Chapter 21 - Decomposition and Nutrient Cycling

Predator populations fluctuate...

-As protist and nematode populations decline, their readily decomposable tissues enter the detrital food chain -protists - as much as 70 percent of soil respiration -nematodes - as much as 15 percent -Globally, rhizosphere processes use about 50 percent of the energy fixed by photosynthesis in terrestrial ecosystems and emit about 50 percent of the total carbon dioxide

The nitrogen content of the fungi and bacteria is much higher than that of the plant material they are feeding on...

-Carbon/nitrogen ratio of plant litter is in the range of 50:1 to 100:1 -Carbon/nitrogen ratio of bacteria and fungi is in the range of 10:1 to 15:1

history of inorganic fertilizers cont.

-In the early 20th century, a German physical chemist (Fritz Haber) developed a process to manufacture ammonia (NH3) from atmospheric nitrogen (N2) -A German industrial chemist (Carl Bosch) developed this into a large-scale process (the Haber-Bosch process) By the 1920s, plants were being built in the United States and other European countries -World War II increased industrial production of ammonia for explosives -After the war, focus shifted to agricultural uses -By 1950, the use of inorganic fertilizer increased exponentially, leading to large gains in productivity -Currently, about 71 percent of fertilizers used are inorganic and about 29 percent are organic

history of inorganic fertilizers

-Inorganic fertilizers were mined from natural geological mineral deposits as early as the 18th century -As the human population increased, the increased demand for food led to an increasing need for fertilizer -By the late 1800s, there was concern about depleting the sources of nitrogen compounds used as mineral fertilizer (KNO3 and NaNO3)

Nitrogen content increases as microbial decomposers immobilize nitrogen from outside the litter

-It can exceed 100 percent -happens when things are going correctly

more on swidden cultivation...

-It is a sustainable form of agriculture as long as there is enough time for natural vegetation to regrow and soil nutrients to recover -Requires enough land to allow for rotation -With increasing population pressure in the tropics, sufficient recovery periods are not always possible -Land is degraded and yields continually decline

The abandoned site is fallow (not used for farmland) for a period of time...

-It reverts to natural vegetation by secondary succession -If left undisturbed long enough, the nutrient status will recover to the levels seen before the plot was farmed -The site can then be cleared and planted again -During this time, other plots have been cleared, burned, and planted -Leads to a patchwork of plots in various stages of cultivation and succession

Decomposition includes many processes:

-Leaching -Fragmentation -Changes in physical and chemical structure -Ingestion -Excretion of waste products

Agriculture disrupts the balance between ______ and ________________

-NPP, decomposition -Plants, and the nutrients they contain, are harvested -Much less organic matter returns to the soil surface to undergo decomposition and mineralization -This leads to a decline in soil fertility and reduces further plant productivity

Bacteria acquire nutrients by breaking down soil organic matter...

-Nutrient immobilization occurs during microbial growth -Predation on bacteria by protists and nematodes remobilizes essential nutrients for plant uptake

change in nitrogen content of plant litter during decomposition (graph)

-The initial phase (A) corresponds to the leaching of soluble compounds -Nitrogen content then increases above initial concentrations (phase B) as the rate of immobilization exceeds the rate of mineralization As decomposition proceeds, the rate of nitrogen mineralization exceeds that of immobilization, and there is a net release of nitrogen from the litter (phase C).

Organic fertilizer must have a C/N that leads to...

-a rate of immobilization < rate of mineralization through decomposition after application -This gives an immediate net release of mineral nitrogen to the soil -The critical C/N is the C/N at which this occurs

Nitrogen availability in the soil limits the rate of uptake...

-affects the rate of NPP -directly influences the rate of photosynthesis

Through digestion of food, organic matter is (three things):

-broken down -altered structurally and chemically -partially released as waste

fertilizers contain chemical elements that:

-improve soil fertility -enhance the growth and productivity of plants -Organic and inorganic forms are available

fallow

-not used for farmland -describes abandoned land for a period of time

inorganic or organic fertilizers?

-organic fertilizers are better -Inorganic fertilizers will kill soil - not a good idea -Need natural processes

The soil microbial loop describes this process of

-plants supplementing carbon to microbial decomposers in the rhizosphere -enhanced decomposition of soil organic material -predator remobilization of mineral nutrients for plant uptake (predators feeding on bacteria)

Inorganic (mineral) fertilizers

-provide nutrients to the soil that can immediately be taken up by the plant -They are also readily leached from the soil -A significant proportion of inorganic fertilizers applied to agricultural fields ends up in groundwater or surface waters -This can have serious environmental consequences

The rhizosphere

-region of the soil where plant roots function -an active zone of root growth and death -It has intense microbial and fungal activity -In fine-rooted grasslands, most of the soil is rhizosphere (about 1 mm between roots) -In forest ecosystems, root density is far less per unit of soil volume (about 10 mm between roots) -roots are doing something to enhance all of this activity

The organic material in the litterbags includes:

-the original dead leaf material -living and dead microorganisms

typical leaf litter ratio

C:N (50:1 - 100:1) -Much more carbon than nitrogen

exchanges between litterbag and soil in a standard litterbag experiment used to quantify the process of decomposition:

