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  1. NH3+ N2 NOx, N2O, N2 Litter SON DON NH4+ NO3- Leaching Erosion Leaching

  2. When is NH3 produced? NH4+ + OH- NH3 + H2O If soil pH >9.3, then >50% of NH4+ goes to NH3 • Ammonia Volatilization • NH3 (ammonia gas) is emitted due to the pH-dependent equilibrium between NH4+ and NH3 What controls the production of NH3? • Depends on pH and [NH4+] What types of ecosystems show lots of ammonia volatilization?

  3. NH3+ N2 NOx, N2O, N2 Litter SON DON NH4+ NO3- Leaching Erosion Leaching

  4. When are NO, N2O, and N2 produced? Nitrification – NO, N2O Denitrification – NO, N2O, N2

  5. NO3- N2 N2O NO2- NOx • Denitrification--reduction of nitrite and nitrate • NO2- and NO3- reduced to NOx, N2O, and N2 • Facultative anaerobic bacteria that use NO2- or NO3- as an alternate electron acceptor when O2 is low; get C from organic matter • Rate depends on  [O2],  [NO3-],  C-availability What types of ecosystems show lots of denitrification?

  6. Denitrification--Fate • Gaseous NOx, N2O, and N2 can be scavenged by microbes and plants, or may be lost from the ecosystem to the atmosphere

  7. What controls the production of NO and N2O? • During Nitrification • Rate: as flow increases, “leaks” increase • 10 to 20 times as much NO is produced as N2O • During Denitrification • Rate: as flow increases, “leaks” increase • When NO3- is relatively more abundant than labile organic carbon, more N2O is produced than N2 • N2O > N2 in conditions of low pH, low temp., and low O2 • Most NO gets reduced

  8. What controls how much NO and N2O leaves the ecosystem? • Rate of production • Soil texture: diffusion path length to escape soil • Stratification of microbial activity • Plant canopy cover

  9. N2 is the major pathway for return of N from ecosystems to the atmosphere • After N2, loss of NOX = NH3 >> N2O

  10. If N trace gasses are << 10% of N losses from most ecosystems, why do we care? Because they have large effects on atmospheric chemistry Global emissions: 23% of NOx and 61% of N2O comes from soil The global atmospheric N budget is unbalanced

  11. In the atmosphere… • Ammonia—NH3 • Interacts with acids in troposphere • NH3 + H2SO4 (NH4)2SO4 • Reduces acidity of rain • Deposited downwind

  12. In the atmosphere… • Nitric Oxide—NOx • Highly reactive in chemistry of the troposphere • Lifetime of <1 day, depending on sunlight... • Lifetime of <1 day, depending on sunlight... • Regulates OH concentration, which determines “scrubbing capacity” of the atmosphere • Regulates concentrations of CO, CH4+ and other hydrocarbons • Regulates the photochemical production of “bad” ozone • Suffers oxidation to HNO3, a component of acid rain

  13. In the atmosphere… • Nitrous Oxide—N2O • Stable, residence time in atmosphere of 150 years • Migrates to stratosphere: • Radiatively active: 150 x more thermal adsorption capacity than CO2 • Undergoes photolysis to NO in stratosphere, resulting in destruction of “good” ozone

  14. Human activities increase gaseous losses of N • Intensification of agriculture • Livestock production Can we manage systems to maintain economic gains and reduce N losses?

  15. Farmer’s fields Alternative practice = fertilizer application Matson et al 1998

  16. Fertilizer application after plant establishment • Reduced gas emissions by 50% • Did not affect grain yield • Increased farmer’s profits 20%

  17. NH3+ N2 NOx, N2O, N2 Litter SON DON NH4+ NO3- Leaching Erosion Leaching

  18. Nitrogen leaching losses • DON is the dominant form of loss in most undisturbed systems

  19. Temperate forests in Chile and Argentina receive little or no N inputs from anthropogenic sources Perakis and Hedin 2002

  20. Perakis and Hedin 2002

  21. Nitrogen leaching losses • DON is the dominant form of loss in most undisturbed systems • NO3- leaching losses dominate • In disturbed systems where plant uptake is reduced • In systems where N is in excess of plant demand, such as agricultural or weathered tropical soils, or sites that receive high deposition inputs

  22. When NO3- leaches out... • Removes a cation (Ca+, K+, NH4+) to balance solution charge • When cations are depleted, NO3- removes H+ or Al 3+