Under what condition denitrification is a major mechanism of nitrogen loss from the soil?

Correct option is C

The correct answer is (C) Submerged soil

Explanation:
• Denitrifaction is the microbial process of reducing nitrate ($NO_3^-$) to nitrogen gases (mostly $N_2$ and $N_2O$).
• This process is carried out by facultative anaerobic bacteria (like Pseudomonas and Bacillus) that require anaerobic (oxygen-free) conditions.
• Submerged or waterlogged soils lack oxygen because the pore spaces are filled with water. In these conditions, bacteria use nitrate instead of oxygen as an electron acceptor for respiration.
• This is a major pathway for nitrogen loss in rice paddies and wetlands, leading to reduced fertilizer efficiency and the emission of nitrous oxide, a potent greenhouse gas.

Information Booster:
• Factors favoring denitrification: High nitrate levels, high organic matter (as a food source for bacteria), warm temperatures, and low oxygen levels.
• The nitrogen cycle is balanced by denitrification, as it returns nitrogen from the soil to the atmosphere.
• Controlled drainage and precision fertilizer application can help reduce these losses.

Additional Knowledge:
• Well-drained soil (Option A): Favors nitrification (conversion of ammonium to nitrate) because oxygen is plentiful.
• Fallow soils (Option B): Soil left uncropped; while $N$ can be lost via leaching here, denitrification is not necessarily the "major" mechanism unless the soil is also wet.
• Pastured land (Option D): Generally aerobic; nitrogen loss here is more commonly through ammonia volatilization from animal waste.