Decomposer organisms (bacteria and fungi) colonize the plant litter. As litter is consumed, a significant proportion of carbon is respired, and nutrients bound in organic matter are mineralized and released to the soil. To convert plant carbon to microbial biomass, mineral nutrients are taken up in the process of immobilization. The difference in the rates of mineral nutrient release (mineralization) and immobilization is the rate of net mineralization. Note that as time progresses, the residual organic matter in the litterbag is composed of a growing proportion of microbial biomass as the original plant material is consumed, respired, and converted into microbial biomass.

plant litter

Many experiments involve the use of litterbags to examine the decomposition of dead plant tissues -These are mesh bags made of synthetic material that does not readily decompose, with 1-2 mm holes -A certain amount of plant litter is placed in each bag

Excess nitrogen is excreted as ____________ so can be taken up by plant roots

ammonia

the sources of these essential nutrients are usually the ______________ or ___________

atmosphere, rocks

the nutrients enter the soil or water and are taken up by ______________

autotrophs

This root exudate is an abundant, high-quality, energy-rich source that supports _____________ __________

bacterial growth

Roots alter rhizosphere chemistry by secreting ___________________ into the soil

carbohydrates -as much as 40 percent of dry matter production -carbohydrates provide food for these microbes and increase their numbers

main readily available source of carbon - not the largest

carbon dioxide

____________ ____________ is usually readily available while __________ isn't always readily available

carbon dioxide, nitrogen

carbon that is fixed through photosynthesis is ultimately returned to the atmosphere through ____________ _____________

cellular respiration

Plant tissues senesce

dead leaves

Nitrogen is returned to the soil surface as

dead organic matter

_________________ and detritovores are organisms that feed on dead organic matter or detritus

decomposers

all heterotrophs are _______________ to some degree

decomposers

When organisms lose parts or die, the nutrients are returned to soil or sediments as dead organic matter and enter the ______________ food chain

detrital

Sycamore is decomposing at a slower rate than the white oak and red maple. Why?

difference in ligament amount

Not enough nitrogen? NPP will...

go down

net mineralization rate (the rate that minerals are supplied to the soil)

is the difference between the rates of mineralization and immobilization

Organic fertilizers

made of organic material that comes from plants or animals -naturally occurring - manure, guano peat, algae, dead plant material (green manure, including crop residues and cover crops) -processed - compost, bone, blood meal, fish meal, plant and algal extracts -a decomposer food source -Decomposition and mineralization make the nutrients available for plant roots

Swidden agriculture

method of traditional subsistence farming, mainly in the tropics -Trees and other vegetation are cut and burned to provide cleared land for planting crops -removes debris -Ash produced is high in mineral nutrients -Plot is cultivated and plants are harvested -Productivity declines with each successive harvest -Each harvest removes plant tissues (nutrients) -Eventually the plot is abandoned -Whatever is left will increase fertility of soil some -With each harvest, less and less nitrogen available

primary production depends on

mineral (inorganic) nutrients taken up by autotrophs -they incorporate the nutrients into their tissues

organic compounds put into an inorganic form

minerals

C/N greater than 30 results in

net immobilization, removing mineral nutrients from the soil

leaching

nitrogen in the litter declines as water-soluble compounds are leached out -generally very short -depends on soil moisture in terrestrial environments -immobilization greater than mineralization

Plants uptake nitrogen (as ammonium and nitrate) from the soil through their roots; This nitrogen is used to make ______________

nitrogen-based compounds, such as proteins and nucleic acids -Producing these compounds converts the nitrogen from inorganic to organic form

Bacterial growth is limited mainly by:

nutrient availability (exudate is low in essential nutrients)

Relationship between microbial decomposers and microbivores determines the rate of ___________ ___________ in the rhizosphere

nutrient cycling -Enhances the availability of mineral nutrients to plants

Nutrients stored in _______________ comprise a significant part of nutrients within ecosystems

organisms (living tissue)

decomposers

organisms that feed on dead organic matter or detritus (bacteria, fungi, and detritivores) -All heterotrophs function to some degree as decomposers

retranslocation

plant is taking back nitrogen from leaves before they fall

Generally, organic materials with a C/N less than 20 have a _____________ net mineralization (rate > 0)

positive

nutrient cycling

process includes the mineralization of organic nutrients by microbial decomposers -These minerals are then available for the autotrophs to take up and use to build new tissues

Plants absorb some of the nutrients from senescing tissues into the perennial parts of the plant through:

retranslocation or resorption -In autumn, deciduous trees can recover up to 70 percent of the nitrogen from green leaves before they drop from the tree -trees have taken back a lot of nitrogen from the trees

fertilizers

substances that supplement soil nutrient supplies

decomposition usually occurs on the...

surface of soil

As plant material is consumed, nitrogen is immobilized to meet the demands of the decomposers...

the C/N ratio declines

mineralization

the process through which microbial decomposers convert nitrogen and other elements from organic compounds into inorganic (mineral) forms -These decomposers also need nitrogen for their own growth and reproduction

immobilization

the uptake and assimilation of mineral nutrients by microbial decomposers -taking it back and putting into organic form

Swidden cultivation is practiced by about 500 million people (7 percent of the world population), mainly in the ____________

tropics

true/false: carbon and nitrogen can be recycled unlike energy

true