Nutrient Cycle

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Net productivity=

gross productivity - respiratory losses

Efficiency % between trophic levels=

(net productivity of a level/net productivity of previous level) x100


1/time or difference/time 

% Difference= (final-initial)/final (x100)

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Nitrogen Cycle

NITROGEN FIXATION- free living nitrogen fixing bacteria in root nodules

AMMONIFICATION- nitrogen compounds from dead organisms are turned into ammonium compounds by decomposers

NITRIFICATION- ammonium ions to nitrate ions by nitrifying bacteria

DENITRIFICATION- nitrates converted to gaseous nitrogen by denitrifying bacteria in anaerobic conditions

1. Nitrates leached from fertilised fields stimulate growth of algae in ponds etc.
2. Large amounts of algae block light from reaching plants below
3. Plants die as they are unable to photosynthesise
4. Bacteria feed on the ded plant matter
5. Increased numbers of bacteria reduce the oxygen concentration in water by carrying out aerobic respiration
6. Fish etc. die as there isn't enough dissolved oxygen

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1.    Less photosynthesis so less removal of carbon dioxide

2.    Usually involves burning unwanted trees, and expanding human populations burn more wood for fuel so combustion creates more carbon dioxide

3.    Less carbon dioxide is removed from the atmosphere and more is added.  This adds to the problem of global warming

4.    With the trees gone there has been large scale erosion due to the high rainfall in these area washing away the soil.

5.    In forests, most of the nitrate ions (and other mineral ions) absorbed by plants come from decomposition of organic remains – the ions are recycled

6.    Many of the decomposing fungi live in association with the roots of trees

7.    The soil is often a poor source of mineral ions. 

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Therefore, deforestation results in: Reduced input to the nitrogen cycle Slower and less recycling of nitrates (and other ions) Increased loss of nitrates by leeching The soil loses fertility can support lower numbers and fewer species of plants à lower diversity 

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Hardy Weinberg

1.    Predicts the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next

2.    No mutations arise

3.    The population is isolated (no emmigration/immigration)

4.    There is no selection, all alleles are equally likely to be passed on

5.    The population is large

6.    Random mating

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Inorganic Fertilisers

žAdvantages: •Are very effective •Contain concentrated amounts of macronutrients and so can be applied in smaller amounts •Are clean and do not smell •Are convenient to handle and apply evenly on the crop ž žDisadvantages: •Cause eutrophication. •Expensive. 

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Organic Fertilisers Advantage

žSlower rate of release: •Last longer and less likely to cause leaching as less soluble than inorganic fertilisers •Therefore the inorganic minerals are released more slowly as they are decomposed. žCheaper: •The organic wastes need to be disposed of anyway, so this is a way of disposing of waste ž žImproves soil structure: •Binds soil particles together and provides food for soil organisms such as earthworms. •This improves drainage and aeration. ž žLess likely to cause nitrate pollution (eutrophication): As less likely to cause leaching

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žBulky •Harder to handle, spread and apply žLess concentrated in minerals •So more needs to be spread on a field to have a similar effect. ž žMay contain unwanted substances •E.g. fungal spores, heavy metals ž žVery smelly! 

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Photosynthesis Efficiency

Efficiency of photosynthesis

chemical potential energy/total amount of light energy on plants

chemical potential= multiply chemical potential energy in 1g of plant by the biomass of plant produced in gm-2year-1

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Chemical Pesticides

Contact pesticides

  • directly onto a crop
  • absorbed by bug through gas-exchange pores
  • cheap but have to be reapplied
  • short lived

Systemic pesticides

  • sprayed onto crop
  • absorbed by leaves
  • insect feeds on leaves
  • doesn't have to come into contact with every pest to work
  • selective

Residual pesticides

  • sprayed onto soil/treat seeds before crop is planted
  • remain active in the soil
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Monoculture -> growing a single type of crop over a wide area

it allows:

-Easier control of pests -> limited by the type of crop -Maximised profit -> growing high gross crops Reduced amount of ecological niches Not a lot of habitats- less organisms can live in one area Less food resources Spreading a large amount of pesticide may have an effect on the environment

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