Nutrient Cycles

• Ecosystems have an essentially inexhaustible supply of energy

• But chemical elements are available in limited amounts

• Life therefore depends on the recycling of these essential chemical elements

• Nutrient cycles involve abiotic and biotic components and are often referred to as biogeochemical cycles

The Nutrient Cycle• Matter cycles between the biotic environment

and in the abiotic environment.• Simple inorganic molecules (CO2, N2 and H2O)

are assimilated (or fixed) from the abiotic environment by producers and microbes, and built into complex organic molecules (carbohydrates, proteins and lipids).

• These organic molecules are passed through food chains and eventually returned to the abiotic environment again as simple inorganic molecules by decomposers.

• Without either producers or decomposers there would be no nutrient cycling and no life.

Nutrient Cycles• A chemicals specific route through a

cycle varies depending on:– Element– Trophic structure

• Generally 2 categories– Global– Local

• Reservoirs defined by 2 characteristics– Organic or inorganic– Directly available for use by organisms or

not

General Nutrient Cycle

Available inorganicmatter (soil,

atmosphere, water)

Availableorganic materials

(in living organismsor detritus)

Unavailableorganic materials(Coal, Oil, Peat)

Unavailableinorganic materials(minerals in rocks)

Photosynthesis

Assimilation

Respiration

Decomposition

Excretion

Fossilisation

Erosion

Burning of fossil fuels

WeatheringErosion

Formation of sedimentary rocks

Organicnutrients

in producers

Organicnutrients in

Primaryconsumers

Organicnutrients inSecondaryconsumers

Decomposers

Inorganic nutrients

General Nutrient Cycle

Carbon• 4th most abundant element• Essential for life on earth• Every organism requires it for structure, energy or

both• Discounting water we are 50% carbon• Found in a variety of forms, from gaseous states

(CO2) to solid states (limestone, graphite, diamonds, coal) The Cycle

• Carbon moves between the atmosphere, oceans, geosphere and biosphere• Much of this recycling occurs during an animals lifetime and not just after its death• Contains large pools of carbon (sinks or sources)• Has biological and geological components

The Carbon Cycle: Geological Components

• Accumulation of detritus often occurs more quickly than it can be broken down

• Remain stable and unavailable for longer periods• These ‘stores’ can lock carbon up for millions of years• These include:

– Coal– Oil– Peat– Carboniferous rocks– Wood

• Eventually released as carbon dioxide

The Carbon Cycle: Biological Components

• Important role in movement of carbon between the land, sea and air

• Mainly through the processes of photosynthesis and respiration

• Photosynthesis bridges the gap from air to land (moving carbon from CO2 to organic compounds)

• Respiration moves carbon in the other direction• Occurs at timescales of days to 1000s of years

Carboncompoundsin producers

Carboncompounds

Primaryconsumers

Carboncompounds

inSecondaryconsumers

Detritus

Atmospheric CO2

The Carbon Cycle

2 2

Carboncompounds indecomposers

3

4

5

1

5

Carbon lockedup in fossil fuels,

peat and limestone 6

7

Fluctuations in atmospheric CO2

•The concentration of CO2 in the atmosphere is not constant but is always fluctuating

On a local scale there are highs and lows that occur daily

Taken fromwww.mlo.noaa.gov

•On a global scale the fluctuations occur on an annual basis, in response to the seasons

BUT…..

•On top of these fluctuations the CO2 levels over the last few hundred years have been consistently rising, with this rise accelerating at an ever increasing rate

•What are the causes of all these fluctuations?

There are potential conflicts between the need/wish to produce things useful to humans in the short term and the conservation of ecosystems in the long term.

Forests•Contain large stores of carbon in plant tissues•Often hundreds of years old

Fossil fuels (coal, oil, peat)•Huge stores of carbon•Carbon has been ‘locked-up’ for millions of years

Limestone•Huge store of calcium carbonate

So how has the use of these resources by man lead to increases in atmospheric CO2?

Human Activities and the Carbon Cycle

Poster and Presentation Task

• Group 1

– What are the causes of the daily fluctuations in local atmospheric CO2 concentrations?

• Group 2

– How has mans activities effected the carbon stored in fossil fuels and carboniferous rocks?

• Group 3

– How has deforestation caused an increase in atmospheric CO2 concentrations?

Daily CO2 fluctuations

Due to an imbalance between photosynthesis and respiration

• What happens: Photosynthesis and occurs at a faster rate than respiration during the day but at night only respiration takes place

• Result:More CO2 is taken up than is released by the plant

during the day but at night CO2 is not taken up at all

• Consequence:Local atmospheric CO2 concentration falls during the

day and increases at night

Seasonal CO2 fluctuations

Due to a seasonal variation in the rate of photosynthesis on a global scale

• What happens: Photosynthesis occurs at its maximum rate during the summer

• Result:More CO2 is taken up during the summer

• Consequence:The northern hemisphere has more land and therefore

more vegetation than the southern hemisphere so global CO2 levels are lower during the northern hemispheres summer

Burning of fossil fuels• The concentration of CO2 in the atmosphere has

been increasing as a result of the combustion of fossil fuels and burning of wood removed during deforestation

• Long-term perspective:

– Just a return to atmosphere of the CO2 that was removed by photosynthesis millions of years ago

• BUT in the millions of years since:– New equilibrium has developed in the global

carbon cycle– This balance is now being disrupted with

uncertain consequences

Limestone

•Contains carbon locked up as calcium carbonate

•All materials that are exposed to the outdoor environment are subject to degradation caused by natural weathering processes releasing CO2 as a result

•Since the mid-19th century, air pollution (and acid rain in particular) has been suspected of accelerating the degradation of natural and man-made materials.

•The use of limestone and marble in the construction of buildings and monuments exposes a greater surface area to weathering

•This all results in a greater amount of CO2 being released into the atmosphere from geological components

Humans have been clearing areas of forest for thousands of years, using the land for buildings or agriculture – leading to deforestation over large areas of Europe, Asia and North America.

Growth in the world population continues to increase demand for land for farming and more recently deforestation has affected tropical rain forests.

Tropical rain forests are particularly important because:

•Contain 50% of the world’s standing timber

•Huge store of carbon

•Sink for carbon dioxide

Deforestation

•Large scale destruction of rainforests greatly reduces the biomass of plants on the planet

•This results in a huge reduction in photosynthetic activity

•As a consequence less CO2 is removed from the atmosphere resulting a global trend of increasing atmospheric CO2 concentration

•Major contributory factor of global warming

•Rainforest destruction may increase atmospheric concentrations of carbon dioxide by up to 50%

Affect of Deforestation on the Carbon Cycle

The Greenhouse EffectMajor implication of increased atmospheric CO2

CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

Results in a warmer atmosphere

But is it all bad?• An increase in CO2 levels in the atmosphere has

obvious positive implications for plant growth• CO2 is a limiting factor for photosynthesis in

bright conditions• Therefore an increase in CO2 should

theoretically increase photosynthesis• Which plants in particular will benefit from

increased CO2 concentration?– C3 plants are affected more by an increase CO2 than

C4 plants

• Could lead to change in agricultural practices• Could effect species composition in non

agricultural communities