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Nutrient Cycles

Nutrient Cycles

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Nutrient Cycles

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  1. Nutrient Cycles • Nutrient requirements • Biogeochemical cycles • Rates of decomposition • Plant adaptations in low nutrient conditions

  2. Nutrient Requirements for Plant Growth • Taken up in gaseous form, Oxygen (O2), Carbon CO2, and from roots - Water (H2O). • Derived from water and carbon dioxide • Rest are taken up from soil solutions • Macro-nutrients –Nitrogen (N), Phosphorous (P), Potassium (K), • Calcium (Ca), Magnesium (Mg), Sulfur (S) • Micro-nutrients – Boron (B), Copper (Cu), Iron (Fe), Manganese (Mn), Molybdenum (Mo), Zinc (Zn)

  3. Nutrient Cycles • Nutrient requirements • Biogeochemical cycles • Rates of decomposition • Plant adaptations in low nutrient conditions

  4. Biogeochemical Cycling The cycling of nutrients through ecosystems via food chains and food webs, including the exchange of nutrients between the biosphere and the hydrosphere, atmosphere and geosphere (e.g., soils and sediments)

  5. Ecosystems produce and process energy primarily through the production and exchange of carbohydrates which depends on the carbon cycle. • Once energy is used, it is lost to the ecosystem through generation of heat • Carbon is passed through the food chain through herbivory, predation, and decomposition, it is eventually lost to the atmosphere through decomposition in the form of CO2 and CH4 . It is then re-introduced into the ecosystem via photosynthesis. • However, the amount of carbon present in a system is not only related to the amount of primary production, as well herbivory and predation (e.g., secondary production), it is also driven by the rates of decomposition by micro-organisms • Atmospheric carbon is rarely limiting to plant growth

  6. When we look at other nutrients, a somewhat different picture emerges than with the energy cycle – e.g., phosphorous in a food chain within a small pond. • Algae remove dissolved phosphorous from the water • The phosphorous is then passed through different trophic levels through herbivory and predation. • At each level there is some mortality, and then the phosphorous is passed to decomposers • These organisms release phosphorous into the water where it is again taken up by primary producers and the whole cycle starts up again

  7. Key Elements of Biogeochemical Cycles • Where do the nutrients that ecosystems use come from? • What happens to the nutrients within the ecosystem itself? • What happens to the nutrients once they leave the ecosystem? • Once nutrients are cycled through an ecosystem, how do they get back? • What are the rates of exchange of nutrients between the different pools?

  8. Nutrient Pools and Nutrient Flux • Nutrient pool – a specific component or compartment where a nutrient resides • Can be a single organism, a population, a community, a trophic level, and an abiotic feature (e.g., lake, soil, atmosphere, etc.) • Nutrient flux – the rate of exchange (e.g., unit of material per unit time) of nutrients between pools

  9. Example of changes in the amounts of tracer phosphorous being exchanged within an aquatic food web • The values themselves represent changes in the pool levels, where each one of the lines represents a different pool • Understanding the feeding relationship allows us to build a nutrient cycle model for this ecosystem

  10. Model of phosphorous cycle for an aquatic ecosystem – flux rates per day shown. • This system is not closed – inputs, probably from run-off from land. • Exports include  herbivores moving outside of system and dead plant/animal material moving out of system, probably through sedimentation. • Rate of uptake by plants is directly proportional to net primary production. • Exchange of nutrients by higher trophic levels is controlled by processes regulating secondary production. • Rates of inputs and outputs of nutrients from an ecosystem are driven by both biotic and abiotic factors.

