MPSC 2023 Prelims Paper-I · Question 17 of 100

Correct — option (3). What soil bacteria do to a nitrogenous fertiliser is called nitrification, and its product is nitrate, the NO3 of the printed option. Follow the sequence. A urea or ammonium fertiliser added to the soil is first converted to the ammonium ion: urea is hydrolysed by the enzyme urease, which soil micro-organisms supply in abundance, to give ammonium. Ammonium is then oxidised in two bacterial steps — Nitrosomonas and related bacteria take it to nitrite, and Nitrobacter and its relatives take nitrite on to nitrate — and both steps require oxygen, so this is what happens in ordinary aerated farmland. Nitrate is where the sequence stops, and it stops there for a reason that matters agriculturally: nitrate is the form in which most crop plants actually absorb nitrogen through their roots. It is also the form that causes the trouble. Being a negatively charged ion, nitrate is not held by the negatively charged clay and humus surfaces of the soil, so it is not retained the way ammonium is; irrigation and rain wash it downwards into groundwater and sideways into streams, where it produces the nitrate contamination of drinking water associated with methaemoglobinaemia in infants, and the nutrient enrichment that drives algal blooms and eutrophication in lakes and coastal waters. That is the whole environmental case against the heavy use of nitrogenous fertiliser, and it rests on the bacterial conversion the stem describes. One caution about the printed option: NO3 is the nitrate ion, written NO3 with a negative charge, and 'nitrogen trioxide' is not the name a chemist would give it — the paper's label is loose, but the formula identifies the substance without ambiguity.

• (1)Nitrogen dioxide (NO2) — Nitrogen dioxide is a reddish-brown gas produced when nitrogen and oxygen combine at high temperature — in vehicle engines, in furnaces and in power stations — and it belongs to the chemistry of combustion and air pollution rather than to the chemistry of soil. It is one of the oxides of nitrogen grouped as NOx, it takes part in the reactions that generate photochemical smog and ground-level ozone, and it contributes to acid deposition. None of that is a bacterial process, and none of it happens to fertiliser in the ground. The option is placed first because its formula looks very like the keyed one, and a candidate reading quickly can confuse a subscript 2 with a subscript 3.
• (2)Nitrous oxide (N2O) — Nitrous oxide does come out of fertilised soil, but by a different route from the one the stem describes. It is a product of denitrification, which happens when soil is waterlogged and oxygen is short: bacteria then use nitrate instead of oxygen as an electron acceptor and reduce it in stages, releasing nitrous oxide and finally nitrogen gas to the atmosphere. That is a loss pathway operating on nitrate after nitrate has been formed, and it requires anaerobic conditions rather than the ordinary aerated soil in which fertiliser is broken down. Nitrous oxide is best known from the climate chapter, as a long-lived greenhouse gas, and that association is what makes this option attractive in a question about what fertiliser releases.
• (4)Nitrogen (N2) — Nitrogen gas is the starting point of the nitrogen cycle rather than the product of fertiliser breakdown. It makes up about seventy-eight per cent of the atmosphere and is almost inert, which is precisely why it has to be fixed — by Rhizobium in root nodules, by free-living soil bacteria, by lightning or industrially by the Haber-Bosch process — before plants can use it at all. Manufacturing fertiliser is the work of converting this unusable nitrogen into a usable form; it would defeat the purpose if soil bacteria simply converted it back. Nitrogen gas is released from soil only at the far end of denitrification, under waterlogged conditions, and that is a loss to the farmer rather than the routine fate of the fertiliser.

The nitrogen cycle is best held as a set of named conversions, each with the organisms that carry it out. Fixation turns atmospheric nitrogen gas into ammonia or ammonium, and is done by Rhizobium in legume nodules, by free-living bacteria such as Azotobacter, by cyanobacteria, by lightning, and industrially by the Haber-Bosch process, which is where fertiliser nitrogen comes from. Ammonification returns the nitrogen in dead tissue and urea to ammonium through decomposers. Nitrification oxidises ammonium to nitrite and then to nitrate, in two aerobic bacterial steps associated with Nitrosomonas and Nitrobacter; nitrate is the form most plants absorb, and it is the form fertiliser nitrogen ends up in after soil bacteria have worked on it. Assimilation is the plant's uptake and incorporation of that nitrogen into amino acids and proteins. Denitrification runs the cycle backwards under waterlogged, oxygen-poor conditions, reducing nitrate through nitrous oxide to nitrogen gas and returning it to the air. The environmental consequences follow from the chemistry: nitrate leaches because it carries a negative charge and the soil cannot hold it, so excess fertiliser reaches groundwater and surface water, while denitrification in wet soils releases nitrous oxide, a potent greenhouse gas.

