MPSC 2024 Prelims Paper-I · Question 34 of 100

Correct — option (1), only statement (a). The question prints two statements, not four, and asks which are correct in the context of global warming. Statement (a) is sound. A paddy field under standing water is cut off from atmospheric oxygen, and in that waterlogged, anaerobic soil the ordinary aerobic decomposers cannot work; the organic matter is broken down instead by methanogenic micro-organisms, whose metabolic end product is methane. The gas then reaches the atmosphere by bubbling up through the water, by diffusion, and most of all by travelling up through the aerenchyma, the air-conducting tissue of the rice plant itself, which acts as a chimney out of the flooded soil. Flooded rice cultivation is on that account one of the largest agricultural sources of methane worldwide, and the statement's word 'anaerobic' is doing real work — it names the precise condition that makes a rice field a methane source, since the same soil drained and aerated would not be one. Statement (b) is where the question is decided, and it fails on its final phrase. The chemistry it gestures at is real: nitrogen applied as urea or another nitrogenous fertiliser is transformed in the soil by nitrifying and denitrifying micro-organisms, and nitrous oxide is one of the products. But the statement says that nitrous oxide 'is released into the soil'. That is the wrong destination. Nitrous oxide is generated within the soil and is then released from the soil into the atmosphere, and it is only once it is in the atmosphere that it matters for global warming at all — it is a greenhouse gas with a very long atmospheric residence and a warming effect per molecule hundreds of times that of carbon dioxide. A gas described as being released into the soil is being described as staying where it was made, which is not an account of a greenhouse gas emission and is not what happens. Statement (a) stands, statement (b) as printed does not, and option (1) is the answer.

• (2)Only (b) — This rejects statement (a) and accepts statement (b), getting both the wrong way round. Statement (a) is a textbook description of how flooded rice cultivation generates methane: standing water excludes oxygen, anaerobic methanogens decompose the organic matter, and the methane escapes largely through the rice plant's own aerenchyma. There is no ground on which to call it false. Meanwhile statement (b), as printed, has nitrous oxide released into the soil rather than from the soil into the atmosphere, which reverses the direction of the emission it is supposed to describe.
• (3)Both (a) and (b) — This is the most tempting wrong option, because both statements point at genuine agricultural sources of greenhouse gases and a candidate scanning for the topic rather than the wording will accept both. Nitrogenous fertiliser use does drive nitrous oxide emissions through nitrification and denitrification in the soil. But the statement as printed sends the gas into the soil, not out of it, and in a question explicitly framed 'regarding global warming' the destination is the whole point: a gas that remains in the soil contributes nothing to atmospheric warming. Reading the preposition rather than the topic is what separates this option from the correct one.
• (4)Neither (a) nor (b) — This rejects both statements, which requires denying that flooded rice cultivation emits methane under anaerobic conditions — a well-established fact and one of the most frequently cited agricultural contributions to greenhouse gas emissions. A candidate who spots the flaw in statement (b) may over-correct and assume both statements have been tampered with, but the standard construction of a two-statement item is one sound statement and one subtly altered one, and the correct response to identifying the altered one is to judge the other on its own merits.

Agriculture contributes to global warming mainly through two gases that are not carbon dioxide, and each has its own mechanism. Methane comes from anaerobic decomposition — decay in the absence of oxygen — which occurs in flooded paddy soils, in the digestive tracts of ruminant livestock, and in landfills and manure heaps. It has a comparatively short atmospheric lifetime of roughly a decade but a strong warming effect while it lasts, put at about 28 times that of carbon dioxide over a hundred-year horizon on the Intergovernmental Panel on Climate Change's Fifth Assessment values. Nitrous oxide comes from the microbial transformation of nitrogen in the soil: applying nitrogenous fertiliser supplies more reactive nitrogen than the crop takes up, and the surplus is processed by nitrifying and denitrifying bacteria which release nitrous oxide as a by-product. That gas escapes to the atmosphere, where it persists for more than a century and carries a warming effect around 265 times that of carbon dioxide over the same horizon, and where it also participates in the destruction of stratospheric ozone. Both pathways can be reduced by management rather than by abandoning the crop: intermittent drying of paddy fields interrupts methanogenesis, and matching fertiliser dose and timing to crop uptake reduces the nitrogen surplus available for conversion to nitrous oxide.

