Clear-cutting and Nitrogen Mineralization

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Clear-cutting and Nitrogen Mineralization

• Brian Strahm

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Nitrogen Cycle
N2
Relative Abundance (g N / m2) N2 1,150 Organic N
725 Plant N 25 NO3- 5 NH4 1
NO3-
NH4
Organic N
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Mineralization / Immobilization

• Ammonification
• R-NH2 --gt OH- R-OH NH4
• conversion of organic nitrogen into ammonium
• mediated by enzymes
• Nitrification
• NH4 --gt 4H energy NO2- --gt energy NO3-
• ? Nitrisomonas spp.
• ? Nitrobacter spp.
• Immobilization
• Inorganic N --gt Organic N (R-NH2)
• conversion of ammonium and nitrate into organic
  nitrogen
• assimilation into microbial biomass

?
?
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Soils and Mineralization

• Soils are the habitat of plant roots and home of
  numerous microflora, including viruses, bacteria,
  fungi, blue-green algae and many other soil
  organisms, from unicellular protozoans to small
  vertebrates.
• Nitrogen mineralization is the result of
  metabolic activities of this diverse group of
  soil organisms.
• How might this be affected by a disturbance?

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Disturbances

• Vary in frequency, size and intensity
• natural vs. anthropogenic
• widespread and chronic (drought)
• or
• site-specific and acute (clear-cutting)
• Magnitude of disturbance
• Canada (1995) - 86 of all logging was
  clear-cutting
• British Columbia (1987-88) - 91 of managed
  forests were
• clear-cut

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Clear-cutting

• Physical effects
• ? restructures vegetation
• ? modifies quantity and quality of litter
• ? alters root exudates
• leaches essential nutrients
• 65 increase in stream flow
• changes microclimate
• increases temperature
• decreases moisture
• increases pH

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Clear-cutting

• Microbial Community Composition / Mineralization
  Rates
• ? Disturbance does not affect microbial abundance
  or biomass
• ? Ammonification result of diverse group of
  soil organisms
• ? rates unaffected by clear-cutting
• large groups of generalists
• little impact on ecosystem
• function
• ? Nitrification - specific organisms
• ? dramatic increase in clear-cut
• small group of specialist
• thrive in clear-cut conditions

(Paavolainen and Smolander, 1998)
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Community Composition
(Marshall, 2000)
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Forest Nitrogen Cycling

• Old Growth Douglas-fir
• typically acidic soils
• low rates of nitrification
• ? acidity inhibits nitrification
• conservative N cycling
• role of soil pool diminished
• greater portion of available N from forest floor?

(Gessel and Cole, 1973)
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Forest Nitrogen Cycling

• Young Douglas-fir Stand
• larger portion of total N pool in soil
• less tied up above ground
• trees access N strictly from soil solution

(Gessel and Cole, 1973)
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Problems

• Why is excess nitrification a problem?
• nutrient loss limits tree growth
• ? denitrification
• ? leaching
• ? counterintuitive to goals of sustainable
  forestry
• ? groundwater contamination
• eutrophication of surface waters

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References
Briggs, R., Hornbeck, C., Smith, C., Lemin, R.,
McCormack, M. 2000. Long- term effects of
forest management on nutrient cycling in
spruce-fir forests. Forest Ecology and
Management. 138, 285-299. Gessel, S., Cole, D.,
Steinbrenner, E. 1973. Nitrogen balances in
forest ecosystems of the Pacific Northwest.
Soil Biol. Biochem. 5, 19-34. Marshall, V.
2000. Impacts of forest harvesting on biological
processes in northern forest soils. Forest
Ecology and Management. 133, 43-60. Morris, S.,
Boerner, R. 1998. Interactive influences of
silvacultural management and soil chemistry
upon microbial abundance and nitrogen
mineralization. Forest Ecology and Management.
103, 129- 139. Myrold, D. 1998.Transformations
of nitrogen. pp. 259-294. In Sylvia, D.,
Fuhrmann, J., Hartel. P., Zuberer, D.
Principles and Applications of Soil
Microbiology. Prentice Hall, Upper Saddle River,
NJ. Paavolainen, L., Smolander, A. 1998.
Nitrification and denitrification in soil from a
clear-cut Norway spruce (Picea abies) stand.
Soil. Biol. Biochem. 30, 775-781.