Effect of rice cultivars on rate of N-mineralization, nitrification and nitrifier population size in an irrigated rice ecosystem

A study was conducted in irrigated rice fields planted to three rice (Oryza sativa) cultivars, Sarju-52, Malviya-36 and Pant Dhan-4, to investigate the influence of rice cultivars on rate of N-mineralization, nitrification and nitrifier population size. Thirty-day-old seedlings were transplanted in the waterlogged condition. Urea was the only fertilizer applied, at a rate of 100 kg N ha(-1) in three split doses. The experiment was laid out in a randomized complete block design with three replicate plots for each cultivar and treatment. Soil mineral-N content, N-mineralization, nitrification and the most probable number of ammonium and nitrite oxidizing bacteria were estimated on six dates within the cropping period. It was observed that the mineral-N content in soil was lowest beneath Pant Dhan-4 under both unfertilized (control) and fertilized conditions. Mineral-N values in plots planted to Malviya-36 had intermediate values, while plots planted to Sarju-52 had highest mineral-N content under both control and fertilized conditions. Throughout the cropping season the lowest rate of N-mineralization, nitrification and nitrifier population was recorded in soil beneath Sarju-52 and highest beneath Pant Dhan-4. The highest vigour in terms of plant growth, grain yield and root porosity, was observed in Pant Dhan-4, followed by Malviya-36 and Sarju-52. Intercultivar differences in plant biomass production, which indicates the differences in nitrogen utilization potential and indirectly the quantity and quality of litter production may explain in part the differences in N-mineralization processes. The nitrifying bacterial population was strongly correlated with root biomass and root air space. The rice cultivars differed significantly in aerenchyma tissue differentiation resulting in different degrees of aerobic conditions in their rhizosphere. This explains the differences in nitrifier populations harboured by each of the cultivars in their respective soils and the consequent differences in soil processes. Hence, apart from fertilizer management, choice of rice cultivar also affects nitrifier populations and their functions, which are responsible for supplying nutrients to the rice soil. (C) 2003 Elsevier B.V. All rights reserved.