BIEB 174: Test 2

What is humus, how is it made, and what is its significance?

Humus is a humic/fulvic acid complex. It is made through decomposition where soil matter is broken down and results in the release of energy and nutrients. It is important because it is stable for millennia and works as an important carbon sink. It also contributes to water holding capacity and CEC.

What soil profile is identified by a spodic layer? What causes it?

Spodosol. This is because confiers release leaf litter that is highly acidic. The acidity causes a highly leached spodic layer that is chalky in color.

What is the pH range of most soils?

4.5-8

What is the relationship between redox potential and energy release?

High redox potential, high energy release

What factors will affect bulk density?

High clay and low soil organic matter (high BD)
Compaction (high BD)
Soil animals/aeration/mixing (low BD)
Low infiltration (high BD)

What causes water loss from leaves

Plants open the stomata in their leaves to allow CO2 to diffuse in for photosynthesis. This results in transpiration

What adaptations do plants make in order to reduce water loss in arid environments?

Evolve leaf hairs to trap humid air and reduce losses via vapor pressure gradient
High leaf angle with reflective surfaces to lower leaf temps (tanning laying down vs standing)
Small, thick leaves with many layers of photosynthetic machinery; low surface area for water loss
Specialized photosynthetic pathways

How is transpiration the major driving force of water through plants?

Water moves continuously through the column of the plant from the water film on the soil to the leaf cells due to the strong matric forces of water molecules themselves. As a result, plants do not spend energy transporting water (passive transport driven by transpiration)

What are the major types of energy transfer in ecosystems?

Conduction, Sensible heat, radiation, and latent heat

How does ecosystem structure influence albedo?

Reflectivity of the biome allows for it to reflect shortwave radiation. Dark and rough biomes will have a lower albedo than light and smooth ones.

Imagine that with rising temperatures there is increased cover of vegetation with a higher Bowen ratio than the original vegetation. What type of vegetation feedback will this produce to regional air surface temperatures?

Amplifying, increasing air temperatures due to increased sensible heat flux to the atmosphere

What is factor k in litter mass equation? What does it mean if k is small?

k is the decomposition constant (littlerfall/litterpool). If k is very small, decomposition is very slow

Why do microbes secrete digestive enzymes into the soil versus consume detritus directly?

They are too small

Which group has the greatest impact on soil properties such as oxygen availability, water infiltration, and compaction?

Macrofauna

Which group is the first to be visible to the naked eye and finally large enough to ingest the detritus itself?

Mesofauna

What are the direct effects of temperature on decomposition?

Generally, increased temperature will mean increasing microbial activity and soil respiration

What are the indirect effects of temperature on decomposition?

High temperatures can dry out soil and result depends on previous conditions

If already dry, then excessive drying will reduce microbial activity
If was waterlogged/anoxic, then drying will increase microbial activity

High temperatures may also increase nutrient availability due to weathering and decomposition (affects quality and quantity of litter)

How do nutrient concentrations affect substrate quality

Microbes still need carbon and nutrients

The C:N ratio is the measure of nitrogen concentration and directly affects decomposition only in presence of labile C

Lignin:N Ratio is an integrated measure of N concentration and substrate size and complexity

You find that fungi in your forest soils have a C:N of 16:1, assuming a 40% C growth efficiency. They will MINERALIZE when their substrate is _____________ than the critical C:N ratio of _________

lower, 40:1

You find that fungi in your forest soils have a C:N of 16:1, assuming a 40% C growth efficiency. They will IMMOBILIZE when their substrate is _____________ than the critical C:N ratio of _________

higher, 40:1

What is SOM decomposition influenced by in the rhizosphere?

Trophic interactions between bacteria and protozoan predators

Give an example of a NUE trade-off strategy

Long lived conifer forests need thick, protected leaves and sacrifice photosynthetic capacity. There is little tissue turnover and thus they must conserve nutrients, so they can afford to have a high NUE to make the most of what they have. As a result this ecosystem is not competitive when resources are high

(Littler decomposes slowly, promotes low nutrient conditions, feedback)

Why would it be better to not have a high NUE?

If your environment has adequate nutrients and rapid growth rates thus with rapid tissue turnover rates and your leaves have high photosynthetic capacity.

But because of this, this is only a good strategy to use when resources are available. Things go downhill quickly if resources are low

(Littler decomposes quickly, promotes high nutrient conditions, feedback)

Which of the following plant traits is most likely to influence soil pH?

