Consider the following statements regarding global warming :
(a) Anaerobic Methane is emitted with rice cultivation.
(b) When nitrogen - based fertilizers are used, nitrous oxide is released into the

Question

Consider the following statements regarding global warming :
(a) Anaerobic Methane is emitted with rice cultivation.
(b) When nitrogen - based fertilizers are used, nitrous oxide is released into the soil.
Which of the above statement/statements is/are correct ?

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2024 Q34 — Environment & Ecology PYQ with Answer

State PCS 2024 Q34 — Environment & Ecology PYQ with Answer