Ecosystems

Page
6

CO2 in atmosphere

Slide 34

The Terrestrial Nitrogen Cycle

Nitrogen is a component of amino acids, proteins, and nucleic acids

The main reservoir of nitrogen is the atmosphere (N2), though this nitrogen must be converted to NH4+ or NO3– for uptake by plants, via nitrogen fixation by bacteria.

Organic nitrogen is decomposed to NH4+ by ammonification, and NH4+ is decomposed to NO3– by nitrification.

Denitrification converts NO3– back to N2

Slide 35

Nutrient Cycles: Nitrogen Cycle

Decomposers

N2 in atmosphere

Nitrification

Nitrifying

bacteria

Nitrifying

bacteria

Denitrifying

bacteria

Assimilation

NH3

NH4

NO2

NO3

+

–

–

Ammonification

Nitrogen-fixing

soil bacteria

Nitrogen-fixing

bacteria

Slide 36

The Phosphorus Cycle

Phosphorus is a major constituent of nucleic acids, phospholipids, and ATP.

Phosphate (PO43–) is the most important inorganic form of phosphorus.

The largest reservoirs are sedimentary rocks of marine origin, the oceans, and organisms.

Phosphate binds with soil particles, and movement is often localized.

Slide 37

Nutrient Cycles: Phosphorous Cycle

Leaching

Consumption

Precipitation

Plant

uptake

of PO43–

Soil

Sedimentation

Uptake

Plankton

Decomposition

Dissolved PO43–

Runoff

Geologic

uplift

Weathering

of rocks

Slide 38

Decomposition and Nutrient Cycling Rates

Decomposers = detritivores play a key role in the general pattern of chemical cycling.

Rates at which nutrients cycle in different ecosystems vary greatly, mostly as a result of differing rates of decomposition.

The rate of decomposition is controlled by temperature, moisture, and nutrient availability.

Rapid decomposition results in relatively low levels of nutrients in the soil.

Slide 39

How does temperature affect litter decomposition in an ecosystem?

Ecosystem type

EXPERIMENT

RESULTS

Arctic

Subarctic

Boreal

Temperate

Grassland

Mountain

P

O

D

J

R

Q

K

B,C

E,F

H,I

L

N

U

S

T

M

G

A

A

80

70

60

50

40

30

20

10

0

–15

–10

–5

0

5

10

15

Mean annual temperature (ºC)

Percent of mass lost

B

C

D

E

F

G