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1. Litter N loss. Physical: leaching can remove 0-20\% (?) of N pool. Biological: net process Fungi and bacteria take up N from the soil solution to “prime” breakdown of high C:N litter ( net immobilization ). Net loss of N occurs when relative availability of C is low (so low C:N).

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slide1
1. Litter N loss
  • Physical: leaching can remove 0-20% (?) of N pool
  • Biological: net process
    • Fungi and bacteria take up N from the soil solution to “prime” breakdown of high C:N litter (net immobilization)
  • Net loss of N occurs when relative
  • availability of C is low (so low C:N)
slide2
Leaves

Twigs and stems

100

90

80

70

60

% initial mass remaining

50

Species

% (initial mass remaining)

40

Dod. Sch.

W

30

Veg. Treat.

20

W+G

10

G

0

150

130

110

% initial N remaining

% (initial N remaining)

90

70

50

0

100

200

300

400

500

600

700

0

100

200

300

400

500

600

700

Day

Day

Mack and D’Antonio 2003

slide3
Proteases Proteins Amino acids

Ribonucleases Nucleic acids Nucleotides

Chitinases Chitins

Cellulase N polymers

  • 2. Production of DON
  • Bacteria and fungi secrete extracellular enzymes to break complex polymers down into to water soluble units that can pass through cell membranes

Microbes have to spend protein (exoenzymes)

to get protein!

slide4
2. Production of DON
  • Controlled by the same factors that control decomposition:
    • Substrate quality (relative availability of C:N)
    • Temperature
    • Moisture
slide5
3. Mineralization
  • (Ammonification or ammonium mineralization)
  • Microbes take up DON and secrete N as NH4+ if it is available in excess of available C
slide6
Microbes only mineralize N when they are starved for C!

Example:

Microbial C:N = 10:1

Microbial growth efficiency = 40% of C can be used for

Growth (@ C:N of 10:1) and 60% is respired as CO2

immobilize N from soil sol’n

microbes break even

microbes mineralize N

Substrate C:N = 50

Substrate C:N = 25

Substrate C:N = 15

attractively simple but
Attractively simple, but…
  • Microbe C:N differs
  • Among microbe taxa
  • With temperature and moisture
  • With resource availability

Microbial growth efficiency differs

  • Among microbe taxa
  • With temperature and moisture
  • With different substrates
slide8
Mineralization terms:

Gross NH4+ mineralization = Total amount of NH4+ mineralized regardless of fate

Gross NH4+ immobilization = Total amount of NH4+ taken up by microbes

Net NH4+ mineralization = Net accumulation of N in soil solution that is presumably available for plant uptake; N in excess of microbial demand

Net NH4+ immobilization = Net reduction of N in soil solution

slide10
Fate of NH4+
  • Immobilized microbes
  • Taken up by a plant
  • Adsorbed on soil exchange complex--available
  • Stabilized on clays or SOM--unavailable
  • Volatilized as ammonia (NH3)
  • Oxidized by microbes to nitrite, nitrate (NO2-, NO3-)

What determines the fate of NH4+?

slide11
4.Nitrification (ammonium oxidation)
  • Oxidation of NH4+ NO2- and NO2- NO3-
  • Chemoautotrophic bacteria-obligate aerobes that derive C
  • from CO2 and get energy from the oxidation of NH4+ NO3-
  • Some heterotrophic bacteria and fungi can also oxidize
  • NH4+ to NO3-, but they do not get any energy from it…
  • NO and N2O produced as gaseous by-products of
  • nitrification
slide12
Fate of NO3-
  • Immobilized by microbes
  • Taken up by a plant
  • Leached out of ecosystem into groundwater
  • May absorb on anion exchange sites in some highly weathered tropical soils
  • Reduction by denitrifying microbes to NOx, N2O, or N2

What determines the fate of NO3-?

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