Lecture: Chapter 22 - Decomposition and Nutrient Cycling Flashcards
Internal cycling
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
Decomposition
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
Nutrient Transformations
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
Decomposition processes:
Leaching - soluble compounds may be washed out from organic dead matter
Fragmentation
Chemical alteration (mineralization)
Litter
Fresh, undecomposed organic matter on the soil surface is known as litter
Fragmentation
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.
Bacteria
The main decomposers of dead animal matter
Fungi
Dominant decomposers of dead plants
Mineralization
The chemical conversion of organic matter into inorganic (mineral) nutrients by decomposers
Immobilization
Decomposers also need nutrients for their own metabolic processes, so they need to uptake and assimilate inorganic (mineral) nutrients (e.g. ammonia)
Net mineralization rate (NMR)
Mineralization rate - immobilization rate
NMR depends on nutrient content of the litter and the nutrient demand of the microbial population
Factors impacting decomposition rate
Litter quality
Environmental factors (e.g. temperature, precipitation - climate)
Litter Quality
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)
Temperature and decomposition
Decomposition and mineralization rates are faster in warm, moist conditions
Requirements for decomposers
Decomposers require energy and nutrients for their own growth and maintenance
C:N ratio of detritus
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
Rhizosphere.
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
Immbilization
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
The Nitrogen Cycle: an atmospheric cycle
Atmospheric N2 — Ocean
|
Land N2
| |
Plant Biomass N2O
| |
Soils ———–Rivers—- Inorganic N
Nitrification
NH3 and NH4 are converted to NO3 by chemoautotrophic bacteria in aerobic conditions
Denitrification
Some bacteria use NO2 as an electron acceptor, converting it into N2 and N20, in anoxic conditions
Mineralization (organic to inorganic)
When C:N is sufficient to meet microbial requirements
Immobilization (inorganic to organic)
When C:N in food is insufficient to meet microbial requirements; microorganisms need nutrients for themselves
Nutrient cycling in open-water systems
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.
Nutrient cycling: summer
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]
Nutrient cycling: Fall
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
Nutrient cycling: Winter
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
Nutrient cycling: spring
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
Upwelling systems
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
Internal cycling
re-cycling of nutrients between organic and inorganic forms driven by primary production and decomposition
Decomposition - VERY IMPORTANT
Decomposers and detrivores break down organic matter into soluble inorganic ocmpounds. They need organic matter for energy + nutrients for themselves
