Lecture 25 - Carbon cycle Flashcards Preview

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Explain the C cycle. Which transfers from atmosphere are fast and which ones are slow?

fast: terrestrial, ocean
slow: sedimentation, organic matter


Rank the pools of CO2

65000000 *10^15g

organic matter

in oceans, dissolved CO2

1e prod 600 *10^15g
soil 1400 *10^15g



At what rate is CO2 rising, what's mostly causing this?

CO2 is now rising at 1.0 ppm (0.5%) per year. Most of this increase comes from burning fossil fuels.


Explain the Greenhouse gases. What gases does it consist of?

Atmosphere’s ability to absorb infra-red energy – a “blanket”

Without it, the average temperature on earth would be 33 degrees cooler than what it is now.

Composed of gases:
Water vapor (H2O)
Carbon dioxide (CO2)
Methane (CH4)
Nitrous oxide (N2O)
Chlorofluorocarbons (CFCs)


Explain the greenhouse effect

Blanket becomes “thicker"
global warming: increase in temperature, 1.4–5.5°C over the next 100 year


What human sources are increasing CO2 in atmosphere?

1. fossil fuels
2. land use change (cutting down forest)


Define atmospheric storage

Atmospheric storage = Human input (fossil, land use)- Biosphere uptake (ocean uptake, unidentified sink)


What is the unidentified CO2 sink suspected to be

Increased forest growth


Explain the experiment that determined the missing CO2 sink

Evidence that forest uptake is part of that missing stink:
• Experiment measured increase in carbon in the Amazonian forest.
• Examined over 6k trees and found a way to estimate the amount of carbon increasing in the tree.
• 0.6 pG is being taken up by plants (About 25% of the unidentified sink)
• Experiment don’t account for the below ground processes because plants have roots.
• Reason: hard measurement to make.
• Supported by NPP from satellites.
• Ex: in tropical systems, plants are close to their temperature optimum. We expected then, as you increase temperature, respiration will be enhanced to a greater extent than photosynthesis. Net effect will be a decrease in photosynthesis. Prediction is that the plants in the tropics will respond negatively to a change in temperature.
High latitudes: increase in temperature might increase the growing season which increases the NPP that we have.
Tropic plants will start moving to the north.


What are 4 causes of soil organic matter buildup?

1. Root biomass increase
2. Litter fall
3. Transfer of C-rich compounds from plants to soil microbes
4. Humus is produced biologically and chemically (recalcitrant form of SOC)


How much has NPP grown in 18 years in 20th century? How can we account for the increased NPP?

Global average shows an increase of about 6% in NPP over 18 years – note that this is opposite to the pattern for NPP in the sea

Enzymatic reactons like photosynthesis are temperature dependent --> more plants!

CO2 fertilization?


What's the optimal temperature for net photosynthesis?

25 C


What are the two types of photosynthesis? Which one is CO2 limited?

C3 photosynthesis


Other resources for growth such as ___, ___ and ___, must be present in excess to observe the (sustained) effect of elevated CO2 levels

water, light and nutrients


What was the Free Air Carbon Enrichment (FACE) experiment? What did it show?

what were its limitations?

Free Air Carbon Enrichment (FACE) experiment - more realistic:
• Plots of natural forest.
• Tower equipped of nozzles that shoot CO2 into the middle of the forest plot.
• Plots are naturally exposed to nutrients.
• Comparing the increase yield of C3 and C4 to the ones in the greenhouse, it’s much less.
• 12-14% (C3) and 0% (C4) vs 20-30% (C3 and C4)

Limitations: only carried on for a decade (short time) but we are interested in identifying a sink might respond over a much longer time.


Why is the NPP also increasing in the high latitude?

• Nitrogen in fossil fuel, so when we burn it, it releases into the atmosphere (enhancing the nitrogen cycle)
• When it rains, nitrates fall into the ground and act as a source of nitrogen for the plants.
• Northern hemisphere acting fertilizing the planet with nitrogen.
• More nitrogen deposition → high values of NEP


Define Net Ecosystem Production (NEP)

NEP = NPP - R(decomp) - R(other heterotrophs) or
NEP = GPP - R(total)

• NPP takes up carbon, respiration by decomposers and heterotrophs release carbon.
• In order for an ecosystem to take up C carbon NEP has to be positive.
• If there is more R than NPP, losing carbon to the atmosphere.
• NPP> R, than the ecosystem will take up CO2.
• How does increase in CO2 affect R?


How does increase in T affect R?
NEP = NPP - R(decomp) - R(other heterotrophs)

What were 2 experiments that showed this?

• Decrease in photosynthesis as T increases.
• Enhancement in respiration as T increases.
• Respiration responds slowly to increase in T.

Experiment: Artificially heated plots of land in the forest using heat cables.
• Compared amount of carbon released by respiration by decomposers.
• Heated plots released 25% more carbon.
• On the short term, heating the soil enhances decomposition of the organic matter. Decreasing caused by reduction of content in the soil → if u reduce all the organic C , there’s nothing left for the microbes to break down → concern for northern hemisphere as the temperature rises.

Another experiment: Heated little ponds, measure the NEP.
• When ecosystem respiration (ER) = gross primary production (GPP), then carbon out = carbon in.
• ER > GPP , more respiration occurring. Source of CO2 to the atmosphere.
• ER < GPP, System is absorbing CO2
• Heating ecosystem makes it less efficient at absorbing carbon.


What is the impact of CO2 on consumers?

• Indirect response.
• Herbivores: High co2, less herbivores.
• When growing plants under high CO2, increase C:N ratio. Increase the ratio, less protein rich → poor quality food for consumers.
• Animals are not able to convert as much of the food they ingest into their own biomass → animals eat more. Survival drops, Abundance drops.Conversion efficiency • • (CE) = Production/Ingestion


What are the effects of CO2 on ocean pH

• CO2 dissolves in water causes decrease in pH, but ph is still way above neutral. Produces H+ protons.
• Ocean contains high amount of carbonate and so as you produce more protons, you reduce the concentration of carbonates.
• Issue because organisms use the carbonates to make hard shells, hard parts, etc.