Organic acid content of litter

Keystone species

When there is only one species in a given functional group

Ecosystem engineers

Alter the ecosystem through their behavior

Beavers create dams that slow streamflow, create sedimentation and create anoxic layers in ponds that result in hotspots of methane production

Elephants knock down trees and maintain open tropical savannahs

Disturbers (ex. worms, gophers)
Stabilizers (ex. plants in sand dunes)

What are the 3 components of diversity?

1. Species richness (# species)
2. Species dominance (equal abundance or no?)
3. Species composition (taxonomic identity)

What causes vertical structure in soil?

Processes in soil development; Parent material and inputs from outside of the ecosystem can be mineral [atmospheric like precipitation, dust, or deposition or horizontal like floods and tidal exchange] or organic [like litterfall and root turnover])

What is the importance of the parent material for plant growth?

Parent material is the source of mineral nutrients and thus influences plant growth

Which cation is most likely to be leached from soil?
Al3+, Ca2+, NH4+, NO3-

NH4+ because it has the highest leaching due to lowest binding affinity.

Where would you find the least variation across soil profiles?

Places where there is a lot of leaching.

What soil type is identified by highly stratified layers and a thick organic layer? Why is it like this?

Gelisols if bottom layer is permafrost histosol if its waterlogged. Highly stratified because less leaching occurring. Thick organic layer due to slow decomposition.

What soil type is identified by little to no organic layer and a calcic layer?

Aridisols

What soil profile is defined by a top layer of well mixed mineral soil with accumulated organic matter and humus? Why is it well mixed? What is it good for?

Mollisols. Well mixed due to lots of soil mixing animals. It is good for agriculture because it is fertile and has a high water holding capacity and high base cation availability.

What are state factors? List examples

Factors that create and control ecosystems without directly being a part of them. Examples include topography, climate, time, parent material, human activities, and potential biota.

How is parent material considered a state factor?

Type of parent material gives resulting weathered soils different properties. For example, soils weathered from sedimentary rocks that were formed from precipitation reactions are more likely to have a higher base cation availability. If soils were weathered from igneous rocks, they are more likely to be more acidic.

How is time considered a state factor?

The amount of weathering that's occurred affects the mineral nutrients that can be found in the the soil. Young soils have more phosphorous, while old soils are often more acidic and have low CEC and plant growth limited by phosphorous.

What happens to CEC when pH is low?

If pH is low, then conditions are more acidic, which is going to cause CEC to decrease. This is because the addition of H+ ions is going to kick cations off their loose bind to the negative clay plates. The cations are then gonna bind to the acid anions and leach off.

How does pH differ from agricultural to forest soils?

Agricultural soils have more neutral pH while forest soils are more acidic

How does pH influence soil properties?

Can decrease CEC

What is a redox reaction?

An oxidation reduction reaction where electrons are transferred from one reactant to another. Process yields energy

What is redox potential?

The tendency of chemical species to be reduced by acquiring electrons

Put these processes in order of high to low energy release
Denitrification, Methanogenesis, Aerobic Respiration, Sulfate Reduction

Aerobic Respiration, Denitrification, Sulfate Reduction, Methanogenesis

Aerobic soils will favor O2 reduction. But for waterlogged soils depleted of O2, what process will they undergo?

High nitrogen, denitrification. Low nitrogen, methanogenesis

List 3 physical properties of soil

1. Texture (Particle size)
2. Bulk Density
3. Water Holding Capacity

What conditions cause soil to have high water holding capacity?

Clay soil and high soil organic matter have higher water holding capacity

What is the relationship between total flux in and out of terrestrial ecosystems?

Equilibrium

Name 2 water inputs into ecosystems

Precipitation and fog

How does water move along an energy gradient?

From high energy to low energy

Name 3 forces that cause water to move

Pressures (ex. gravity; vapor pressure gradients), osmotic gradients, and matric forces (ex. surface tension, adsorption)

What is infiltration

Gravity driven water movement in soil

What are some scenarios that can affect infiltration

Impermeable layers like the calcic layer can prevent infiltration. Rate of infiltration depends on hydraulic conductivity of soil. Runoff occurs when infiltration is slower than precipitation

What is field capacity

Quantity of water remaining in a saturated soil after draining due to gravity

What is plant available water and what kind of soil has the most?

