Theme C Flashcards
Why is the northern hemisphere producing more carbon dioxide than the southern hemisphere?
- It has more land mass and is more industrialised
Why is there correspondence between el nino records and C02 concentrations?
- El Nino caused by climatic oscillations
- C02 spikes in El nino years because tropics become sources rather than sinks
In what ways is carbon lost due to disturbance?
Photosynthesis (respiration)
Fires
Deforestation
What makes a terrestrial sink a terrestrial sink
Carbon sink means photosynthesis has to be > than respiration year after year
The last thing a carbon sink needs means is that the carbon has to be stored with a slow turnover rate (soils,trees etc)
How does demography cause ecosystems to act as carbon sinks?
- young trees grow fast in new forests
- Younger, regenerating forests in mid lats
- Old growth forests in tropic and high lats
How does physiology cause ecosystems to act as carbon sinks?
Hotter weather = increases in productivity, but not in drought
- Evidence of weakening amazon carbon sink, increasing productivity but increased tree turnover
- In African forests there is a net positive trend with increased productivity and no change in mortality
What is helicopter science?
Scientists wouldn’t engage with scientists in the country they were working in
How does C02 fertilisation of growth depend globally on mycorrhizas?
- Mutualistic relationship increases plant C uptake
- High N negatively impacts AMF and good levels of P is needed for AMF to work properly
How do droughts weaken tropical carbon sinks?
- Peak mortality of trees correlate to el nino events
- In the southern, dry amazon, the species adapted to wet climates are at higher risk of mortality
Which processes control carbon sinks in mature tropical forests?
- Rising C02 boosts photosynthesis if there is nutrient bioavailability
- Rising temps and drought limit wood production - could cause saturation of Amazonia C sink
- El Nino increasing tree mortality
- African forests less impacted by heatwaves as they are at higher altitude
When is amazon deforestation aiming to end by?
2030
Why do tropical forests surprisingly remain more intact than other ecosystems?
- They havent yet been transformed
- Savannas are used for pasture land and temperate ecosystems have had much more transformation over a much longer period of time
When did deforestation for agriculture begin and how can we tell?
- intensive land use beginning 3000-1000 BC
- Pollen evidence of olive cultivation from 6000 BC decline of Oak pollen at the same time as well as bones of livestock farming
E.g. Yellow river region in China where there is evidence of land clearance for agriculture 5000-3000 years BC
How is carbon emitted from deforestation?
Decay of felled trees
Disturbance releases soil carbon
Burning to clear the felled area
How much has deforestation been responsible for human C02 emissions?
A third
What are the feedbacks from deforestation mediated via surface energy balance?
The amount of dissipated longwave energy from the sun depends on albedo
High albedo= reflectance (ice)
Low albedo = absorbance (ocean)
Deforestation increases albedo and cools the land
Also a cooling effect as loss of trees=loss of evapotranspiration (significant as convectional rainfall recycles 80% of water over tropical forests)
What is the Bonn Challenge?
to restore 150 million ha of deforested and degraded lands worldwide by 2020
(didn’t happen)
Why is it important that tree restoration is targeted to degraded and not native ecosystems?
-Trees can deplete groundwater reserves
- Most of the C in a savanna is stored in soil and evidence shows that at higher rainfall areas you get a decrease in the amount of soil C after you plantation
How does forest that covers snow warm the earths surface?
- Forest blocks the high albedo of the snow- more light is absorbed
What do we use to fertilise with in the UK?
Ammonium nitrate
- Other places use ammonium or urea
When is N20 released throughout the agricultural process?
- peaks of N20 emission following fertillisation
- after harvest because the plant root is turning over and releasing nitrogen into the system
- tillage also releases N20 as its releasing N20 throughout the soil
What is the natural balancing of N20 emissions
Denitrification produces N20 emissions under anaerobic conditions
The activity of N2O reductase can mitigate its release by converting N2O into N2
N20 reductase is expressed more in anaerobic conditions
- What is effecting this balance is the excessive addition of N through fertilisation
How can fluctuating water systems cause release of N20?
