Nitrogen fertilizers in soils releases ______ when broken down by bacteria.

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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.  (b) 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.  (d) 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.

Concept: 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.

State PCS 2023 Q17 — Environment & Ecology PYQ with Answer

State PCS 2023 Q17 — Environment & Ecology PYQ with Answer