PAW is the difference between field capacity and permanent wilting point. Silt loam has the greatest amount of water from soils

What is permanent wilting point

When plants wilt because they cannot access water from soils

How is water movement in plants reminiscent of water movement in soils

It's the same equation

What is the vapor pressure gradient and what is its relationship with temperature and humidity?

Leaf interior is saturated, air is dry, and drives evapotranspiration. It increases with high temperatures and low humidity

How does water potential gradient influence water movement through plants? (Think back to the equation)

Water potential describes the strength of tension on water; the potential energy of water relative to pure water in an open surface (which is zero). Water potential can be measured through a pressure bomb, where we measure the amount of pressure to force the water out

What is a limiting factor for plants in dry soils?

Transpiration rates plateau due to soil moisture limits in dry soils

What is the driving force for water movement through stem? What other factors cause resistance?

Driving force is difference in water potential between leaf and root. Water moves through the stems to replace what was lost by the transpiring leaves. Resistance depends on the path/column length and stem structure (ex. think of boba straw, coffee stirrer example)

What happens to plant water potential and soil water potential when stomata close?

They are in equilibrium

You measure pre-drawn water potential during the dry season for 4 species, which is likely to have the shallowest roots?
chi=2, 0, -2, or -4

chi=-4. 0 is top of the soil and 2 is not possible because it implies that water is shooting out of the soil

What is the deep roots and hydraulic lift system?

The upward movement of water through roots from moist deep soils to dry surface soils along a gradient in water potential. It occurs in many arid ecosystems and moist forests. During the day, the water moves from the soils to the atmosphere, but at night the water moves from deep soil to dry surface soil because of the roots.

Why are dry conditions dangerous to the transport system?

It can collapse under very dry conditions. The water column can break and bubbles can get inside the xylem cavity. If this happens, it isnt possible for transpiration to suck water up anymore.
To try and prevent this, plants in drier environments have narrower xylem. This is a trade-off between hydraulic conductance and the risk of cavitation

In moist, undisturbed ecosystems, _________ is constant so precipitation __________ regulates streamflow

evapotranspiration, directly

Streamflow is directly linked with precipitation

Why does cutting down forests increase streamwater supply for agriculture?

Without the forest, there will be declining evapotranspiration rates which will increase the quantitity available for runoff

_______ cools the air and latent heat is _____ while ________ warms the air and latent heat is ______

evaporation, absorbed; condensation, released

What causes the variation seen in Rnet? (net radiation)

Varies bc of incoming solar radiation because longwave radiation is fairly constant and unchanging

What does surface albedo depend on?

Structure; Smaller leaves with more complex structure allow for less reflection

How are water and energy budgets coupled?

Through latent heat flux

Evapotranspiration accounts for 80% of heat transfer from earth surface to the atmosphere

Where is latent heat flux the greatest?

In tropical regions

Explain the Bowen Ratio

(Sensible heat flux/Latent Heat flux)

If B<1, then the greater proportion of available energy at the surface is passed down to the atmosphere as latent heat than as sensible heat
If B>1, then more energy transfer as sensible heat

When B is small, water and energy balance and tightly coupled

Which ecosystem has the highest Bowen Ratio?
Tundra, agricultural fields, evergreen temperate forest, desert

(Sensible HF/Latent HF). Deserts have very high Sensible and very low latent. So they will have the highest Bowen ratio.

Are energy and water budgets more or less coupled with a low Bowen ratio?

Highly coupled

Because more heat flux

Boreal forests have lower albedo than tundra. What will be their feedback to warming?

Amplifying feedback (aka positive)

Due to low albedo

An increase in boreal forest fires will likely produce a _______ feedback to climate warming via increased atmospheric CO2

Amplifying.

Fires lead to increased greenhouse gas emissions. Amplifying feedback loop via greenhouse gases leads to warming

Deciduous forests grow back first after a fire, with higher albedo and evapotranspiration rates than conifer forest they replaced. What kind of feedback is this?

Stabilizing

This is because the regrowth of high albedo deciduous species and increased ET(evapotranspiration)

What is the effect of tropical deforestation on climate warming (but also specifically albedo, Bowen ratio, rainfall, and temps?)

Amplifies climate

Higher albedo, much higher Bowen ratio
Less rainfall, higher temperatures

Higher temperatures with higher albedo because albedo cannot offset the agricultural's plants' evapotranspiration which dwarfs the wild plants.

What processes make up decomposition?

