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Nutrient cycling & Nutrient cycling & Ecosystem HealthEcosystem Health
READINGS for this lecture series:READINGS for this lecture series:
• KREBS chap 27. Ecosystem Metabolism III: Nutrient Cycles
• KREBS chap 28. Ecosystem Health:
Human Impacts; Pp 590 – 600
• WEB Downloads
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NUTRIENT CYCLINGNUTRIENT CYCLING• Energy – 1-way flow
- eventually gets “lost”
• Nutrients – cycle
Organic
(living organisms)
Inorganic
(rocks, air, water)
assimilation
mineralization
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Two main types of cycles: 1. Biochemical cycles:
• Redistribution within an individual organism• This relates to r- and K-selection (Biol 303)
2. Biogeochemical cycles:• “Local” - exchange occurs within and between terrestrial/aquatic ecosystems• “Global” – exchange occurs between atmosphere and terrestrial/aquatic ecosystems
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Two main types of cycles: 1. Biochemical cycles:
• Redistribution within an individual organism• This relates to r- and K-selection (Biol 303)
2. Biogeochemical cycles:• “Local” - exchange occurs within and between terrestrial/aquatic ecosystems• “Global” – exchange occurs between atmosphere and terrestrial/aquatic ecosystems
e.g. CO2, SO2, NOx
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Krebs Fig. 27.12; p573
SULPHUR CYCLE
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Krebs Fig. 28.8; p591
CARBON CYCLE
respiration
photosynthesis
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Krebs Fig. 27.17; p579
NITROGEN CYCLE
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78% of air
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These figures have:
• All sorts of rates of transfer
• We can compare between systems
More interesting:
• What influences the rates?
• What are the impacts of altering the rates?
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These figures have:
• All sorts of rates of transfer
• We can compare between systems
More interesting:
• What influences the rates?
e.g. forms of nutrients, types of organisms
• What are the impacts of altering the rates?
e.g. disturbance, pollution, etc.
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Compartment Models
Quantitative descriptions of storage and movement of nutrients among different compartments of an ecosystem
• “Coarse” – few broad compartments
e.g. plants, herbivores• “Fine” – many detailed compartments
e.g. separate species
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Compartment Models
POOL – “the quantity of a particular nutrient in a compartment”
FLUX – “the quantity moving from one pool to another per unit time”
TURNOVER TIME – “the time required for movement of an amount of nutrient equal to the quantity in the pool” (POOL/FLUX)
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Krebs Fig. 27.2 p562 Phosphorus cycle in a lake (simplified)
Turnover time (water):
9.5 (pool) /152 (flux) = 0.06 day
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NUTRIENT PUMP• Any biotic or abiotic mechanism
responsible for continuous flux of nutrients through an ecosystem
• Biotic – tree roots, sea birds,
Pacific salmon
• Abiotic – lake overturn, ocean upwelling
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Nutrient pump (Terrestrial)
Mycorrhizae
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Mycorrhizae
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Mycorrhizae
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Soil micelles
“CEC” Cation Exchange Capacity
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Nutrient pumps (Marine)
Microbial loop
Upwelling
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Nutrient pump (temperate lake turnover)
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BIOGEOCHEMICAL CYCLES: A few major points (general principles): 1. Nutrient cycling is never perfect i.e.
always losses from system• input and output (terrestrial systems)
• Precipitation • Runoff & stream flow
• Particle fallout from atmosphere • Wind loss
• Weathering of substrate • Leaching
• Fertilizer & pollution • Harvesting
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3. Relatively 'tight' cycling is the norm
2. Inputs and outputs are small in comparison to amounts held in biomass and recycled
4. Disturbances (e.g. deforestation) often uncouple cycling
5. Gradient from poles to tropics
terrestrial systems cont’d…
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HUBBARD BROOK FORESTHUBBARD BROOK FOREST
Experiments done to:
1. Describe nutrient budget of intact forest
2. Assess effects of logging on nutrient cycles
catchments
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Annual Nitrogen budget for the undisturbed Hubbard Brook Experimental Forest. Values
are Kg, or Kg/ha/yr
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Disturbances (e.g. deforestation) often uncouples cycling, and a consequent:
loss of nutrients (Krebs Fig 27.7 p567) x13 normal loss of NO3 in Hubbard Brook
reduction in leaf area 40% more runoff (would have transpired) more leaching more erosion, and soil loss
decouples within-system cycling of decomposition and plant uptake processes all the activities (and products) of spring decomposition get
washed away
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Logging causes decoupling of nutrient cycles and losses of nitrogen as nitrates and nitrites
Nitrate losses after logging
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Concentrations of ions in streamwater from experimentally deforested, and control, catchments at Hubbard Brook.
