The Terrestrial Carbon Cycle: Soils

The Global Carbon Budget

• With increasing carbon emissions, both atmospheric carbon and carbon sinks (land and ocean) are increasing.
• The net terrestrial uptake has a big uncertainty in the future of the global carbon budget.

The Keeling Curve

• The Keeling Curve shows the change in concentration of carbon dioxide in Earth's atmosphere since the late 1950s.
• There is an annual cycle in the data due to greenhouse gases.
• Peak value each year is around 420ppm.
• The annual increase in peak values, D, is around 2.5ppm.
• Drop due to Northern Hemisphere summer sequestration (d) is around 7ppm.
• Since 2008, d is decreasing.

Natural Sequestration

• Natural sequestration by plants, drawing down atmospheric CO2, during the Northern Hemisphere summer (d), increased for many years with increasing atmospheric CO2 concentrations, until 2008.
• d is now in decline, which means atmospheric CO2 will now rise more rapidly.
• The current rate of increase of atmospheric CO2 of around +2.5ppm per year (D) would have been around +1.9ppm if natural sequestration had not begun to fall.

Objectives

• Understand how atmospheric carbon can be stored in soils.
• Learn the basics of soil science (formation, texture, and horizons).
• Understand how land-use-change alters the capacity of soils to store carbon.
• Understand why there is a big uncertainty in the future of the net terrestrial uptake and why d is declining.
• Understand how carbon gets from the atmosphere into the soil.

Carbon Movement Through Land Surface

• Photosynthesis
• Litter fall and dead roots ('litter' refers to dead plant material, such as leafs, needles, barks, twigs, fallen trees…)
• Plant respiration: ratio of photosynthesis to respiration about 2 to 1.
• Soil respiration: decay (microorganisms consume/decompose the dead plant material), but also root respiration
• Run-off and erosion
• Residence time of carbon in plant biomass: Days-100 years
• Residence time of carbon in soil: 10-500 years
• Less than 1% of the primary production is stored long term away from the atmosphere, however, over time this carbon accumulates.

Carbon Storage

• The amount of carbon stored above ground (in vegetation/plant biomass) and below ground (in soils) depends on soil type, vegetation and climate.

Pedosphere

• Derived from the Greek words πέδον (pedon) meaning "soil" or "earth" and σφαίρα (sfaíra) meaning "sphere."
• It is the outermost layer of the Earth composed of soil and subject to soil formation processes.
• The pedosphere is the interface of the lithosphere, atmosphere, hydrosphere, and biosphere.

Definition of Soil

• Soil is the mixture of minerals, organic matter, gases, liquids, and the countless organisms that together support life on earth.

Soil Development

• Soil is both a factor in weathering and a product of weathering.
• Weathering types:
  • Physical weathering (e.g., through temperature, freezing and thawing, wind, rain, waves)
  • Chemical weathering (e.g., through acid rain)
  • Biological weathering (e.g., through plant roots, burrowing animals, paths…)

Rates of Weathering

• Rates of weathering vary from place to place and from rock type to rock type.
  • Marble (soft rock):
    • Cold climate: $20 \, \mu m/year$
    • Warm, humid climate: $200 \, \mu m/year$
  • Basalt (medium rock):
    • Cold climate: $10 \, \mu m/year$
    • Warm, humid climate: $100 \, \mu m/year$
  • Granite (hard rock):
    • Cold climate: $1 \, \mu m/year$
    • Warm, humid climate: $10 \, \mu m/year$
• It takes (on average) about 100 years to generate 1 mm of soil.

Soil Profile

• Typical soil profile for a healthy matured soil includes:
  • O Horizon: Organic matter
  • A Horizon: Topsoil
  • E Horizon: Eluviation layer (in some soils)
  • B Horizon: Subsoil
  • C Horizon: Parent rock
  • R Horizon: Bedrock

Carbon Storage Within Soil

• Microbes transform fresh organic matter into stable humus (O-Horizon).
• Downward movement and accumulation in regions of the soil with high clay content (organo-mineral complexes form).
• Less oxygen in the lower parts of the soil makes it difficult for microbes to decompose it further.
• In average, the top one metre of the soil contains roughly three times as much carbon as the above-ground biomass of plants, and twice as much as the atmosphere.

Soil Texture

• The maximum capacity of soil to store organic carbon is determined by soil texture.
• Soil texture: proportions of sand, silt and clay content.
• Loam is the right mixture of all three that it holds nutrients well, retains water but still drains properly and allows oxygen to infiltrate
• Each soil type has a different soil organic carbon capacity because the three growth factors for plants (water, nutrients, and oxygen) are supported differently.
  • Large particles cause lots of air and space between each grain. Water and nutrients flow through easily but aren’t retained.
  • Small particles, close together -> very dense and sticky. It holds water very well. Poor drainage and high density makes it difficult for roots and organisms to break through. Water and nutrient holding capacity is medium to high but air content is poor.

Clay Content and Carbon Storage

• Particles of organic matter can:
  • adsorb to clay surfaces,
  • be coated with clay particles
  • buried inside the small pores or aggregates.
• This makes it difficult for microorganisms to access and decompose the organic matter.
• Clay content has special significance for long term carbon storage because of its high specific surface area and negative charge
• E-Horizon (Eluviation, lat. wash): no clay, only silt and sand => organic matter, minerals and nutrients are washed out
• Downward movement and accumulation in regions of the soil with high clay content (organo-mineral complexes form).

