Clear-cutting and Nitrogen Mineralization Brian Strahm

Nitrogen Cycle N2N2 NO 3 - NH 4 + Organic N Relative Abundance (g N / m 2 ) N 2 1,150 Organic N 725 Plant N 25 NO NH 4 + 1

Mineralization / Immobilization Ammonification: R-NH 2 --> OH - + R-OH + NH 4 + conversion of organic nitrogen into ammonium mediated by enzymes Nitrification: NH > 4H + + energy + NO > energy + NO 3 -  Nitrisomonas spp.  Nitrobacter spp. Immobilization: Inorganic N --> Organic N (R-NH 2 ) conversion of ammonium and nitrate into organic nitrogen assimilation into microbial biomass 

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?

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 ( ) - 91% of managed forests were clear-cut

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

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)

Community Composition (Marshall, 2000)

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)

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)

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

References Briggs, R., Hornbeck, C., Smith, C., Lemin, R., McCormack, M Long- term effects of forest management on nutrient cycling in spruce-fir forests. Forest Ecology and Management. 138, Gessel, S., Cole, D., Steinbrenner, E Nitrogen balances in forest ecosystems of the Pacific Northwest. Soil Biol. Biochem. 5, Marshall, V Impacts of forest harvesting on biological processes in northern forest soils. Forest Ecology and Management. 133, Morris, S., Boerner, R Interactive influences of silvacultural management and soil chemistry upon microbial abundance and nitrogen mineralization. Forest Ecology and Management. 103, Myrold, D Transformations of nitrogen. pp 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 Nitrification and denitrification in soil from a clear-cut Norway spruce (Picea abies) stand. Soil. Biol. Biochem. 30,