1.6 - Soil Health

Question 1

Why is the presence of carbon in our atmosphere important?

Answer 1

• Traps incident and reflected longwave radiation (heat) from Earth’s surface
• without CO₂ and CH₄, average temperature of Earth would be 20ºC lower

Question 2

What affects oceanic productivity (rate of photosynthesis + growth of organisms) ?

Answer 2

• light availability for photosynthesis diminishes rapidly with depth
  
  • only available in top 200m
• Nutrient availability varies with location
  
  • cold water holds more carbon, thus better for productivity
    
    • therefore most productivity occurs at depth of 50-75m
    • however is seasonal, large burst of activity occurs in summer
  • in polar regions, surface + deep water are well mixed all year
    
    • thus more nutrients available, productivity is less seasonal
  • water in tropical regions is stratified (no mixing), leaving surface water poor in nutrients, as carbon is released to atmosphere
• coastal estuaries have largest concentration of productivity
  
  • due to availability of sediment from coastal erosion
  • open ocean is still larger overall due to size of ocean itself

Question 3

How is climate change affecting oceanic productivity?

Answer 3

• increasing stratification of ocean to wider latitudes
  
  • resulting in more carbon being released into atmosphere
  • less carbon for photosynthesis
  • overall global decline in productivity

Question 4

What factors control terrestrial productivity?

Answer 4

• Latitude
• Sequestration/emission of carbon from soil (on local scale)
  
  • dependant on local conditions
• decomposition and weathering rates
• amount of biomass
• amount of biota (living content of soil which respires)
• clay content of soil
  
  • clay protects carbon from decomposition from biota, allowing soil to be more carbon rich
• extent of soil erosion
  
  • can remove carbon from active layer of soil

Question 5

How does latitude affect terrestrial productivity?

Answer 5

• controls temperature and precipitation patterns
  
  • thus photosynthesis, thus productivity

Question 6

How much of global carbon is stored in soil?

Answer 6

• 20-30%

Question 7

How does terrestrial productivity vary across different biomes?

Answer 7

• Tropical: high
  
  • large biomass, high decomposition and uptake rates (limits size of litter)
• Temperate: mid
  
  • high organic matter content in soil —> large capacity to store carbon —> results in decent productivity
• Taiga: low
  
  • slow decomposition rate, upper soil is frozen, litter is largest carbon store
  • thus productivity is low as plants have very little access to carbon

Question 8

What is NPP?

Answer 8

• a measure of ecosystem productivity
  
  • rate of generation of biomass
  • reflects how well an ecosystem acts as a carbon sink

Question 9

How has the role of plants as carbon sinks changed in recent history?

Answer 9

• pre 1800s: sink, human disturbances to plant productivity were localised
• 1800s-1950s: source, scale of degradation was more global eg. deforestation, resource extraction, soil erosion, desertification
• 1950s to today: sink, net carbon sequestration despite carbon loss due to land use changes
  
  • due to afforestation + reforestation in Europe + North America as well as improved agricultural practices
• future: potential increasing source?, warmer temps result in faster decomposition and recycling of carbon stored in dead plants and soil, moving more carbon back into atmosphere

Question 10

What are the projections for NPP in the future?

Answer 10

• Human induced rises in CO₂ should speed up photosynthesis and NPP
• however, plant growth is limited as CO₂ use is also dependent on nitrogen and phosphorus content in soil
  
  • which is about 20%