UPSC Prelims 2022 Q21: Environment & Ecology – Climate Change & Greenhouse Gases

UPSC Prelims 2022, General Studies Paper I · Environment & Ecology

Among the following crops, which one is the most important anthropogenic source of both methane and nitrous oxide?

Answer: (b) Rice

Explanation

Rice — a major source of both methane and nitrous oxide

Gas	Why rice produces it
Methane (CH₄)	Flooded paddy fields have no oxygen; methanogenic bacteria break down organic matter and release methane
Nitrous oxide (N₂O)	Heavy use of nitrogen fertilisers; wet-dry cycles boost N₂O release from soil

Scale of the problem:

Fact	Detail
Rice and global methane	Paddy fields give about 10–12% of human-caused methane
Rice in India's agriculture emissions	A major share of agricultural methane
Solutions	Alternate wetting and drying (AWD), System of Rice Intensification (SRI), direct-seeded rice

Other crops: cotton, sugarcane and wheat are grown on dry land, so they emit N₂O from fertiliser but little methane.

Answer: (b) Rice.

(a) Incorrect. Cotton is a dryland crop; it emits N₂O from fertiliser but little methane.
(b) CORRECT. Flooded rice releases methane, and its nitrogen fertilisers release nitrous oxide.
(c) Incorrect. Sugarcane is not grown in standing water, so methane emissions are small.
(d) Incorrect. Wheat is a dryland rabi crop with low methane emissions.

Flooded paddies → methane (anaerobic bacteria); nitrogen fertiliser → N₂O. Rice is the top crop source of both.

Rice is the most important crop source of both methane and nitrous oxide.

Thinking all crops emit methane; methane needs waterlogged, oxygen-free soil.
Forgetting that rice also emits N₂O from fertilisers and wet-dry cycles.
Overlooking livestock (enteric fermentation) as the largest agricultural methane source overall.

Standing water + no oxygen = methane. Only rice among these crops grows in flooded fields.