro plot with a retainer was found to be the best substitute for the field plot.