  11. Types of Biogeochemical Cycles Three major categories of biogeochemical cycles based on slowest-changing pool(=reservoir): • Gaseous cycles of C, O, H20 • Gaseous cycle of N, (S) • Sedimentary cycles of the remaining nutrients Global scale Local scale

  12. Sedimentary Cycles Gaseous Cycles

  13. Major Components of Nitrogen Cycle

  14. Biological Nitrogen Fixers • Cyanobacteria – blue-green algae • Free living soil bacteria • Mycorrhizae • Symbiotic bacteria living in root nodules

  15. Root nodules on ? Cassia fasciculata

  16. NO from lightning Lightning + N2 + O2 NO + O2  Nitrate (NO3)

  17. Phosphorous Cycle Phosphate – PO4-3

  18. Potassium

  19. Sources of Nutrients Atmosphere Parent Material Run-off, Ground water Floods

  20. Nutrient Cycles • Nutrient requirements • Biogeochemical cycles • Rates of decomposition • Plant adaptations in low nutrient conditions

  21. Simple Model of Soil Decomposition/ microbial respiration H2O, O2 CO2 or CH4 Litter Energy Microbial Population Organic Soil Nutrients Dissolved Nutrients

  22. Factors Controlling Microbial Respiration • Availability of oxygen CO2 versus CH4 production • Temperature • Moisture • Quality of material comprising dead organic matter

  23. Simple Model of Simple Model of Soil Decomposition/ microbial respiration H2O, O2 CO2 or CH4 Litter Energy Microbial Population Organic Soil Nutrients Dissolved Nutrients

  24. k is the fraction of a material that decomposes in a given year Decomposition as a Function of Lignin Content

  25. Residence Time • Residence time is the length of time it takes for biomass or a nutrient to be completely decomposed or recycled from the forest floor

  26. Residence times Coniferous forests have longer residence times than deciduous  C/N control Boreal forests have longer residence times than temperate forests  temperature control

  27. Nutrient Cycles • Nutrient requirements • Biogeochemical cycles • Rates of decomposition • Plant adaptations in low nutrient conditions

  28. Tree Nutrient Content

  29. Translocation of Nutrients • Prior to shedding leaves in the fall, translocation of nutrients often takes place in trees • This allows tree to retain essential nutrients that are hard to come by • Spruce trees remove more nutrients than other coniferous trees • An adaptation to poor nutrient sites

  30. Question – do plants growing on sites with low soil nutrients have low nutrient contents as well? The answer is no – • Plants on sites with low nutrients tend to have higher nutrient contents • They have a higher nutrient use efficiency

  31. Nutrient Use Efficiency (NUE) • Some plants are more efficient at using nutrients because it gives them selective advantages in low nutrient conditions NUE = A / L A – the nutrient productivity (dry matter production per unit nutrient in the plant) L – nutrient requirements per unit of plant biomass

  32. A common pattern found in ecosystem productivity is saturation curve. Productivity increases linearly with N availability, up to a certain point, when other resources become limiting (e.g., light, water, temperature, other nutrients)

  33. Three types of relationships with respect to limitations of nutrients: • Production is independent of resource availability • Production is a linear function of resource availability • At some point, another resource becomes limiting

  34. Factors Influencing Nutrient Availability • Presence of nitrogen fixers • Microbial activity • Fire • Precipitation patterns • Soil drainage • Soil temperature, moisture

  35. H2O - Precipitation CO2 Fire GHG Photosynthesis Aeolian, Atmospheric Deposition Internal translocation N2, O2 Litterfall nutrients N fixers CH4, CO2 Organic soil Dissolved nutrients Through-fall nutrients Nutrients Energy, Nutrients Upper mineral soil Microbes Lower mineral soil Leaching, run off

  36. Forest Type Living Biomass Pool Primary Production Rates Soil Carbon/ Nutrient Pool Decomposition Rates Tropical Highest Highest Lowest Highest Temperate Middle Middle Middle Middle Boreal Lowest Lowest Highest Lowest Boreal forest has the largest available nutrient pool in soil, but lowest rates of production, where as tropical forest has lowest soil pool, and highest production.

  37. Role of Disturbances in Nutrient Cycling • Type of disturbance important • Fire versus logging versus large-scale mortality • Disturbances directly alter biotic and abiotic controls on nutrient cycling • Rates of primary production • Controls on evapotranspiration • Influences on surface runoff • Soil temperature/moisture  decomposition rates • Linkages between terrestrial/aquatic systems

  38. Hubbard Brook watershed, upstate New Hampshire.