3 Fig. 3.1 shows a simplified diagram of the nitrogen cycle in an agricultural system. Some stages are labelled with the letters P, Q, R and S. Fig. 3.1 (a)...

Labelled answer diagram:

(a) The four processes in the nitrogen cycle: [4]

1. P = Nitrogen fixation - the conversion of atmospheric nitrogen gas (N\(_2\)) into ammonium (NH\(_4^+\)) or nitrate by lightning, free-living soil bacteria (e.g. Azotobacter) or symbiotic bacteria (Rhizobium) in legume root nodules. [1]
2. Q = Nitrification - the conversion of ammonium ions (NH\(_4^+\)) into nitrite and then nitrate (NO\(_3^-\)) by nitrifying bacteria (Nitrosomonas and Nitrobacter) in aerobic soil conditions. Nitrate is the form most readily absorbed by plant roots. [1]
3. R = Denitrification - the conversion of nitrate (NO\(_3^-\)) back into nitrogen gas (N\(_2\)) by denitrifying bacteria under anaerobic (waterlogged) conditions. This returns nitrogen to the atmosphere. [1]
4. S = Decomposition / ammonification - the breakdown of dead organic matter (plant and animal remains, faeces) by decomposer bacteria and fungi, releasing ammonium ions (NH\(_4^+\)) back into the soil. [1]

(b) Why a rice farmer in waterlogged paddy fields loses nitrogen more rapidly: [3]

1. Waterlogged (flooded) soils are anaerobic (lacking oxygen), which creates ideal conditions for denitrifying bacteria to thrive. [1]
2. These bacteria convert nitrate in the soil into nitrogen gas (N\(_2\)) or nitrous oxide (N\(_2\)O), which escapes into the atmosphere and is lost from the soil. [1]
3. Well-drained wheat fields are aerobic, so denitrification proceeds slowly and much less nitrogen is lost as gas. [1]

(c) Two ways nitrogen is returned to the soil naturally (without fertilisers): [2]

1. Decomposition: dead plant material, animal remains and faeces are broken down by soil bacteria and fungi, releasing nitrogen compounds (ammonium) into the soil. [1]
2. Biological nitrogen fixation: nitrogen-fixing bacteria in the root nodules of legumes (e.g. groundnuts, beans) convert atmospheric N\(_2\) into ammonium, which enriches the soil. [1]

(d) How crop rotation with legumes reduces the need for nitrogen fertiliser: [2]

1. Legumes (such as groundnuts, beans or clover) have root nodules containing Rhizobium bacteria that fix atmospheric nitrogen into a plant-usable form. [1]
2. When the legume crop is harvested and its residues are ploughed into the soil (or left to decompose), the nitrogen that was fixed in the roots and plant tissue is released into the soil, making it available for the next crop in the rotation. [1]