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1. Litter N loss

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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1. Litter N loss

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  1. 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)

  2. 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

  3. 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!

  4. 2. Production of DON • Controlled by the same factors that control decomposition: • Substrate quality (relative availability of C:N) • Temperature • Moisture

  5. 3. Mineralization • (Ammonification or ammonium mineralization) • Microbes take up DON and secrete N as NH4+ if it is available in excess of available C

  6. 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

  7. 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

  8. 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

  9. 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+?

  10. 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

  11. 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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