1. Leaching by water (fast)
2. Fragmentation by soil animals (increases surface area for microbial attack)
3. Chemical alteration (releases CO2 and nutrients)

What factors affect decomposition rate?

Type of substrate, age of substrate (exponential decline)

What is fungi able to decompose that most other decomposers cannot? What is it the main decomposer of? Do they require aerobic or anaerobic environments for that?

Cell walls/lignin. Wood. Aerobic

What is the importance of hyphae?

Hyphae are fungal networks that bind to aggregates in the soil and can transport water and metabolites. Can be used to import resources to poor soil so that enzymes can be made

What do bacterial decomposers specialize in? Require aerobic or anaerobic? Dependent on resources or can they import them? How do bacteria break down complex substrates?

Labile substrates/sugars/easy to break down. Can function anaerobically. Dependent on resources already in the soil and will go dormant if the substrate is exhausted of resources (and will reactivate if substrate present). Different species specialize in different breakdowns, so they work together to break down complex substrates

What are 3 microfauna soil animals?

Protozoans (bacterial predators), Nematodes (herbivores that eat as much as grazers above ground), Mites

What's significant about soil mesofauna?

Finally big enough to be visible to the naked eye and can actually ingest litter and the microbial community. They fragment litter and make more surface area for decomposition. They also excrete mineralized nutrients to fuel plant and microbial growth.

Give some examples of soil macrofauna, what is special about them, and their "job" in the ecosystem

Earthworms, termites, etc. They not only fragment litter but also ingest the soil itself. Their job is to be ecosystem engineers because they mix soil and carry organic matter to depth. They also reduce compaction and create channels for air, O2, water, and roots

Which groups most often secretes exoenzymes responsible for breaking down woody substrates (ex. lignin)

fungi

Which group is most active in anaerobic soils

Bacteria

What is the correlation between soil respiration and NPP (net primary productivity)?

Strong positive correlation

How does soil moisture affect decomposition?

Decomposition declines at both very high and very low decomposition. But more sensitive to high levels of soil moisture than net primary productivity

How and why do respiration-temperarture in lab versus spatial settings vary?

In the lab, there is an increase till about 20deg C where then there is a decline in respiration at high temperatures. In the spatial gradient, there is only an exponential increase.

This is because among spatial scales, there is greater diversity compared to whatever was grabbed from a single sample and tested in the lab. As a result, there are likely some heat tolerant or extremophiles present in the spatial group that will create this trend

What other environmental effects influence decomposition?

Soil texture - clay slows decomposition bc binds to SOM
Soil Disturbance- SOM protection by clays. Breaks up soil aggregates and increases O2 availability
pH-low pH=low decomposition. Highly acidic soils tend to be highly leached, nutrient poor, and dominated by slow growing plants with slow decomposition

With rising temperatures associated with climate change, how will decomposition rates change in waterlogged soils and why?

Increase due to direct effects of warming and indirect effects of soil drying

Why is climate change changing the Danish peat bogs?

Decomposition is increasing due to warming, microbial activity is increasing and soil is drying.

Many things are preserved in the bog and now theres a race to recover them before they decompose

What is substrate quality and what does it depend on

"Susceptibility to Decomposition"

Size of molecule, type of chemical bonds, regularity of structure, toxicity, and nutrient concentrations

How does size of molecule affect substrate quality?

Large molecules cant pass through membranes and must be broken down outside of the cell, so require exoenzyme decomposition

So large molecules have low decomposition while smaller ones have higher decomposition

How do types of chemical bonds affect substrate quality?

Some bonds are easier to break than others (ex. peptide bonds in protein are relatively easy to break). Most of litter nitrogen is in protein (abt 80%)

As opposed to bonds in aromatic rings that are hard to break. Most N in old soil organic matter is in armoatic rings, typically defense compounds

Microbes typically specialize in either fresh or old soil organic matter because need to produce different enzymes

Regularity of structure also changes substrate quality because...

Irregular structures slow decomposition

Ex. lignin and humic acids have very irregular structure and thus dont fit the active sites of most enzymes

How does toxicity influence substrate quality?

Plant evolved defense compounds can be either C based (phenolics or tannins) or N based (caffeine, nicotine). Toxic to herbivores and pathogens but also deomcposers

If well defensed, then more difficult to decompose

What compounds in litter will be the easiest to decompose (ie. highest quality)

Aminoacids, small compounds with easily broken peptide bonds and high N content