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NITROGEN FIXATION and DENITRIFICATION Reading: Text: Coleman et al. Ch 5

NITROGEN FIXATION and DENITRIFICATION Reading: Text: Coleman et al. Ch 5 Dan Binkley, Yowhan Son,and David W. Valentine. 2000. Do Forests Receive Occult Inputs of Nitrogen? Ecosystems 3:321-331. Lecture Topics. A. Nitrogen fixation 1. Importance

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NITROGEN FIXATION and DENITRIFICATION Reading: Text: Coleman et al. Ch 5

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  1. NITROGEN FIXATION and DENITRIFICATION Reading: Text: Coleman et al. Ch 5 Dan Binkley, Yowhan Son,and David W. Valentine. 2000. Do Forests Receive Occult Inputs of Nitrogen? Ecosystems 3:321-331

  2. Lecture Topics A. Nitrogen fixation 1. Importance 2. N fixation – the process • Organisms responsible 4. N fixation rates 5. Methods 6. Environmental factors controlling N fixation – The Daviesia story from Australia

  3. Lecture topics - continued B. Denitrification 1. Importance 2. Denitrification – the process 3. Organisms responsible 4. Denitrification rates 5. Methods 6. Environmental factors controlling denitrification

  4. A. Nitrogen fixation • Importance – N in soil comes from microbial N fixation. • Important in natural and managed ecosystems – occurs in leaves, stems, roots, soil, rotting logs, rhizosphere, nodules • Important in agriculture and forestry Legumes with pines to add lost Nitrogen

  5. (Brady & Weil 2002)

  6. 2. Nitrogen fixation – the process N2 + 6 H 2NH3

  7. 3. Organisms responsible

  8. Family/Genera of N fixing plants • Betulaceae: Alnus • Casuarinaceae: • Allocasuarina ,Casuarina ,Gymnostoma • Coriariaceae: Coriaria • Datiscaceae: Datisca • Elaeagnaceae: • Elaeagnus ,Hippophae ,Shepherdia • Fabaceae (Leguminosae) – legumes – soy beans, beans, peas, alfalfa, peanuts, lupin, clover, Scots broom, kudzu, Acacia, • Myricaceae:... • Morella arborea ,Myrica, Comptonia • Rhamnaceae: • CeanothusColletiaDiscaria, Kentrothermos, Retanilla, Trevoa • Rosaceae: • Cercocarpus,Chamaebatia,Purshia,Dryas

  9. Legumes – 18,000 species/650 genera – only 200 cultivated – not all fix N Bacteria in nodules – Rhizobium spp. Gram –ve rods Nodulation process – invasion through root hairs – Less than 5% of infections form nodules – bacteroids in nodules.

  10. 5

  11. Rhizobia produce extracellular polysaccharides (exopolysaccharides) that may be responsible for regulating oxygen flow to cells (slime layer) and signaling the plant to start the infection process. • Plants also produce root exudates that stimulate bacterial nodulation genes to turn on. • Many plant and bacterial genes are involved.

  12. The nodule interior is surrounded by a physical barrier to gas diffusion. barrier Nodule interior Depth from nodule surface (mm) Rhizobium nodules on white lupin (David Patriquin) R. F. Denison The root nodules can contain oxygen-scavenging molecules such as leghaemoglobin, which shows as a pink color when the active nitrogen-fixing nodules of legume roots are cut open. (Jim Deacon)

  13. Alnus glutinosa nodules http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm

  14. Alder nodule longitudinal section showing cells packed with Frankia genoscape.cns.fr Alder nodules

  15. Ceanothus nodules (Biology.kenyon.edu)

  16. Anabaena in cycad roots

  17. 4. RATES

  18. 5. Methods a. Acetylene reduction N2 + 6H - 2NH3 – uses same enzyme – nitrogenase 3C2H2 (acetylene) + 6H  3C2H4 (ethylene) b. Nitrogen accretion with time (e.g., after glaciation) Red alder over time – chronosequence. Hubbard Brook sand box study c. 15N - tracer experiments

  19. Hubbard Brook sand box study Occult N inputs -unexplained N inputs beyond nodulated plant inputs Trees planted in sandboxes 7.5 m x 7.5 m and N measured In soil and vegetation after 5 yrs Alder, Locust, Red pine, Pitch pine, No vegetation

  20. 6. Factors influencing N fixation – Nutrients - Mo, Fe, Ca, Co, Vitamin B12) pH (little below 6) – except for alder Oxygen – anaerobic process Temperature (opt 35 C) Moisture

  21. The Daviesia mimosoides N-fixation story in Australia Daviesia is a legume with about 200 spp. native to Australia

  22. Acetylene reduction

  23. In the field N fixation rates were estimated to be 4.5-7 kg/ha/yr based on No. nodules per plant, no. plants/ha, N fixation rates and temperature and moisture

  24. B. Denitrification 1. Importance 2. Denitrification – the process 3. Organisms responsible 4. Denitrification rates 5. Methods 6. Environmental factors controlling denitrification –

  25. 1. Importance • respiration in absence of oxygen • Loss of N following fertilization • Reduction of nitrate leaching following sewage disposal • Important in riparian areas for reducing nitrate contamination • in streams • - More important in tropical areas than temperate areas • Ozone degradation by N2O • - Greenhouse gas contributing to global warming

  26. (Brady & Weil 2002)

  27. 2 2 2 2

  28. 3. Organisms involved Killham 1996

  29. 4. RATES OF DENITRIFICATION

  30. Old-growth Fresh gap

  31. Denitrification likely to be highest in the Pacific Northwest in: • Riparian areas • Bogs • Areas with clay soils • Clearcuts • In areas with applied biosolids • In general denitrification in the PNW is much lower than in tropical areas or in southeastern U.S.

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  34. HIERARCHY OF REGULATING FACTORS (Davidson et al. 1990) Denitrifiers present DENITRIFICATION Yes IMPROBABLE Anaerobic conditions Yes Organic carbon Yes Water available Yes Temperature, pH and nutrients, etc? Yes DENITRIFICATION PROBABLE No

  35. How can denitrification occur in an aerated field????

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