Nutrient cycles

What are the four main stages of the nitrogen cycle?

1) Nitrogen fixation

2) Ammonification

3) Nitrification

4) Denitrification

What is nitrogen fixation?

Nitrogen fixing bacteria convert nitrogen gas in the atmosphere to ammonia (then into ammonium ions), using a reduction reaction catalysed by enzyme nitrogenase 

Mutualistic/symbiotic relationship between bacteria and legumes

Legumes provide glucose/proteins/DNA for bacteria, whilst bacteria provides nitrogen containing compounds e.g ammonium 

What is ammonification? 

Saprobionts e.g bacteria/fungi decompose dead, organic material and convert the organic nitrogen into ammonia, then into ammonium ions for absorption

What are saprobiotic microrganisms and how do they cause decay? 

Bacteria that carry out extracellular digestion by secreting enzymes and digesting nitrogen containing compounds externally 

Absorbs products of digestion and use them e.g ammonia, but some are released back into the atmosphere 

What is nitrification?

Nitrifying bacteria convert ammonium ions into nitrites, then into nitrates (oxidation reactions)

Releases energy which they use for the synthesis of organic molecules e.g carbohydrates

Aerobic conditions

What are phototrophs?

Green plants use light energy to synthesise organic matter

What are chemoautotrophs? 

Some bacteria oxidise chemical compounds e.g nitrites to release energy they need to synthesise organic matter

What is denitrification?

Dentrifying bacteria convert nitrates into nitrogen gas (reduction reaction)

Anaerobic conditions

This occurs in water logged soil, so it can be aerated/drained/ploughed

Which organisms consume nitrogen in living plant material? 

Primary consumers, herbivores and omnivores 

Which types of organisms consume nitrogen in dead plant and animal material? 

Saprobionts 

Which types of organisms consume nitrogen in living animals? 

Secondary/tertiary consumers, carnivores and omnivores 

Why do crops grown with mycorrhizhae have more growth? 

Due to their hypnae which increase surface area- more water uptake and absorption of nutrients needed for growth 

Why are fertilisers used? 

To replace the lost mineral ions and maintain crop productivity

What are the most important inorganic ions found in fertilisers? 

Nitrates (needed for proteins and nucleic acids)

Phosphates (needed for phospholipids, nucleic acids and ATP)

Potassium 

What are the advantages of natural fertilisers?

Useful, cheap means of disposal of farm waste

Adds humus to soil- improve structure and drainage

Nutrients not readily leeched

What are the advantages of artificial fertilisers?

Nutrients concentrated form- can be applied in smaller amounts- makes it more cost effective

Nutrients readily released into soil 

Clean chemicals that lack odour 

What are the disadvantages of natural fertilisers?

Minerals may be released quickly

May be difficult/require heavy machinery to spread 

Unknown quantities/concentration of nutrients 

What are the disadvantages of artificial fertilisers?

Readily leeched from soil,can damage ecosystems of ponds/lakes

Risk of fertiliser spray spreading to other areas 

Expensive to buy/manufacture 

Why will there be reduced species diversity?

Fertilisers increase growth of species e.g nettles. These rapidly growing species outcompete many other species, which die as a result 

They compete for light, mineral ions and water 

This will lead to fewer species of animals as there will be a reduced variety of food and animals

What is leaching? 

Rainwater will dissolve any soluble nutrients e.g nitrates and wash them out of the soil 

The leached nitrates find their away into watercourses, e.g streams and rivers 

This then causes eutrophication 

What is eutrophication? 

In most rivers and lakes, there is naturally very little nitrates- limiting factor for plant and algal growth 

Rapid algal growth 

Absorbs light and prevents it from reaching the plants below the surface- cannot photosynthesise and die 

When the plants die, they are decomposed by saprobionts, which use the oxygen in water to respire aerobically

This decreases oxygen concentration in water, therefore organisms aren’t able to respire and die