logging
Calcium
Potassium
Nitrate-N
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H+ >Ca++>Mg++>K+>Na+
NH3, NH4 NO2- NO3
-
1) Logging causes increased nitrification:
2) H+ displace nutrient cations from soil micelles
Uncoupling of N-cycle
H+ H+
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POLAR TROPICS
Decomposition Slow Rapid
Proportion nutrients in living biomass
Low (mostly in dead organic
matter)
High
Cycling Slow Rapid
5. Gradient from poles to tropics
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“laterites”
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Relative proportion of Nitrogen in organic matter components
ROOTS
Polar
Tropics
Non-forest Forest
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Relative proportion of Nitrogen in organic matter components
SHOOTS
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DECOMPOSITIONIF TOO SLOW:
• Nutrients removed from circulation for long periods
• Productivity reduced
• Excessive accumulations of organic matter (e.g. bogs)
IF TOO FAST:
• Nutrient depletion
• Poor chemistry and physics of soil (e.g. decreased soil fertility, soil moisture and resistance to erosion) (e.g. tropical laterites)
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WHAT DETERMINES DECOMPOSITION RATES IN FORESTS?
moisture and temperature pH of litter and the forest floor
more acid promotes fungi, less bacteriaspecies of plant producing the litter chemical composition of the litter
C/N ratio - high gives poor decomposition microbes need N to use C
N often complexed with nasties (e.g. tannin)
optimum is 25:1
Douglas fir wood 548:1 Douglas fir needles 58:1 alfalfa hay 18:1
activities of soil fauna e.g. earthworms
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Decomposition Rates influenced by:• temperature• moisture• pH, O2
• quality of litter• soil type (influences bugs)• soil animals• type of fauna / flora
• rapid if bacterial• slow if fungal
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RATE OF DECOMPOSITION• humid tropical forests about 2 - 3 weeks• temperate hardwood forests 1 - 3
years• temperate / boreal forests 4 - 30 yr• arctic/alpine / dryland forests >40 years
• generally, rate of decomposition increases with increased amount of litterfallResidence time … the time required for the
complete breakdown of one year’s litter fall
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Residence times (years)
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Residence times (years)
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Decomposition Rates influenced by:• temperature• moisture• pH, O2
• quality of litter• soil type (influences bugs)• soil animals• type of fauna / flora
• rapid if bacterial• slow if fungal
(mineral content, C/N ratio)
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Plant species
% weight loss in 1
year
C/N ratio
# bacterial colonies
#
fungal colonies
Bact / Fungi ratio
Mulberry 90 25
Redbud 70 26
White Oak 55 34
Loblolly pine
40 43
Relationship between rate of litter decomposition Relationship between rate of litter decomposition and litter quality (C/N ratio)and litter quality (C/N ratio)
Faster decomposition at lower C/N ratiosFaster decomposition at lower C/N ratios
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Decomposition Rates influenced by:• temperature• moisture• pH, O2
• quality of litter• soil type (influences bugs)• soil animals• type of fauna / flora
• rapid if bacterial• slow if fungal
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(J) J A S O N D J F M A
100
90
80
70
60
50
40
30
20
10
0
% leaf litter
remaining
0.5 mm mesh bags
7.0 mm mesh bags
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Litter decomposers
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Decomposition Rates influenced by:• temperature• moisture• pH, O2
• quality of litter• soil type (influences bugs)• soil animals• type of fauna / flora
• rapid if bacterial• slow if fungal
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Plant species
% weight loss in 1
year
C/N ratio
# bacterial colonies
#
fungal colonies
Bact / Fungi ratio
Mulberry 90 25 698 2650 264
Redbud 70 26 286 1870 148
White Oak 55 34 32 1880 17
Loblolly pine
40 43 15 360 42
Relationship between rate of litter decomposition Relationship between rate of litter decomposition and the balance between bacteria and fungiand the balance between bacteria and fungi
Faster decomposition at higher bact/fungi ratiosFaster decomposition at higher bact/fungi ratios
x102
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