Soil Water & Air
Why is the concentration of oxygen lower in soils than in the open atmosphere? Why is the concentration of carbon dioxide lower in the open atmosphere than in soils?
The concentration of oxygen is lower in soil than open atmosphere because the rate at which oxygen is being diffused into the soil is outpaced by the rate at which soil organisms are expelling carbon dioxide through cellular respiration.
The concentration of CO2 is lower in the open atmosphere because the rate at which plants are releasing oxygen through photosynthesis outpaces the rate at which organisms are releasing CO2 through respiration.
What does it mean to be anaerobic? Name one organism that functions under anaerobic conditions
Anaerobic Respiration- ability to break down sugars in the absence of oxygen. Does not need oxygen to function
Anaerobic Organisms: Denitrifying bacteria, Pseudomonas (primarily aerobic, can adapt), protozoa (can be ~ aquatic organisms), archaea,
What is “immobilization” of nitrogen or phosphorus?
Immobilization- the process in which soil microbes absorb nutrients from the soil, making them unavailable for plants to use
How can a farmer increase soil diversity?
Diverse Crop Polycultures: Different crops support different soil organisms, so more plant diversity = more organism diversity.
Integrated Pest Management: Act as an alternative to chemical pesticides which serve to decrease soil organism populations
Cover Crops: support soil organisms while cash crops aren't growing
Detail the relationship between plants and soil.
“Soil is effectively the accumulated residue of dead plants comingling with broken down rock at the Earth’s surface.” Soil would not exist without land plants (and fungi), and plants continue to be the main driver of the biological processes within the soil. This is because plants photosynthesize, capturing the sun’s energy and then providing that solar energy to all other soil organisms.
The name given to this concept: vapor-pressure gradient from the leaf surface to the atmosphere is the driving force for water movement through plants.
Soil-plant-atmosphere continuum
Water transport from the soil through the plant to the atmosphere takes place in a soil–plant–atmosphere continuum that is interconnected by a continuous film of liquid water. Water moves from the soil through the plant to the atmosphere along a gradient in water potential. The low water potential of unsaturated air outside leaves, relative to the water potential of saturated air inside leaves, is the major driving force for water loss from leaves which in turn drives water transport along a water-potential gradient from the roots to the leaves, which in turn drives water movement from the soil into the plant.
This class of organisms affect decomposition rates indirectly. A significant fraction (most?) of the litter these organisms consume is not fully digested, and the fragmented litter that is a waste product is more easily and completely decomposed by the guild of decomposers.
Detritivores
Once inside an ecosystem, what is the most common “circuit” that a nitrogen or phosphorus atom will travel?
Soil organic matter gets decomposed -> decomposers mineralize the waste (nitrogen or phosphorus) and it becomes part of the soil mineral nutrient pool -> plants uptake the mineral nutrients -> the plants die and become soil organic matter
What are the major types and causes of soil degradation (name 3)? What human activity is most commonly associated with soil degradation?
Land use change, water and wind erosion, loss of soil biodiversity, loss of SOM and nutrients, soil salinization, soil contamination, soil compaction, soil acidification
Food production is most commonly associated with soil degradation
These are the major components of soil and their basic proportions
Water-25%
Air-25%
OM/Biota-5%
Mineral Matter-45%
Why do water molecules adhere to charged surfaces and cohere to each other?
Polarity: the hydrogen molecules are slightly positive while the oxygen molecules are slightly negative. This causes hydrogen bonding between water molecules, explaining why they cohere to each other, as well as why they can adhere to the surface of soil solids
Surface Tension: cohesion of water molecules have greater attraction to each other than to the air above
What are the differences between EM and AM fungi?
1. Ectomycorrhizal fungi can decompose organic matter and thus obtain nutrients directly. Arbuscular mycorrhizal fungi scavenge nutrients released by microbes.
2. AM fungi penetrates the roots of the plants whereas EM fungi forms a net on the outside of the root
3. EM fungi is found mostly in boreal forests whereas AM fungi prefers grasslands or tropical forests. AM fungi can colonize far more species than EM
What is ecological stoichiometry and what does it means for an organism to be “limited” by a given nutrient (e.g. C, N, or P)?
Ecological Stoichiometry: the idea that organisms generally need elements in some ratio
- being limited by a given nutrient means that the organism's growth and development are restricted by the lack of a given nutrient in its environment
What are cover crops and how do they help regenerate soil?
Cover crops- crops that are planted to cover the soil for the duration of the year when the cash crops aren't growing
- Keep living roots in the soil: contribute to SOM, break up soil compaction, contribute to aggregation, provide habitat and food for soil organisms, cover the soil to minimize wind and water erosion, suppress weeds, some can fix nitrogen
Name at least two ways that soil biological processes contribute to good aggregation.
1. burrowing/molding activities of soil animals like earthworms and growth of plant roots. Earthworms form worm castings which aggregate soil. Channels created by plant roots and soil animals serve as large conduits for new root growth. These channels also break up clods and help to define larger soil structural units plus, in the act of burrowing, they are pushing soil bits together.
2. The enmeshment of particles by sticky networks of roots and fungal hyphae. Plant roots (particularly root hairs) and fungal hyphae exude sugarlike polysaccharides and other organic compounds, forming sticky networks that bind together individual soil particles and tiny micro aggregates into larger macro aggregates. Fungi that associate with plant roots (mycorrhizae) produce a sticky sugar-protein called glomalin, which is thought to be an effective cementing agent
3. The production of organic glues by microorganisms, especially bacteria and fungi. Bacteria produce organic glues such as the polysaccharides intermixed at a very small scale with clay. Many of these root and microbial organic glues resist dissolution by water and so not only enhance the formation of soil aggregates but also help ensure their stability for months or a few years.