Two-statement questions in this paper are usually decided by a single word, and this one is decided by a preposition. That is a pattern worth internalising, because it changes how the statements should be read: not 'is this the right topic' but 'is every element of this sentence, including the direction and the destination, correct as written'. The Commission is testing whether a candidate can distinguish the process, which happens in the soil, from the emission, which is the movement of the gas out of the soil and into the atmosphere — a distinction that matters in the science and not only in the wording, since a greenhouse gas exerts its effect only in the atmosphere. Note too that only two statements are printed here; there is no statement (c) or (d) to reason about, and the four numbered choices are the answer options, not further statements.

• Flooded paddy fields become anaerobic because standing water excludes oxygen, and methanogenic micro-organisms decomposing organic matter in that condition produce methane.
• Methane escapes a flooded rice field by bubbling, by diffusion, and largely through the aerenchyma, the air-conducting tissue of the rice plant itself.
• Nitrogenous fertiliser applied in excess of crop uptake is converted by nitrifying and denitrifying soil micro-organisms into nitrous oxide, which is then emitted from the soil into the atmosphere.
• On the IPCC Fifth Assessment values, methane has a global warming potential of roughly 28 times that of carbon dioxide over a hundred years, and nitrous oxide roughly 265 times; nitrous oxide also persists in the atmosphere for over a century and contributes to stratospheric ozone depletion.
• Both agricultural sources are reducible by management — intermittent wetting and drying of paddy fields for methane, and matching fertiliser dose and timing to crop uptake for nitrous oxide.

• Reading a statement for its topic rather than its wording; both statements here concern real agricultural greenhouse gas sources and only one describes the process correctly
• Overlooking the destination of an emission — a gas released into the soil and a gas released from the soil into the atmosphere are different claims, and only the second describes a greenhouse gas emission
• Assuming methane from rice fields escapes only by bubbling through water, when the plant's own aerenchyma is a principal pathway
• Reasoning about a statement (c) or (d) that this question never printed; only statements (a) and (b) appear

MPSC's environment section asks greenhouse gases through short statement pairs and through single-fact recall of sources, lifetimes and warming potentials. The statement-pair form is the harder one, because the Commission's usual method is to leave the science recognisably correct and alter one small element — a preposition, a direction, a unit, or the identity of the gas. Preparation that stops at 'fertiliser causes nitrous oxide' will not survive it; preparation that holds the full pathway, from application through microbial transformation to atmospheric release, will.

No directly related past PYQ was found.

• practice — not a real PYQ

  Methane emission from a flooded paddy field is chiefly the result of which of the following ?

  • (a)Photosynthesis in the rice plant under high temperature
  • (b)Anaerobic decomposition of organic matter by methanogenic micro-organisms in waterlogged soil
  • (c)The oxidation of urea applied as fertiliser
  • (d)Evaporation of water from the flooded field surface

  Answer(b) Anaerobic decomposition of organic matter by methanogenic micro-organisms in waterlogged soil — standing water excludes oxygen, aerobic decomposers cannot operate, and methanogens break the organic matter down with methane as the end product. The gas then reaches the atmosphere partly by bubbling and diffusion but largely through the aerenchyma of the rice plant.

• practice — not a real PYQ

  Nitrous oxide emitted from agricultural soils is a matter of concern for which two reasons together ?

  • (a)It is a potent greenhouse gas and it contributes to stratospheric ozone depletion
  • (b)It is a potent greenhouse gas and it causes acid rain
  • (c)It depletes soil nitrogen and it forms photochemical smog
  • (d)It is toxic to crops and it acidifies surface water

  Answer(a) It is a potent greenhouse gas and it contributes to stratospheric ozone depletion — nitrous oxide persists in the atmosphere for over a century and has a warming effect per molecule around 265 times that of carbon dioxide over a hundred-year horizon, and it also takes part in reactions that destroy stratospheric ozone. Acid rain is chiefly caused by sulphur dioxide and nitrogen oxides other than nitrous oxide.