UPSC Prelims 2022 Q21: Crops as Sources of Methane and Nitrous Oxide
Among the following crops, which one is the most important anthropogenic source of both methane and nitrous oxide?
- Cotton
- Rice
- Sugarcane
- Wheat
Answer: (b) Rice
Statement-wise explanation
- a: incorrect — Cotton grows in well-drained, aerated soil. Methane-producing microbes need oxygen-free conditions, so a cotton field produces almost no methane. Nitrogen fertiliser on cotton does release some nitrous oxide, but that covers only one of the two gases the question asks about.
- b: correct — Flooded paddy fields cut oxygen off from the soil. In those conditions, methanogenic microbes break down organic matter and release methane. FAO puts rice cultivation at about 8% of global anthropogenic methane. The heavy nitrogen fertiliser used on paddy, and the wet–dry swings in the soil, also release nitrous oxide. So rice is a major source of both gases.
- c: incorrect — Sugarcane is a heavily fertilised crop that is irrigated but not kept flooded. Its soil stays mostly aerated, so it is not a significant methane source. Burning its residue adds small amounts of both gases, but nowhere near the scale of flooded rice.
- d: incorrect — Wheat is a rabi crop grown on upland, aerated soil. Urea applied to wheat releases nitrous oxide, but without standing water the field releases almost no methane. Wheat therefore cannot be the top source of both gases.
How to eliminate
The key word is 'both'. Nitrous oxide comes mainly from nitrogen fertiliser, which all four crops receive, so N2O alone cannot separate the options. Methane does separate them. It forms only where soil is waterlogged and oxygen-free, and of these four crops only rice is grown under standing water. If you know the textbook link 'flooded paddy → anaerobic decomposition → methane', the answer follows directly. The two facts to remember: methane comes from anaerobic conditions (paddy, ruminant guts, wetlands, landfills), and nitrous oxide comes from nitrogen in soil (fertilisers, manure).
Know more
Why agriculture emits methane and nitrous oxide
Agriculture produces only a small share of India's carbon dioxide. Its main climate impact comes from two non-CO2 gases with much higher warming power. Per IPCC AR6, over 100 years methane warms about 27–30 times as much as CO2, and nitrous oxide about 273 times.
| Gas | How it forms | Main farm sources |
|---|---|---|
| Methane (CH4) | Microbes break down organic matter where there is no oxygen | Enteric fermentation in cattle and buffalo, flooded rice, manure |
| Nitrous oxide (N2O) | Soil microbes convert nitrogen (nitrification and denitrification) | Urea and other N fertilisers, manure, crop residues |
FAO estimates that agriculture produces about 40% of anthropogenic methane: livestock about 32% and rice about 8%. In India, enteric fermentation, rice cultivation and nitrous oxide from farm soils together make up most of the agriculture sector's emissions. Livestock is the larger methane source, but it is not a crop, which is why rice is the answer here.
The rice trade-off. Draining paddy fields lets oxygen back into the soil, which cuts methane. Alternate Wetting and Drying (AWD), mid-season drainage and Direct Seeded Rice (DSR) all work on this principle. The same wet–dry cycling can increase nitrous oxide, so the net benefit depends on how water and nitrogen are managed together. ICAR's NICRA project and the National Mission on Sustainable Agriculture promote these low-methane rice practices.
For similar questions: link each gas to the condition that produces it. Methane means 'no oxygen' (paddy, wetlands, landfills, cattle rumen). Nitrous oxide means 'nitrogen' (fertiliser, manure).
In the news
At COP26 (November 2021), the Global Methane Pledge put farm methane in the news. India stayed out of the pledge and argued that most of its methane comes from small farmers' rice paddies and livestock. The question itself tests a static concept.
NCERT Concepts to Revise
- Methane: The question turns on which crop's growing conditions produce methane, so you need to know what methane is and how it forms.
- Greenhouse gases: Methane and nitrous oxide are both greenhouse gases; this entry explains why emissions from crops matter for climate.
- Greenhouse effect: Explains the heat-trapping process that makes farm methane and nitrous oxide a climate concern.