In what ways is oxygen availability heterogeneous in soils?
1. The insides of large aggregates can be anaerobic
2. There is more oxygen availability at the top of the soils because most oxygen that diffuses into the soil is used by organisms before it can get very deep
3. Concentrations of oxygen are greater in soils with bigger pore spaces (sandy soils vs. clay)
What are the three types of earthworms, where do they reside in the soil, and how do they move?
Anecic Earthworms: vertical motion. Can be found deeper than endogeic and epigeic
Epigeic Earthworms: Reside mostly in plant litter layer at the surface of the soil. They don't make burrows
Endogeic Earthworms: Reside in the upper layer of the soil. They move horizontal and make temporary burrows
What are the approximate C:N ratios of plants, soil organic matter, and soil biota, and under what conditions will soil biota tend to mineralize nitrogen? Are those conditions common or rare?
Wood - 300:1
Roots - 50:1
Dead leaves - 50:1
Green leaves- 28:1
Soil organic matter- 12:1
Soil bacteria, fungi, and most non-plant living things- 8:1
- 30:1 is generally the threshold at which a higher ratio is considered nitrogen limited and a lower ratio is considered carbon limited
What is biochar? Is all biochar basically the same? What two big factors can lead to biochar with different properties? How can biochar help improve soil health?
Biochar- what remains after organic matter is combusted with very little oxygen at intermediate temperatures.
- No two biochars are exactly the same
- Temperature and oxygen concentrations can affect the biochar's surface area and pH. Different feedstocks yield very different biochar properties
- Biochar is rich in carbon and can remain in soils for centuries, so it can improve soils with low organic carbon. Because it acts like platelets, it can also improve soil structure, improve water and nutrient retention, stabilize soil pH, and immobilize contaminants
The name for this danger to plants: If the pull of water from the foliage is stronger than the supply of water from the roots, the continuous column of water can break, introducing air bubbles and most likley devestating the plant.
Xylem Cavitation
Explain the relationship between evapotranspiration, gravity, and matric forces on soil water. How are these related to “field capacity” and the “permanent wilting point”?
Evapotranspiration pulls soil water up, gravity pulls soil water down, and the matric forces of adhesion and cohesion hold soil water to soil particles. All three of these can make water inaccessible to plants
Field Capacity: Films of water surrounding soil particles are thin enough that gravity cannot pull them away but the water is able to be taken up by plants. Transpiration is also strong enough to pull water particles away
Permanent Wilting Point: Films of water surrounding soil particles are too thin for plants to access and transpiration cannot pull them away
What does it mean to say an organism (e.g. plants, soil microbes) is “carbon limited” or “nitrogen limited” or “phosphorus limited”?
To be "limited" means that the organism's growth and development are restricted by the lack of a given nutrient in its environment
- Carbon limited organisms will hold onto carbon and release nitrogen as a waste product
- Nitrogen or phosphorus limited organisms (like plants) will hold onto nitrogen or phosphorus and release carbon through cellular respiration
What are the major inputs and outputs of nitrogen to terrestrial ecosystems and how do those impact nitrogen availability over geologic time?
What are the major inputs and outputs of phosphorus to terrestrial ecosystems and how do those impact phosphorus availability over geologic time?
Nitrogen Inputs: lightning, fertilizer, nitrogen fixation
Nitrogen Outputs: leaching, denitrification
- Nitrogen inputs from fixation and lightning slightly outpace losses from leaching and denitrification, so nitrogen in local ecosystems builds over geologic time
Phosphorus Inputs: weathering of rock, dust, fertilizer
Phosphorus Outputs: chemical occlusion, leaching
- Phosphorus losses from leaching and occlusion slightly outpace inputs from weathering and dust, so phosphorus in local ecosystems decreases over geologic time
Why are local nutrient cycles so broken by industrial agriculture? What are CAFOs and why are they so problematic?
Food systems have moved away from an integrated farm model. On an integrated farm, animal waste is a resource that can help maintain soil health, but in industrial agricultre, waste is more akin to industrial pollution that has to be dealt with. Crops are taking up nutrients that aren't being replenished, leading to the use of chemical fertilizers that contribute to leaching.
CAFO: Concentrated Animal Feeding Operation. For each pound of live weight, a farm animal produces 2-4 pounds of manure per year that contains ~80% of the nutrients it consumed. Because animals are carbon limited, they release nitrogen as a waste product that is not being returned to planted systems.
Mechanisms by which SOC is protected from further decomposition.
Cold but productive environments store soil carbon. Decomposers and other soil organisms are exothermic, meaning their body temperature is determined by the environmental temperature (unlike us). Thus, as temperature decreases, their biological activity also decreases. The boreal forest soils of Canada and Russia store a lot of carbon for this reason. Worryingly, this carbon may be rapidly decomposed as the planet warms, creating a vicious cycle.
Anoxic (low oxygen) but productive environments store soil carbon. Anaerobic decomposition is vastly less efficient than aerobic decomposition. Wetlands typically contain productive plants but almost no aerobic decomposition. Wetland soils store a lot of carbon for this reason.
Well-aggregated soils store soil carbon. Small bits of soil can get trapped inside aggregates and thus become protected from decomposition.
Bits of organic matter can become adsorbed to clay surfaces and sandwiched into clay domains where they become protected from decomposition.
Plant bits subjected to fire in low-oxygen conditions become “char,” which is difficult for microbes to decompose. This leads to some long-term carbon storage after natural wildfires and is one of the benefits of using “biochar” as a soil amendment