soils quiz 4

What are the main roles of the macrofauna

create porosity, mix soil with organic matter, fragment litter

What is the difference between chemo-heterotrophic and chemo-autotrophic organisms

heterotrophic: need organic C source for energy; autotrophic: don’t need organic C source for energy

How does the redox ladder work

If O₂ runs out, other electron acceptors are used to break down organic C including NO₃^-, Fe³⁺, Mn⁴⁺ etc.

As you go down the ladder, electron acceptors become less efficient; i.e. gain less energy per unit C consumed. 

What is nitrification and under what conditions does it occur

conversion of NH₄⁺ into NO₃^-, occurs when there is NH₄ and O₂

What are the consequences of nitrification

soil acidification, higher N leaching potential because NO₃ is highly soluble and not sorbed to negatively charged solid surfaces.

What are the main roles of fungi

breaking down large C compounds including cellulose and lignin

What are mycorrhizal fungi and why do they form symbiotic relationships with plants

fungi that live in association with plants; plants provide c compounds and fungi take up nutrients that can be used by plants.

What is decomposition

breakdown of organic matter

What is N fixation and under why is it often done by symbiotic organisms

conversion of N₂ gas into NH₄⁺; it costs a lot of energy and plants can provide easily decomposable C compounds for fungi to use, the plants can take up NH₄ produced by N-fixing organisms

What is denitrification and when does it occur

breakdown of organic matter using NO₃ instead of O₂ as electron acceptor; it occurs when no O₂ is available such as in waterlogged soils. Also organic matter and NO₃ have to be available (heterotrophic process).

When does iron reduction/oxidation occur and how can it be detected in soils

when soils are waterlogged when no O2 is available; it can be detected by looking at the soil soil color. When the soil is red/brown, no Fe is used to break down organic matter and soils are oxic/aerobic. When soils are grey, Fe is reduced to break down organic matter; soils are anaerobic anoxic, typical for waterlogged conditions

What are microbiotic crusts

assemblages of different microorganisms (algae, fungi, cyanobacteria, etc.) present on top of desert soils. The are critical for C and N cycling in desert soils.

What are the main consequences of decomposition

breakdown of organic matter, release of gases, release of nutrients into the soil

What are the main sources of organic matter

dead plant materials (shoots, roots), dead organisms, excrements

What is the approximate C concentration of organic matter

50%

What is the relationship between substrate complexity and decomposability

the larger the molecule, the harder it is to decompose

What are some of the most and least decomposable compounds

most: sugars, amino acids, proteins

least: cellulose, lignin, wax, phenol

What are the main mechanisms by which organic matter decomposes

fragmentation (often by macrofauna) followed by exocellular breakdown. Other mechanisms include leaching, UV radiation, rain impacts (especially important in the initial stages)

What are the main controls on decomposition

Substrate quality, climate (temperature/moisture), nutrient availability, O₂ availability

What is the role of C:N ratio in determining decomposition rates

the higher the C:N ratio of fresh litter, the more difficult it is to decompose

What is humification

accumulation/new formation of stable compounds during decomposition. These compounds are very difficult to decompose and persist in the soil for a long time (up to 1000s of years)

How does humus affect soil physical and chemical properties

chemically very reactive, it affects aggregate formation/stability, nutrient sorption/retention, water retention

Globally, how much C do soils contain compared to the atmosphere

soils contain twice as much C as the atmosphere

What is N mineralization/immobilization

Mineralization: NH4 release by microorganisms during decomposition

Immobilization NH4/NO3 uptake by microorganisms during decomposition

How do mineralization/immobilization depend on C:N ratio of the substrate

substrate has low C/N ratio (high N content): N mineralization

substrate has high C/N ratio (low N content): N immobilization

What are the consequences of mineralization/immobilization for soil fertility

mineralization: increases soil fertility (NH4 is added to the soil)

immobilization: reduces soil fertility (NH4/NO3 are removed from the soil and taken up by microbes)

Understand the main process of the N cycle: N fixation, mineralization/immobilization, nitrification, leaching, denitrification, ammonia volatilization

ammonia volatilization happens in alkaline soils and is not common except for intense livestock farms with high manure production.

Fresh organic matter primarily consists of

water

The carbon concentration of dry organic matter is approximately:

50%

True or False: The macrofauna plays a minor role in OM decomposition.

False

What fraction of original OM remains as humus?

25%

What happens to the N concentration of remaining materials during decomposition?

It increases

What happens with N when straw (C/N = 80) decomposes?

N will first be immobilized, then mineralized.

Why does inorganic N typically not accumulate in soils?

N is required in large quantities by biota

What can cause slow recovery of forests following a wildfire?

Removal of biomass (N), nitrification followed by leaching of N, and Combustion of organic matter. (aka “all of these”