Theory question 107
Nutrient Cycling(For candidates in Nigeria only)
(a) Explain the following terms:
(i) putrefaction; (ii) nitrogen fixation; (iii) nitrification; (iv) denitrification
(b) State three differences between nitrogen fixation and denitrification.
(c) Explain how nitrogen from urine gets to the roots of plants.
Show the model answer
(a)(i) Putrefaction: breakdown of dead organisms and wastes by bacteria and fungi, releasing ammonia/ammonium compounds.
(ii) Nitrogen fixation: conversion of free atmospheric nitrogen into nitrogen compounds (ammonia, nitrates) by bacteria such as Rhizobium, Azotobacter, or by lightning.
(iii) Nitrification: oxidation of ammonium compounds to nitrites (Nitrosomonas) and then to nitrates (Nitrobacter).
(iv) Denitrification: conversion of nitrates back to free nitrogen gas by denitrifying bacteria (e.g. Pseudomonas).
(b) Fixation adds nitrogen compounds to the soil / denitrification removes them; fixation turns N2 into compounds / denitrification turns nitrates into N2; fixation is done by nitrogen-fixing bacteria (and lightning) / denitrification by denitrifying bacteria; fixation favoured by aerated soil / denitrification by waterlogged, poorly aerated soil; fixation increases soil fertility / denitrification reduces it.
(c) Urea in urine is broken down by bacteria into ammonia/ammonium compounds; nitrifying bacteria convert ammonium to nitrites then nitrates; nitrates dissolve in soil water; roots absorb them by active transport.
Name the bacteria involved at each step.