Soil Organic Carbon Density

• The fraction of total carbon in living biomass increases towards the equator, but the proportion in soils shows the opposite pattern, reflected in the soil organic carbon density.

Tropical Forest: Oxisols

• Almost complete absence of soluble minerals leached by the wet and humid climate leading to infertile soils.
• Plants are forced to get their nutrition solely from decaying litter, leading to highly efficient use of organic matter.
• This results in no/low accumulation of humus and nutrient or carbon reserves.
• Most nutrients and carbon are contained in the standing vegetation and decomposing plant material.

Temperate Grassland: Black Soils

• Temperate grasslands have soils that are nutrient-rich from the growth and decay of deep, many-branched grass roots.
• The rotted roots hold the soil together and provide a food source for living plants.
• The world's most fertile soils underlie the eastern prairies of the U.S., the pampas of South America, and the steppes of Ukraine and Russia.

Northern Wetlands: Hydric Soils

• Formed under conditions of saturation, flooding or ponding during the growing season which leads to anaerobic conditions.
• Anaerobic decomposition is less effective than aerobic decomposition, leading to accumulation of organic matter.
• Anaerobic microorganisms tend to utilise the oxide forms of nutrients and carbon, resulting in reduced forms are dominant in strongly anaerobic soils (e.g., CH4).
• Wetlands are currently a net sink of greenhouse gases.

Ecosystem Comparison as Carbon Sinks

• Comparison of Oxisols, Black Soils, and Hydric Soils in terms of average stored carbon in t/ha at a ground depth of 1 m.

Wetlands and Methane Emissions

• Roughly 40% of methane emissions are from wetlands.
• Wetlands are the world’s largest natural source of methane emissions.

Methane Emission Trends

• Methane emissions have risen by 1.2-1.4m tonnes per year, faster than the average projection under the most pessimistic emission scenario (RCP8.5).
• Greatest current increase in global wetland methane emissions comes from tropical wetlands (temperature driven).
• Especially tropical wetlands are spreading, getting wetter and warmer, releasing more and more methane.
• As the changing climate causes shifts in rainfall patterns, some wetlands will expand and new soils become waterlogged in certain areas.
• Warming undermines the mitigation potential of pristine wetlands even for a limited temperature increase of 1.5-2C.

Land Use Change

• About 3/4 of the ice-free land globally are altered and used by humans.
• The biggest uncertainty in the future of the global carbon budget is the net terrestrial uptake, largely because of uncertainties in land-use change and the extend of climate extremes.
• Breakdown of land use:
  • Growing crops: 12-14 %
  • Managed and planted forest: 22%
  • Grassland adopted for grazing and other uses: 37%

Trade-offs Among Ecosystem Services

• Deciding whether trade-offs among land-use types are negative or beneficial depends on values and priorities, which is usually a socio-political decision.
• Sources suggest there are few, if any, beneficiaries from extreme degradation and the permanent loss of function and services.
• There are always trade-offs among ecosystem services and biodiversity with land use intensification.

Land Use Change - Deforestation

• Deforestation acts in two ways as a carbon source in the climate system:
  • loss of plant carbon
  • loss of soil carbon

Soil Erosion

• Land cleared or overgrazed -> vulnerable to erosion by wind and water -> exposure of carbon which was stored in the ground back to the atmosphere.
• Crops offer poor soil protection:
  • they are present only part of the year
  • the plowed ground readily gives way to runoff and wind
  • reduced infiltration and absorption of rainfall -> increased runoff
• Plants:
  • secure the soils against the force of running water and wind
  • induce infiltration and absorption of rainfall so that less water is available for runoff

Soil Organic Carbon Loss

• Over the past two centuries, soil organic carbon has seen an estimated 8 per cent loss globally (176 gigatons of carbon (Gt C)) from land conversion and unsustainable land management practices
• Projections to 2050 predict further losses of 36 Gt C from soils from the expansion of agricultural land into natural areas (16 Gt C), degradation due to inappropriate land management (11 Gt C) and the draining and burning of peatlands (9 Gt C) and melting of permafrost.

Net Terrestrial Carbon Uptake

• Disturbance from both natural and anthropogenic sources leads to further release of CO2.
• It takes thousands of years to create a soil and only a small fraction of organic carbon is stored long term.

Ecosystem Disturbances

• Natural:
  • forest fires
  • storm-based vegetation removal
  • sudden pest and disease infestation
• Human-induced:
  • land cover change
  • de- and re-forestation
  • agriculture
  • over-grazing
  • invasive species
  • pollution
• Ecosystem Disturbances are all predicted to increase with climate change
• IPPC special report Land (August 2019): The most important links between climate change and land degradation are the result of increasing temperatures, changing rainfall patterns and intensification of rainfall.

Summary

• It takes about 100 years to generate 1 mm of soil, once this sink is gone (e.g. due to erosion) it is hard to replace.
• Higher density of soil carbon correspond to areas where soil carbon accumulates, generally regions where vegetation growth and decomposition is slower.
• The soils which are the best carbon sinks for mitigating climate change are the soils we use and need for agriculture and therefore degrade most.
• Land use change such as deforestation, agriculture and pasture are potentially turning the terrestrial carbon sink into a carbon source.
• The wetland methane feedback might turn wetlands from effective net carbon sinks to net carbon sources through increased release of methane.
• Climate change is strengthening the natural ecosystem disturbances, weakening the land carbon sink even further.