Saturday, 15 July 2006

Spatial and Temporal Distribution of Nitrifying Bacteria and N Transformation Rates in Tropical Rainfed Rice Soil.

Paromita Ghosh1, A.K. Kashyap2, and P. P. Dhyani1. (1) G. B. Pant Institute of Himalayan Environment and Development, Kosi - katarmal, Almora, India, (2) Dept of Botany, Banaras Hindu Univ, Varanasi, India

In a tropical rainfed rice field, the rate of N-mineralization, nitrification and nitrifier population fluctuations in planted (bulk), unplanted (bare) and rhizosphere rice soil was quantified. The rate of N-mineralization and nitrification were not measured in rhizosphere soil. The experiment was laid out in a randomized complete block design with three replicates. The factors assessed were soil type (bare, bulk and rhizosphere soil) and fertilization (control and urea). The control plots received no nitrogenous fertilizers whereas the plots designated as fertilized received urea at the rate of 100 kg N per hectare in three split doses. The nitrogen transformation rates, nitrifier population size, mineral-N left in soil and plant growth parameters were measured at intervals of 10-15 days throughout the cropping season. The soil sampling coincided with phenological events in the rice plants. Sampling was done simultaneously in the adjacent plots without rice plants (bare soil). There were significant differences in the rates of N-mineralization, nitrification, soil mineral N pool and nitrifier population size due to soil type. The growing rice plants made the soil more aerobic and increased the availability of substrates for the growth of the nitrifiers, which resulted in a ten-fold higher nitrifer population in the bulk soil in comparison to bare soil. The creation of an aerobic zone around the rice rhizosphere, due to presence of aerenchyma tissue, resulted in a ten fold higher nitrifier population than in the bulk soil and a hundred fold higher nitrifier population than in bare soil. The rate of N-mineralization and nitrification was significantly higher in the bulk soil in comparison to bare soil. The mineral-N pool was highest in bare soil in the absence of a plant sink followed by bulk soil. It was lowest in the rhizosphere soil. Thus the presence of rice plants created a soil gradient in respect of nitrifier population distribution which consequently affected their processes and mineral N availability in soil. Fertilization enhanced the nitrifier population and their processes by acting as substrate for the nitrifiers as well as affecting proliferation of aerenchyma tissues and led to better plant growth characteristics.

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