Lecture: Chapter 22 - Decomposition and Nutrient Cycling

Question 1

Internal cycling

Answer 1

refers to recycling of nutrients within an ecosystem as they are transformed between inorganic and organic forms

internal cycling is driven by primary production and decomposition

Nitrogen will be used as an example for internal cycling

Question 2

Decomposition

Answer 2

Chemical and biological transformations in ecosystems alter the chemical form and supply of nutrients

Foremost among nutrient transformations is the decomposition of organic matter, which releases nutrients back into the ecosystem

Question 3

Nutrient Transformations

Answer 3

Nutrients in detritus are made available by decomposition: the process by which detrivores break down detritus to obtain energy and nutrients

Decopoisiton releases nutrients as simple, soluble, inorganic compounds that can be taken up by other organisms

Question 4

Decomposition processes:

Answer 4

Leaching - soluble compounds may be washed out from organic dead matter

Fragmentation

Chemical alteration (mineralization)

Question 5

Litter

Answer 5

Fresh, undecomposed organic matter on the soil surface is known as litter

Question 6

Fragmentation

Answer 6

Animals such as earthworms, termites and nematodes consume the litter, breaking it up into progressively finer particles. This fragmentation increases surface area, which facilitates chemical breakdown.

Question 7

Bacteria

Answer 7

The main decomposers of dead animal matter

Question 8

Fungi

Answer 8

Dominant decomposers of dead plants

Question 9

Mineralization

Answer 9

The chemical conversion of organic matter into inorganic (mineral) nutrients by decomposers

Question 10

Immobilization

Answer 10

Decomposers also need nutrients for their own metabolic processes, so they need to uptake and assimilate inorganic (mineral) nutrients (e.g. ammonia)

Question 11

Net mineralization rate (NMR)

Answer 11

Mineralization rate - immobilization rate

NMR depends on nutrient content of the litter and the nutrient demand of the microbial population

Question 12

Factors impacting decomposition rate

Answer 12

Litter quality

Environmental factors (e.g. temperature, precipitation - climate)

Question 13

Litter Quality

Answer 13

Depends on types of carbon compounds present

• High : small molecules with high energy bonds (simple sugars)
• Moderate: Strucutrally more complex, harder to decmopose (cellulose)
• Low: Large molecules with much complexity, hardest to decompose (Lignin)

Question 14

Temperature and decomposition

Answer 14

Decomposition and mineralization rates are faster in warm, moist conditions

Question 15

Requirements for decomposers

Answer 15

Decomposers require energy and nutrients for their own growth and maintenance

Question 16

C:N ratio of detritus

Answer 16

How much nutrients are released into soil depends on the C:N ratio of detritus. Decomposition of organic matter with a C:N ratio of

• > 25:1 all N used by the microorganisms
• <25: net release of N (Nurtients) into the soil available for plant uptake

Lignin content and secondary compounds also affect decomposition rates

Question 17

Rhizosphere.

Answer 17

Decomposition rates are higher in the rhizosphere. It is the region in the soil with high root density and where roots function

Roots alter soil chemistry as they secrete carbohydrates into the soil

Question 18

Immbilization

Answer 18

The availability of high-quality carbon favors bacteria growth. Plants enhance decomposition rate

Bacteria still limited by limited nutrient content immobilization during bacteria growth

But protozoa and nematodes eat bacteria and fungi biomass and remobilize nutrients for plant uptake

Question 19

The Nitrogen Cycle: an atmospheric cycle

Answer 19

Atmospheric N2 — Ocean
|
Land N2
| |
Plant Biomass N2O
| |
Soils ———–Rivers—- Inorganic N

Question 20

Nitrification

Answer 20

NH3 and NH4 are converted to NO3 by chemoautotrophic bacteria in aerobic conditions

Question 21

Denitrification

Answer 21

Some bacteria use NO2 as an electron acceptor, converting it into N2 and N20, in anoxic conditions

Question 22

Mineralization (organic to inorganic)

Answer 22

When C:N is sufficient to meet microbial requirements

Question 23

Immobilization (inorganic to organic)

Answer 23

When C:N in food is insufficient to meet microbial requirements; microorganisms need nutrients for themselves

Question 24

Nutrient cycling in open-water systems

Answer 24

In shallow water along the shoreline sediment-rooted vascualar plants dominate.

Away from shores, the epilimnion is home to algae and zooplankton. Here the layers of production and decomposition are disconnected.

Question 25

Nutrient cycling: summer

Answer 25

Deep lakes and oceans stratify in presence of sunlight

Surface waters, zone of primary producton, “photic zone”, low nutrient content [top waters]

Benthic zone, zone of decomposition, high nutrient content [water along the bottom]

Question 26

Nutrient cycling: Fall

Answer 26

Sun is weaker causing epilimnion to cool off. Thus, water density increases and the heavier water sinks and mixes

Mixing permits Oxygen and nutrients to mix across the whole water column

Fall turnover

Question 27

Nutrient cycling: Winter

Answer 27

Ice covers the surface

Water cannot mix in lack of wind under the ice

No Oxygen supply to hypolimnion for decomposers

When Oxygen runs out in the hypimnion, in the anoxic environment organisms die

Question 28

Nutrient cycling: spring

Answer 28

Spring turn over: ice melts, wind mixes

Oxygen and nutrients mixed across entire water column

PEAK PRODUCTIVITY

Once surface water becomes warmer, stratification re-establishes

Question 29

Upwelling systems

Answer 29

Due to the Coriolis effect, wind-driven surface waters are pushed offshore

This causes the nutrient-rich deeper waters to move vertically to the surface.

Vertical circulation of water is called upwelling

Coastal upwelling of nutrient rich cold deep water support highly productive fisheries

Question 30

Internal cycling

Answer 30

re-cycling of nutrients between organic and inorganic forms driven by primary production and decomposition

Question 31

Decomposition - VERY IMPORTANT

Answer 31

Decomposers and detrivores break down organic matter into soluble inorganic ocmpounds. They need organic matter for energy + nutrients for themselves