The role of soils Flashcards
(32 cards)
How much carbon is estimated to be stored in soils globally (excluding permafrost)?
Soils are estimated to contain 1500-2400 PgC (petagrams of carbon, where P = 10¹⁵).
How much carbon is stored in permafrost, and why is this a concern?
Permafrost contains an additional estimated ~1700 PgC. The fear of this thawing due to rising temperatures means this carbon could be released to the atmosphere, contributing to climate change.
How does the amount of carbon stored in soils compare to other major carbon pools?
Soils (including permafrost) contain significantly more carbon than all vegetation (450-650 PgC) and oil reserves (173-264 PgC). The atmosphere contains ~730 PgC.
Even without permafrost, soil is a greater store than other major carbon pools.
What are three key services provided by soil?
- Soil acts as biomass recycling systems, returning biomass and nutrients for future life.
- Soil is a source of nutrients, supporting plant growth by providing a medium for roots and essential nutrients.
- Soil properties influence the types, diversity, and biomass of vegetation and microorganisms, and indirectly the animals, they can support.
What is humus?
Humus is carbon of plant and microbial origin that has resisted degradation. It is slowly decomposed and provides soil with its dark colour.
What layer of soil is humus most concentrated in?
It is most concentrated in the A horizon, the most biologically active layer with higher organic matter and nutrients.
What does recalcitrant mean?
Not easily decomposed.
What are colloids? What is their charge?
The tiny, most chemically active particles in soil, primarily consisting of clay minerals and organic matter (e.g., humus).
Typically negatively charged.
What is Cation Exchange Capacity (CEC), and why is it important for soil fertility?
Cation exchange capacity is the ability of soil colloids (the smallest clay and humus particles, which are mainly negatively charged) to hold onto and exchange positively charged nutrients (cations) in the soil solution and with plant roots.
A high CEC means high nutrient retention and good soil fertility
What does soil fertility depend on?
- Nutrient abundance and balance/bioavailability
- Type of clay (proportions and sizes)
- pH
- Presence of toxic elements
- Salinity
- Microbes
How does soil pH affect nutrient availability and toxicity?
Soil pH affects soil fertility.
In acidic soils (low pH), H+ ions (which are cations) can displace positively charged nutrients from cation exchange sites, causing them to be leached away and become unavailable for plant growth.
Toxic aluminium is also soluble and toxic at low pH.
What are the two main topographical types of soil (linking to water drainage)? Which do crops typically grow best on?
Upland soils (well drained; often forests)
Wetland soils (often partially flooded, with unique vegetation tolerant of anoxic conditions)
Crops typically thrive the best on well-drained upland soils (exceptions include rice paddies).
Describe the GHG fluxes in forest soils.
Forest soils can also take up some atmospheric methane (many methanotrophs with high activity in these forest upland soils).
Can, however, can be a source of nitrous oxide (more powerful than methane).
What is the sequestration of C in soil equal to?
Biomass and C from roots.
Does carbon storage increase with forest tree size? How does this compare with “stand level productivity”?
The larger the trees, the more carbon they take up and the more they grow (was thought that with age there would be limiting returns, but this is not the case).
Note: when looking at forests/stand level productivity, it decreases with age as tree density declines as trees get larger as there is more shading.
How does Gross Ecosystem Productivity (GEP), Ecosystem Respiration (R) and Net Ecosystem Productivity (NEP) change with age in a plantation?
GEP increases with age (gain of C)
R becomes more negative (loss of C)
Tends to equilibrate – R = GEP, so NEP is 0
Shows how returns decline with age.
What are peatlands?
Peatlands are wetlands with a high water table where slow decomposition of plant material (due to anoxia/anaerobic conditions) leads to the accumulation of organic matter over time.
Why are peatlands significant carbon stores despite emitting methane
They hold one-third of the total soil carbon globally despite covering only 3% of the Earth’s surface. Although peatlands emit methane, the amount of CO2 they store greatly outweighs the CH4 released.
Which moss is commonly associated with peat bogs? Why does this peat decompose slowly?
Sphagnum peat bogs:
- It is flooded, so is anoxic/anaerobic
- It is acidic (releasing protons, so further slowing decomposition)
- It is nutrient poor (very leached) known as an ombrotrophic bog
What causes peat subsidence?
Peat subsidence is caused by draining peatlands, often for agriculture. Draining makes the peat spongy initially, then denser and more compact as it dries.
Why is peat subsidence problematic?
This loss of peat is linked to a CO2 increase in the atmosphere
What percentage of agricultural land is affected by soil degradation, and how much land is lost to drought and desertification annually?
52% of land used for agriculture is moderately or severely affected by soil degradation.
Each year, 12 million hectares are lost to drought and desertification.
How many of the world’s poor are directly affected by land degradation?
74%
Where is soil typically stable globally (i.e., where is soil degradation limited)? Why is this unhelpful for agriculture?
The northern latitudes - however, limited crop growth occurs here due to conditions
