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SOIL MICROBES AND THE NITROGEN CYCLE Topics covered 1. General nitrogen cycle 2. Nitrogen mineralization and immobiliz

SOIL MICROBES AND THE NITROGEN CYCLE Topics covered 1. General nitrogen cycle 2. Nitrogen mineralization and immobilization 3. Nitrification 4. Nitrogen fixation 5. Denitrification. Reading: Killham - p. 108-141, 182-186 Articles

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SOIL MICROBES AND THE NITROGEN CYCLE Topics covered 1. General nitrogen cycle 2. Nitrogen mineralization and immobiliz

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  1. SOIL MICROBES AND THE NITROGEN CYCLE Topics covered 1. General nitrogen cycle 2. Nitrogen mineralization and immobilization 3. Nitrification 4. Nitrogen fixation 5. Denitrification

  2. Reading: Killham - p. 108-141, 182-186 Articles Aber, J., W. McDowell, K. Nadelhoffer, A. Magill, G. Bernston, M. Kamaka, S. McNulty, W. Currie, L. Rustad and I. Fernandez. 1998. Nitrogen saturation in temperate forest ecosystems: hypotheses revisited. BioScience 48:921-934. Hart, S. E., G. E. Nason, D. D. Myrold and D. A. Perry. 1994. Dynamics of gross nitrogen transformations in an old-growth forest: the carbon connection. Ecology 74:880-891. Neff, J. C., F. S. Chapin III, and P. M. Vitousek. 2003. Breaks in the cycle: dissolved organic nitrogen in terrestrial ecosystems. Front Ecol. Environ. 1(4): 205-211. Articles referenced in notes Aber, J. D. and J. M. Melillo. 1994. Litter decomposition: measuring relative contributions to organic matter and nitrogen to forest soils. Can. J. Bot. 589:416-421. McClaugherty, C. A., J. Pastor and J. D. Aber. 1985. Forest litter decomposition in relation to soil nitrogen dynamics and litter quality. Ecology 66:266-275. 2

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  6. Sylvia et al. 2005 6

  7. Neff et al. 2003 7

  8. Neff et al. 2003 8

  9. 2.0 Nitrogen Mineralization and Immobilization 2.1 Forms of N in litter and humus Amino acids (20-50% of humus N) Amino sugars Proteins Nucleic acids (cytosine) Urea N bound in soil complexed w polyphenolics in clay Structures from http://en.wikipedia.org/wiki/ 9

  10. 2.0 Nitrogen Mineralization and Immobilization (cont’d) 2.2 Mineralization of N Organic N to NH4 (and NO3 also is considered) Accumulated NH4 represents quantity of substrate N in excess of microbial demand Net change of inorganic N = organic N mineralized – (N assimilated by microbes + plant uptake + N leached + N denitrified) 2.3 N mineralization can be aerobic or anaerobic Aerobic: protein → CO2, NH4, SO4, and H2O Anaerobic: protein → amines, NH4, organic acids, CO2, mercaptans, and H2S (putrefaction) 10

  11. 2.0 Nitrogen Mineralization and Immobilization (cont’d) • 2.4 Methods for determining available N • useful for predicting correct fertilizer additions • Incubation method of soil • aerobic - cores (w/wo resins) or bags in field or lab • anaerobic - generally lab w/ saturated soil & controlled temp • Resin bags in soil - cation &/or anion resins • Extraction of N • 2 M KCl extraction of soil (mineral &/or organic) • Microbial biomass N 11

  12. 2.0 Nitrogen Mineralization and Immobilization (cont’d) • 2.5 Environmental influences of N mineralization e.g., • Nitrogen – high soil N results in high N mineralization (1 – 4 % of humus N mineralized per year) • Moisture – optimum 50-75% of moisture holding capacity • (e.g., 20-30% in loam soil) • pH – greater N mineralization in neutral than acid soil • Temperature – optimum 30-60 C • Cultivation causes large decline in soil organic N content 12

  13. 2.6 Net N mineralization rates in forests (29-125 kg/ha/yr in Wisconsin). In sugar maple and aspen much of this nitrified, but there is very little nitrification in white oak and hemlock stands. McClaugherty et al. 1985 13

  14. 2.7 N immobilization – results from microbial assimilation of inorganic N. Critical substrate C:N ratios for immobilization are<20:1 = mineralization; >30:1 = immobilization (Fig 1) 14

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  17. Edmonds 17

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  19. 2.9 N mineralization by earthworms – up to 72% of the dry weight of earthworms is protein and that 1.75% of the freshweight is N. In a Fraxinus/Quercus woodland with 364 g m-2 of earthworms, annual N return from worms dying in this population is 6-7 g m-2 (60-70 kg/ha/yr). About 25% of the nitrogen from decomposing worms is nitrate, 45% is ammonia and the remainder as soluble organic matter in components such as setae and cuticle. Mineralizable N is also produced by living earthworms in their excreta and earthworm casts generally contain more N than the surrounding soil. 19

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