- Through creating anaerobic conditions through water pore-filled spaces
- In anaerobic conditions, organic matter decomposition stops so when the soil is aerated again it can increase
- N20 can accumulate in water saturated soils
How much N added to agriculture is collected by the plant?
30%
What factors effect denitrification?
- pH
- 02 availability which is affected by rainfall and soil structure
- Availability and conc. Of nitrate (no nitrate = no denitrifiers)
- Available respirable carbon
- N03 and carbon availability effects are interlinked and can affect both the relative flux through denitrification and N fate
- Temp. is important too due to the enzymes (warmer=faster)
What are the effects of temperature on denitrification activity?
- Warmer temps generally accelerate microbial processes
- Peak around 20-30 degrees
- Extreme temps inhibit microbial activity
- Different species of denitrifying bacteria have different temperature optima+tolerances, can shift the balance of microbial populations
How can the formation of soil aggregates/compaction lead to increased denitrification?
- Pores fill up with water, limited 02 diffusion
- if you compress or plough the soil all the time you compact the soil so the air gaps are removed but the same amount of water
- This means the soil will become more anaerobic
What are FACE experiments
- Assessment of controlled C02 release, measuring the effects on plant growth and other plant traits
What do the FACE experiments show regarding plant growth and increased C02 levels?
- Increased C02 = plant becomes more nutrient restricted as its growing faster
- Found bacterial community changed in structure and increased in size with increased C02
What is meant by the rhizosphere?
area of soil plant root is influencing
What are rhizobox experiments and what are they used for?
- Transparent containers designed to observe and study plant root systems
- Part of the rhizobox not containing any roots was removed and replaced
- isotopic tracing conducted to track the movement of certain substances into the rhizobox system
- Looked to see how far the nutrients moved from the root surface
Explain what is meant by artificial root exudate addition
- process of adding artificial solution to simulate the organic compounds that are released by plant roots into the soil
- Can add a line of carbon up the soil, realistic nitrate addition, water, different water filled pore spaces etc.
What are the genes used to study N20?
nirK and nirS are both nitrite reductase genes - increase of these = increase n20 emissions
nosz= gene encoding for reduction of N20 to N2 - limiting N20 emissions
What is an ordination plot?
visualize the relationships between samples or variables in a dataset
- can be used to plot the abundance of different genes involved in the N cycle
Give a brief overview of the N cycle
Ammonification = conversion of organic nitrogen to ammonium (NH4+)
Nitrification = converts ammonium to nitrate (NO3-)
Denitrification = reduces nitrate to nitrogen gas (N2), completing the nitrogen cycle
How does the carbon form in soil affect the nitrogen gas flux rate?
- Labile (simple) vs Recalcitrant (complex)
- Labile C is more readily available to microbes and so stimulates N20 more by producing bigger pool of available nitrate
- Recalcitrant C takes longer to decompose and so supports microbes but over longer time periods
The problem is we fertilise with simple N forms*
Why is organic fertilisation better?
- less N20 because its far more complex forms of nitrogen being added
- reduces leeching and soil erosion
- yield increases due to increased C and pH
What is the experiment that was used to see how organic fertilisation affected soil aggregates and therefore N20 emissions?
- Soil was collected from fertillised treatments and separated into aggregate classes
- Soil was flushed with helium so its completely anaerobic then they assessed N20 and N2 levels over 72 hours
How does tilling effect soil carbon?
- Reduces it by exposing it to the microbial population in higher intensities through aeration
- Mixes deeper organic soil layers
- Disrupts formation of stable soil aggregates
What are the preferences of N uptake for microbes and plants?
Plants - prefer nitrate as ammonia can be toxic
Microbes - prefer ammonium but can change to nitrate (problem if it starts taking up too much
How does soil denitrification activity change throughout the crop cycle?
- Early in the crop cycle, denitrifiers have selective advantage and are favoured in soil environment
- Later on DNRA is used which means nitrate is reduced to ammonium to conserve bioavailable nitrogen within the system and less gas