MPSC's science section takes most of its environment-linked questions from cycles — nitrogen, carbon, phosphorus and water — and the nitrogen cycle is the one that recurs most, because it connects agriculture, water pollution and climate in a single diagram. Questions are usually of two kinds: name the organism responsible for a named step, or name the product of a named step. Both are answered by holding the cycle as a chain of conversions rather than as a picture, and by remembering which steps need oxygen and which need its absence — that single distinction separates nitrification from denitrification and answers a large proportion of what is asked. This question is printed as a fill-in-the-blank, a format the Commission uses freely, and its chemical formulae are set with true subscripts on the page. The names attached to the formulae are not always the names a chemist would use, so the safest reading is to take the formula as the identification and to treat the words beside it as a label.

• Nitrification is the aerobic bacterial oxidation of ammonium to nitrite by Nitrosomonas and of nitrite to nitrate by Nitrobacter, and it is what soil bacteria do to a urea or ammonium fertiliser.
• Nitrate is the form in which most crop plants absorb nitrogen through their roots, which is why the sequence stops there in ordinary aerated farmland.
• Because the nitrate ion carries a negative charge it is not held by the negatively charged clay and humus surfaces of the soil, so it leaches readily into groundwater and runs off into streams.
• Nitrate contamination of drinking water is associated with methaemoglobinaemia in infants, and nitrate enrichment of surface water drives algal blooms and eutrophication.
• Nitrous oxide and nitrogen gas leave the soil by denitrification, a different pathway that operates on nitrate under waterlogged, oxygen-poor conditions rather than in aerated soil.
• The option keyed here prints the formula NO3 under the name 'nitrogen trioxide'; NO3 is the nitrate ion, and the formula rather than the printed name is what identifies the substance.

• Confusing the oxides of nitrogen with one another; a hurried reading can turn a subscript 3 into a subscript 2 and change the answer entirely
• Mixing up nitrification and denitrification, when the first is an aerobic build-up to nitrate and the second an anaerobic breakdown of it
• Assuming that fertiliser breaks down to nitrogen gas, when converting unusable atmospheric nitrogen into a usable form is the whole purpose of making fertiliser
• Reading the printed name beside a formula as authoritative; in this option set the formula identifies the substance more reliably than the words do

Biogeochemical cycles are asked in MPSC papers as short recall items — which bacterium performs a named conversion, what a named process produces, which form of a nutrient plants absorb — and occasionally as statement-combination questions about the environmental effects of fertiliser use. The nitrogen cycle appears more often than the others because it supports questions in three different sections at once: general science, agriculture and environment. Expect fill-in-the-blank stems, expect chemical formulae to be printed with subscripts that may or may not survive reproduction, and expect the four options in a chemistry item to be four closely similar formulae, so that the mark turns on reading accurately rather than on knowing more. It is worth being able to write the cycle out from memory in five steps with an organism and a product against each.

No directly related past PYQ was found.

• practice — not a real PYQ

  In the soil, the oxidation of ammonium first to nitrite and then to nitrate is carried out by which bacteria, and the process is called what ?

  • (a)Rhizobium and Azotobacter; nitrogen fixation
  • (b)Nitrosomonas and Nitrobacter; nitrification
  • (c)Pseudomonas and Thiobacillus; denitrification
  • (d)Clostridium and Bacillus; ammonification

  Answer(b) Nitrosomonas and Nitrobacter; nitrification — Nitrosomonas and related bacteria oxidise ammonium to nitrite, and Nitrobacter and its relatives carry nitrite on to nitrate. Both steps require oxygen. Denitrification is the reverse process, carried out under waterlogged conditions by organisms such as Pseudomonas, which reduce nitrate through nitrous oxide to nitrogen gas.

• practice — not a real PYQ

  Nitrate from fertilised fields reaches groundwater far more readily than ammonium does, chiefly for which reason ?

  • (a)Nitrate is a gas at soil temperature
  • (b)Nitrate is negatively charged and is not held by soil colloids
  • (c)Nitrate is insoluble and moves as suspended particles
  • (d)Nitrate is absorbed by plants faster than ammonium

  Answer(b) Nitrate is negatively charged and is not held by soil colloids — clay and humus surfaces carry a negative charge, so they retain positively charged ammonium but repel the nitrate ion, which therefore moves freely with percolating water into groundwater and with runoff into streams. This is the basis of nitrate contamination of drinking water and of the eutrophication of surface waters.