The process which reduces the nitrogen content of the soil is

Step 1: Analyze the question The question asks which process reduces the nitrogen content of the soil. That means we are looking for a process that removes usable nitrogen from the soil.

Step 2: Review each process in the nitrogen cycle

• A. Ammonification: Decomposers (bacteria and fungi) break down dead plants, animals and waste into ammonia/ammonium compounds. This adds nitrogen compounds to the soil.
• B. Denitrification: Denitrifying bacteria (e.g., Pseudomonas denitrificans) convert nitrates in the soil into free nitrogen gas (N₂), which escapes into the atmosphere. This removes usable nitrogen from the soil. It is common in waterlogged, poorly aerated soils.
• C. Nitrogen fixation: Bacteria such as Rhizobium (in legume root nodules) and Azotobacter convert atmospheric nitrogen into nitrogen compounds. This increases soil nitrogen.
• D. Nitrification: Bacteria (Nitrosomonas and Nitrobacter) convert ammonia to nitrites and then to nitrates. This makes nitrogen available to plants and does not reduce the nitrogen content.
• E. Putrefaction: Decay of proteins in dead organisms by bacteria, releasing ammonia and other compounds. This also returns nitrogen to the soil.

Step 3: Conclusion Only denitrification takes nitrogen compounds out of the soil and releases them as nitrogen gas into the air.

Answer: B. denitrification

Exam tip: Remember "Denitrification = decrease". The prefix de- signals removal. Also, farmers reduce denitrification by draining waterlogged soils and ploughing to improve aeration.