Nitrogen, Phosphorus and Potassium Dynamics Questions and Answers

Answer: A

The process is denitrification.

In denitrification, anaerobic bacteria such as Pseudomonas and Thiobacillus denitrificans use nitrate in place of oxygen and reduce it, through nitrite, to gaseous nitrous oxide (N₂O) and nitrogen (N₂), which escape to the atmosphere. It is the step that returns fixed nitrogen to the air, and it is common in waterlogged soils.

Ammonification is the release of ammonia from decaying organic matter, and nitrification is the oxidation of ammonia to nitrite and nitrate by bacteria such as Nitrosomonas and Nitrobacter, the opposite direction.

Answer: B

Ammonical fertiliser should be placed in the reduced zone.

A flooded rice soil develops two layers: a thin oxidised layer of a few millimetres at the surface, where oxygen from the floodwater is present, and the reduced zone below it, which is oxygen-free. Ammonium placed in the oxidised layer is nitrified to nitrate, and that nitrate then moves down into the reduced zone, where denitrifying bacteria convert it to nitrogen gases that are lost.

If the ammonium is placed deep in the reduced zone, for example as urea super granules, it stays as ammonium, is held on the soil colloids and is not nitrified, so denitrification loss is avoided. 'Calcified' and 'hydronized' are not recognised zones of a rice soil.