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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