13- Nutrient Cycles

Why is it important that elements such as carbon, nitrogen, and phosphorus are recycled?

There is limited availability of nutrients in a usable form

Why do organisms require nitrogen?

• Amino acids

• Nucleic acids

Why is there a need for the nitrogen cycle, despite 78% of the atmosphere being made up of nitrogen?

Very few organisms can use nitrogen gas directly

What form do plants take up nitrogen in?

Nitrate ions

What is the chemical formula of a nitrate ion?

NO₃^-

Plants take up nitrate ions by which process?

Active transport (by the roots)

How do animals obtain nitrogen-containing compounds?

By eating and digesting plants

How are nitrate concentrations restored in natural ecosystems?

Through the nitrogen cycle

Other than through the nitrogen cycle, how can nitrate concentrations be increased in agricultural ecosystems?

Nitrogen-containing fertilisers

What are the four main stages in the nitrogen cycle?

• Ammonification

• Nitrification

• Nitrogen fixation

• Denitrification

Draw the nitrogen cycle from memory.

Describe the process of ammonification.

• The production of ammonia from organic nitrogen-containing compounds

• Saprobionts decompose proteins in faeces and dead organisms, releasing ammonia

• Ammonia forms ammonium ions in the soil

Describe the process of nitrification.

• The conversion of ammonium ions to nitrate ions

• Carried out by nitrifying bacteria in the soil

• First, ammonium ions are oxidised to nitrite ions

• Then, nitrite ions are oxidised to nitrate ions

How do farmers increase the rate of nitrification and decrease the rate of denitrification?

• Nitrifying bacteria are aerobic, denitrifying bacteria are anaerobic

• Farmers keep soil well aerated by ploughing (produces air spaces)

• Farmers drain soil to prevent air spaces filling with water

Which two types of microorganisms carry out nitrogen fixation?

• Free-living nitrogen-fixing bacteria

• Mutualistic nitrogen-fixing bacteria

Describe the process of nitrogen fixation by free-living nitrogen-fixing bacteria.

• Bacteria in the soil reduce nitrogen gas to ammonia

• Ammonia forms ammonium ions in the soil

Describe the process of nitrogen fixation by mutualistic nitrogen-fixing bacteria.

• Bacteria live in nodules on the roots of plants such as peas and beans

• Reduce nitrogen gas to ammonium compounds and convert these to nitrates

• Bacteria obtain carbohydrates from the plant

• Plant obtains amino acids from the bacteria

Describe the process of denitrification.

• Denitrifying bacteria convert nitrates in the soil to nitrogen gas

• Reduces the availability of nitrates for plants

Draw the phosphorus cycle from memory.

What do organisms need phosphorus for?

• ATP

• Phospholipids

• Nucleic acids

What is guano?

A substance made up of excretory products of some sea birds

What can mycorrhizae between fungi and plants be described as?

Mutualistic

Why is the mycorrhizae between fungi and plants described as mutualistic?

• Plant benefits from improved water and inorganic ion uptake provided by the fungi, enabling it to better resist drought

• Fungi receives sugars and amino acids from the plant

What is mycorrhizae?

• The mutualistic relationship between fungi and the roots of plants

• In which the fungi act as extensions of the plant’s root system

• Increases surface area of the plant’s roots so that it absorbs water and minerals more readily

Why are fertilisers needed in agricultural systems?

• Mineral ions are removed from the soil as they are taken up by crops

• Crop is harvested and transported away for consumption

• Mineral ions do not return to the soil through decomposition or excretion in same area of land

• Fertilisers need to be added to soil to combat loss of mineral ions

What are the two types of fertiliser?

• Artificial fertilisers

• Natural fertilisers

What are natural fertilisers?

• Fertilisers that contain the dead and decaying remains of plants and animals

• As well as animal waste such as manure, slurry and bone meal

What are artificial fertilisers?

• Fertilisers that are mined from rocks and then converted into different forms

• Almost always contain nitrogen, phosphorus and potassium

What are the three main harmful consequences of using nitrogen-containing fertilisers?

• Reduced species diversity

• Leaching

• Eutrophication

How do nitrogen-containing fertilisers reduce species diversity?

• Nitrogen-rich soils favour the growth of grasses, nettles and other rapidly growing species

• These outcompete many other species, which die as a result

What is leaching?

• Rainwater dissolves any soluble nutrients such as nitrate ions

• Removes nutrients from the soil and carries them to watercourses such as streams and rivers

What are potential harmful consequences of leaching?

• Could prevent efficient oxygen transport in babies (if the watercourses are a source of drinking water)

• Could cause stomach cancer in humans (if the watercourses are a source of drinking water)

• Can cause eutrophication

Describe the process of eutrophication. (10 marks)

• Nitrate ions are leached into watercourses such as streams and rivers

• As nitrate ion concentration increases, population size of algae increases

• Upper layers of water become densely populated with algae- this is called an ‘algal bloom’

• Algae absorbs light and prevents it from reaching water below

• Plants at lower depths of water die as they cannot photosynthesise

• Population size of saprobionts increases, as they use dead plants as food

• Saprobionts respire, decreasing the concentration of oxygen in water

• Aerobic organisms like fish die, as there is no oxygen available

• Now less competition for anaerobic organisms, so population sizes increase

• Anaerobic organisms decompose dead material further, releasing more nitrates and some toxic wastes like hydrogen sulfide