3.5.4 Nutrient cycles

There is a limited availability to nutrient ions (that are in a usable form, e.g. most living things cannot use nitrogen in it's gaseous state). The simple nutrient cycle is as follows:

• Nutrient taken up by producers as inorganic molecules
• Producer incorporates the nutrient into complex organic molecules
• The producer is eaten and the nutrient passes into the consumer
• It passes along the food chain when these animals, in turn, are eaten.
• When producers/consumers die their molecules are broken down by saprobionts that release the nutrient in it's original state.

We need to know the nitrogen cycle and the phosphorous cycle:

Nitrogen cycle
Organisms require nitrogen to manufacture nucleic acids/proteins/other nitrogen-contain compounds. Plants need nitrogen in the form  NO₃- which is actively transported from the soil into their roots.

• Ammonification: the production of ammonia from organic nitrogen-containing compounds. These include urea, proteins, nucleic acids, and vitamins. Saprobionts feed on faeces and dead organism. This releases ammonia which forms ammonium ions in the soil.
• Nitrification: Ammonium ions are converted to nitrate ions by means of an oxidation reaction(this releases energy which is where nitrifying bacteria get their energy from). There are two stages:
• oxidation of ammonium ions to nitrite ions (NO₂-)
• oxidation of nitrite ions to nitrate ions (NO₃-)
• Nitrogen fixation: Nitrogen gas is converted into nitrogen-containing compounds (this occurs naturally when lightening strikes). We need to know about two microorganisms that carry this out (there are other forms of nitrogen fixation but the ones by these microorganisms are the most important):
• Free-living nitrogen-fixing bacteria reduce gaseous nitrogen to ammonia which they can use to manufacture amino acids. Nitrogen-rich compounds are released from them when they decay upon death.
• Mutualistic nitrogen-fixing bacteria live in nodules on the roots of plants (such as peas/beans). The plant obtains amino acids from the bacteria and the bacteria obtain carbohydrates from the plants.
• Denitrification: When soil is waterlogged few aerobic nitrifying and nitrogen-fixing bacteria are found and more anaerobic denitrifying bacteria are found. These convert soil nitrates into gaseous nitrogen (this is BAD!!!). This is bad as it reduces the availability of nitrogen-containing compounds for plants.
  

NOTE: nitrifying bacteria require oxygen to oxidise ammonium ions to nitrate ions and consequently to raise productivity farmers plough their soil to aerate it. They also ensure a good drainage system as denitrifying bacteria can work anaerobically so thrive in waterlogged fields.

The phosphorous cycle

Phosphorous is a component of ATP, nucleic acids, and phospholipids.

• Phosphorous exists as phosphate ions (PO₄₃-) in sedimentary rock deposits. Geological uplifting of rocks brings these rocks to the surface (above sea level). Erosion and weathering dissolves the phosphate ions.
• Plants absorb the dissolved ions and incorporate them into their biomass
• Animals feed on the plants. The phosphate ions travel along the food chain.
• Excess phosphate ions are excreted - these are eroded and end up back in the sea forming new sedimentary rock deposits.
• Upon death, decomposers break down animals/plants releasing the phosphate ions back into the soil/water
• The phosphate ions are transported back into the sea where they form sedimentary rock again.

Mycorrhizae
Okay so we need to know a tinsey winsey bit about mycorrhizae. They are associations between certain types of fungi and the roots of some plants. Basically, they act like extensions which increases the surface area of the plant roots which aids the absorption of water and minerals. They hold water and minerals next to the roots which enables the plant to resist drought (to a certain extent). The fungus receives organic compounds (sugars/amino acids) in return so the relationship is mutualistic.


Fertilisers
Okay so we need to know about fertilisers and how they aid farmers etc. Basically, intensive farming means that mineral ions are continuously being taken up by crops but when the crops are harvested the minerals can not get back to the soil - this disrupts the nutrient cycles. This means that is it necessary to replenish these ions because their reduced concentrations will pose a limiting factor to plant growth meaning productivity will reduce.

There are two types of fertiliser a farmer could add:

• natural/organic fertilisers consist of dead/decaying matter as well as animal wastes
• artificial/inorganic fertilisers are mined from rock deposits and converted into different forms and blended together to give the appropriate balance of minerals for a particular crop.

So, how do fertilisers actually increase productivity I hear you ask. Well, when there is a good concentration of nitrogen (for example) in the soil, plants develop earlier, grow taller, and have greater leaf area (which means an increased rate of photosynthesis).

However. fertilisers are not all fun and games. There are some not so good effects including:

• reducing species diversity as nitrogen-rich soils favour the growth of grasses/rapidly growing species such as nettles. These species out-compete some slower growing ones
• leaching which has the potential to pollute watercourses. This is the process by which rainwater dissolved any soluble nutrients (e.g nitrate ions) deep into the soil and eventually into water courses. These can even affect humans as a high nitrate ion concentration in drinking water can prevent efficient O2 transport in babies (there is also a potential link to stomach cancer). Leaching can also cause eutrophication.
• eutrophication (caused by leaching) is the process by which nutrient concentrations increase in bodies of water...
• nitrate ions are a limiting factor for plant/algal growth
• nitrate ion concentration increases due to leaching and nitrate ion concentration ceases to be a limiting factor - plants and algae grow more
• Algae mostly grow at the surface (algal bloom) and this layer absorbs light preventing it from penetrating the lower depths
• Light becomes a limiting factor for plants and lower level growing algae and they die
• saprobionts use this decaying matter as food and their populations grow rapidly increasing the demand for O2
• The O2 concentration in the water is reduced  and nitrates are released from decaying matter
• O2 becomes a limiting factor for aerobic organisms and fish etc eventually die
• without aerobic organisms there is less competition for anaerobic organisms whose populations rise
• anaerobic organisms further decompose dead matter which releases more nitrates and some toxic wastes such as hydrogen sulphide making the water putrid.