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Topic%203%20Nutrient%20Cycling%20slides - Nutrient Cycling...

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Unformatted text preview: Nutrient Cycling in Forest Ecosystems Biomass Removal Erosion Gaseous Inputs/Outputs Atmospheric Deposition Atmosphere Decomposition Dry Wet Carbon and Nitrogen Cycles Microbes Im m Litterfall Litter ob iliz ati Ex ud Vegetation Mycorrhizae Soil Organic Matter on Le a fw Geosphere Primary minerals Secondary minerals tio n Weathering Synthesis Soil Solution Up t Mi ner al ati o n as h& iza Biosphere ak e Exchan NH4+ ge Exchangeable Ions Major exchanges with atmosphere Carbon = photosynthesis, respiration Nitrogen = N fixation, denitrification Hydrosphere Leaching (Surface & Ground Waters) Organic matter is the major storage pool 1 Nitrogen Fixation N2(g) + 6H+ + 6eRhizobium spp. 2NH3 Frankia spp. Reduced compounds High energy (electron-rich) Oxidized compounds Low energy (electron poor) Nitrogen Fixation N2(g) + 6H+ + 6e2NH3 Nitrogen Fixation is the Primary Source of Nitrogen on Many Highly Disturbed Landscapes Symbiotic and free-living (asymbiotic) forms Plants "spend" about 2.5% of NPP on N2 fixation Free-living heterotrophic N2 fixing bacteria are usually found in organic-rich soils N fixation rates appear to be controlled by the N:P ratio (lower N:P = higher N fixation) Ceanothus rates up to 100 kg/ha/yr Forest regrowth was limited by availability of nitrogen Disturbance gradient Primary succession on highly disturbed landscapes Volcanic deposits are N-limited with respect to plant growth Buried O horizon 2 Atmospheric Deposition Great Smokey Mountains, NC Buried OM NO3-N NH4-N kg/ha/yr Ca 7.4 2.6 6.0 16.0 Gas Particulate Precipitation 8.6 1.9 3.2 3.7 17.4 1.6 3.1 5.1 9.8 Soil Erosion Enhanced Vegetation Establishment Fog/Cloud Total Deposition California N Deposition N deposition (kg/ha) Fenn et al 2010 Nitrogen Mineralization/Ammonification Organic N NH4+ Release of organically bound N (e.g., protein) to a mineral form (ammonium) Part of the decomposition process Nitrogen Immobilization NH4+/NO3- Organic N Assimilation of mineral N by microbes Immobilization is often dominant when the C/N of substrate is greater than 20-30. 3 Effect of the C/N Ratio Low C/N High C/N Nitrification -3 +3 + most microbes +5 Chemoautotrophs (Nitrosomonas & Nitrobactor) Aerobic process Converts N to a more mobile form Nitrate can be lost to denitrification Acidifying reaction NO3 is more susceptible to ecosystem loss than NH4 & NO3 requires more energy for protein synthesis than NH4 Nitrification is an acidifying reaction Denitrification (organic matter) NO N2O (greenhouse gas) Anaerobic process Organic matter is source of energy Wet soils and anaerobic soil microsites Rates are a function of labile C, NO3-, T & H2O Loss of N from ecosystem 4 Phosphorus Calcium, Magnesium & Potassium Young/Weakly Weathered Soils = High Nutrient Release by Weathering Old/Highly Weathered Soils = Low Nutrient Release by Weathering Mineral weathering is the dominant source Sorption (P) and exchangeable (Ca, Mg, K) are major source of plant available nutrient Low leaching of P & higher leaching of Ca, Mg & K Mineral weathering primary source of nutrients (except N) Atmospheric deposition is primary source of nutrients (except N) Apatite (Ca-PO4) in volcanic deposits (Mt. Pinatubo) Phosphorus Dynamics over Time (or Increased Weathering Intensity) New volcanic ash Old volcanic ash a (Apatite) AP Sorbed P Very low availability Slow availability Soil Biota Drive Nutrient Cycling 5 Temperature & Moisture Effects on Organic Matter Decomposition High High Decomposition Rate Low Low lignin High N High lignin Low N Decomposition Rate Low High lignin & Low nitrogen decrease decomposition A Which sample has a higher nitrogen-use efficiency? Based on the nitrogen-use efficiency, which plant would best represent a conifer vs a deciduous tree? Mycorrhizae Fungi B Which sample’s litter will decompose more quickly (think C/N ratio)? Mycorrhizae Biomass vs Root Carbohydrate Content Role of Mycorrhizae Enhance nutrient availability enzymes, acids, chelates, & increased root surface area Enhance water availability Pathogen protection Reduce toxicity from metals (e.g., Al3+) Resorption or Re-translocation of Nutrients 6 Net Primary Production during Ecosystem Development Nutrients in excess of growth requirement are potentially leached What can you say about the ability of an oldgrowth forest to store additional carbon? NPP Allocation Change in nutrient demands in a forest over time When is the best time to fertilize a forest? By fertilizing a forest, your change the shoot:root ratio resulting in greater wood production Food webs in forest streams Terrestrial nutrient inputs Algae Terrestrial organic matter inputs 7 ...
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