nutrient pump (temperate lake turnover). biogeochemical cycles: a few general points (terrestrial...
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Nutrient pump (temperate lake turnover)
BIOGEOCHEMICAL CYCLES: A few general points (terrestrial systems): 1. Nutrient cycling is never perfect i.e.
always losses from system
• Precipitation • Runoff & stream flow
• Particle fallout from atmosphere • Wind loss
• Weathering of substrate • Leaching
• Fertilizer & pollution • Harvesting
Inputs Outputs
(i.e. relatively “tight” cycling is the norm)
2. Inputs and outputs are small in comparison to amounts held in biomass and recycled
3. Disturbances (e.g. deforestation) often “uncouple” cycling
4. Gradient in rates of decomposition and nutrient cycling from poles to tropics
terrestrial systems cont’d…
HUBBARD BROOK FORESTHUBBARD BROOK FOREST
Experiments done to:
1. Describe nutrient budget of intact forest
2. Assess effects of logging on nutrient cycles
catchments
Annual Nitrogen budget for the undisturbed Hubbard Brook Experimental Forest. Values
are Kg, or Kg/ha/yr
Deforestation is a major change in community structure, with a consequent:
loss of nutrients (Krebs Fig 27.7 p567) x20-30 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
Logging causes decoupling of nutrient cycles and losses of nitrogen as nitrates and nitrites
Nitrate losses after logging
Concentrations of ions in streamwater from experimentally deforested, and control, catchments at Hubbard Brook.
logging
Calcium
Potassium
Nitrate-N
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+
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
“laterites”
Relative proportion of Nitrogen in organic matter components
ROOTS
Polar
Tropics
Non-forest Forest
Relative proportion of Nitrogen in organic matter components
SHOOTS
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)
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
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
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
Residence times (years)
Residence times (years)
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)
Litter accumulation in forest floor
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
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
(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
Litter decomposers
micro meso macro
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
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
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