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Month Day Topic Sept. 25 Decomposition 30 Soil organic matter Oct. 2 Exam 7 Nutrient cycles 9 Nutrient uptake and use 14 Nitrogen inputs 16 Nitrogen dynamics 21 Nitrogen loss 23 Phosphorus C,M&M 7 7 1-7 8 8 9 9 9 9 NH3+ N2 NOx, N2O, N2 Litter SON DON Erosion Leaching NH4+ NO3- Leaching 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) Leaves Twigs and stems % (initial mass remaining) 90 80 70 60 50 Species 40 Dod. Sch. 30 W 20 W+G 10 G Veg. Treat. % initial mass remaining 100 % (initial N remaining) % initial N remaining 0 150 130 110 90 70 50 0 100 200 300 400 Day 500 600 700 0 100 200 300 400 500 600 700 Day Mack and D’Antonio 2003 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 Proteases Ribonucleases Chitinases Cellulase Proteins Nucleic acids Chitins N polymers Amino acids Nucleotides Microbes have to spend protein (exoenzymes) to get protein! 2. Production of DON • Controlled by the same factors that control decomposition: • Substrate quality (relative availability of C:N) • Temperature • Moisture 3. Mineralization (Ammonification or ammonium mineralization) • Microbes take up DON and secrete N as NH4+ if it is available in excess of available C 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 Substrate C:N = 50 Substrate C:N = 25 Substrate C:N = 15 immobilize N from soil sol’n microbes break even microbes mineralize N 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 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 Fate of NH4+ 1. Immobilized microbes 2. Taken up by a plant 3. Adsorbed on soil exchange complex--available 4. Stabilized on clays or SOM--unavailable 5. Volatilized as ammonia (NH3) 6. Oxidized by microbes to nitrite, nitrate (NO2-, NO3-) What determines the fate of NH4+? 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 Fate of NO31. Immobilized by microbes 2. Taken up by a plant 3. Leached out of ecosystem into groundwater 4. May absorb on anion exchange sites in some highly weathered tropical soils 5. Reduction by denitrifying microbes to NOx, N2O, or N2 What determines the fate of NO3-?