Lecture 5: Biogeochemical Cycles

January 19, 2010

9:00	1. Kolbert chap 4	Reading for today was second half of Chapter 3 and Kolbert Chapter 4 scanned images from text - not available online
	2. cycles
	3. reasons to study cycles
	4. water
	5. carbon
	6. phosphorus
	7. nitrogen
	8. inter-related cycles

 

1. Discuss chapter 4 in Kolbert

multiple lines of evidence from species distribution shifts

• comma butterfly
• Wyeomyia smithii
• golden toad
• pine and other tree species
• Mountain ringlet butterfly

compared to climate changes (rainfall and temp)

2. Cycles

molecule and element cycles

• water
• carbon
• nitrogen
• phosphorus
• sulfur
• iron

need to track and understand

• the different forms or compartments of the molecule or element and the amount in each category
  • such as:
    • water in the ground, water in the lake
    • carbon in CO2, carbohydrate in trees, fossil fuels
    • N in plants, soil inorganic compounds, atmospheric NO2
• the processes that connect these compartments
  • such as:
    • evapotranspiration of water from soil to atmosphere
    • carbon fixation of trees (ATM C as CO2 --> Plant C as biochemical)
• rate control relationships for these processes
  • such as:
    • evapotranspiration controlled by sun light, wind, temp, humidity, plant characteristics, etc.
    • net growth rate of tree biomass from photosynthesis and usual factors light, temp, CO2, nutrients, etc.)
• interactions between cycles
  • water cycle - major control over flows
  • plant nutrition (C:N:P:S:Si:Fe)
  • others

3. Reasons why these cycles are important

understanding the "baseline" that used to be a fairly stable set of interacting cycles

how they have changed with human activities

• acceleration of specific transformations
  • fossil fuels - C input
  • erosion - trace elements accelerated 100x
  • mining of specific elements, co-traveling such as release of Cd when mining other elements
• limitation of some transformations
• substitution of some land use actions

how these might change on their own

• threshold
• catastrophe
• self accelerating

 

4. Water cycle - Hydrologic cycle

key concepts:

• evaportation, precipitation, transpiration
• groundwater, aquifers

ways in which humans effect the cycle

• withdrawal of freshwater
• clearing natural cover for agriculture or forestry
• increase flooding

 

5. Carbon cycle

key concepts:

• CO2 is 0.038% of the atmosphere (380 ppm)
• CO2 is a greenhouse gas, traps heat
• photosynthesis, aerobic respiration
• fossil fuels (coal, oil, natural gas, oil-shale)

ways we change the cycle

• burning fossil fuels
• deforestation

6. Phosphorus cycle

key concepts:

• phosphate ion is the main compound
• even in biological use
• no gas phase
• travels with water and sediments

ways we change the cycle:

• mining for fertilizer
• sewage into water

phosphorus is being buried in lakes and sediments around the world, as it is becoming a scarce resource

excess P in lakes can lead to toxic cyanobacteria blooms and degrade water

 

7. Nitrogen cycle

key concepts:

• enormous reservoir in the atmosphere as N2
• nitrification - ammonia to nitrate
• denitrification - ammonia to N2 or N2O
• nitrogen fixation N2 to ammonia by bacteria and symbiotic associations

ways in which we interfere:

• NO from fuel combustion
• N2O from livestock and fertilizers
• N release from soils
• upset nitrogen cycle in lakes
• deplete soil through agriculture
• doubled the release of nitrogen from the land

water resource degradation from increased N load and aquatic organisms

 

8. Inter-relatedness of these cycles

Redfield ratio - the average composition of biological material

• 106 C - 256 H - 106 O - 16 N - 1 P - 0.7 S
• elements are taken up in this ratio in the surface oceans
• degraded by respiration
• long time scale sediment burial and geological processes complete the cycle

energy is stored in organic carbon compounds and released in aerobic respiration

liquid water is a major player in these processes

• dissolving rocks
• carrying compounds
• supporting plant growth
• storage (in the oceans)
• Goldilocks hypothesis (planetary temperature with/without water)