Decomposers, Aquatic and Nutrient Cycles

Heat Heat Heat Heat

Heat

Heat

Heat

First TrophicLevel

Second TrophicLevel

Third TrophicLevel

Fourth TrophicLevel

Solarenergy

Producers(plants)

Primaryconsumers(herbivores)

Tertiaryconsumers

(top carnivores)

Secondaryconsumers(carnivores)

Detritvores(decomposers and detritus feeders)

Three Major Types of Nutrient Cycles

• Hydrologic (or water) Cycle – water in the form of ice, liquid water and water vapor cycles through the biosphere.

• Atmospheric Cycle – a large portion of a given element exists in a gaseous form in the atmosphere.

• Sedimentary Cycle – An element does not have a gaseous phase, or its gaseous compounds do not make up a significant portion of its supply.

Hydrologic Cycle• Collects, purifies, and distributes the

Earth’s fixed supply of water – powered by the sun.

• Distribution of Earth’s Water Supply:– Salt water (oceans) = 97.4%

– Freshwater = 2.6%• 80% in glaciers and ice caps• 20% in groundwater• 0.4% in lakes and rivers (0.01% of all water!)

– Anytime of year, the atmosphere holds only 0.0001% of water on the planet.• Although large quantities are evaporated and

precipitated each year• About 84% of water vapor comes from the ocean

1. Evaporation – conversion of water into water vapor

2. Transpiration – evaporation from leaves of water extracted from soil by roots

3. Condensation – conversion of water vapor into droplets of liquid water

4. Precipitation – rain, sleet, hail, and snow

5. Infiltration – movement of water into soil

6. Percolation – downward flow of water through soil and permeable rock formations to groundwater storage areas called aquifers

7. Runoff – downslope surface movement back to the sea to resume cycle

Main Processes of the Hydrologic Cycle

Hydrologic Cycle

12

4

3

5

6

7

Global Air Circulation & Regional Climates• Uneven heating of the Earth’s Surface– Air is more heated at the equator and less at

the poles.

Global Air Circulation & Regional Climates• Seasonal changes in temperature and

precipitation

Insolation

A CB

Solar Energy

Rainy Season

Seasonal shift in rainy/dry seasons

Matter Cycling in Ecosystems

• Nutrient – any atom, ion, or molecule an organism needs to live, grow, or reproduce– Some (such as C, O, H, N, P, S, and Ca) are

needed in fairly large amounts– Some (such as Na, Zn, Cu, and I) are only

needed in trace amounts.

Nutrient Cycles

• Compartment – represents a defined space in nature

• Pool – amount of nutrients in a compartment

• Flux rate – the quantity of nutrient passing from one pool to another per unit time.

Major Nutrient Cycle Pathways

Pool

Flux rate

Hypothetical Phosphorus Nutrient Cycle

Plants Herbivores

Water100

81126

91.4

7

133

9.519

45

Flux rate and pool size together define the nutrient cycle within any particular ecosystem

Nitrogen Cycle

• Nitrogen is used to make essential organic compounds such as proteins (amino acids), DNA, and RNA.

• Nitrogen is the atmosphere’s most abundant element (global gaseous cycle).• 78% of the volume is chemically un-reactive

nitrogen gas N2.

• Takes a lot of energy to break the triple covalent bonds holding N N

• Microbes mostly responsible for N cycle

Have You Hugged Your Microbes Today? Besides making beer, they are responsible for:

• Nitrogen fixation –conversion of gaseous nitrogen (by Rhizobium, Azotobacter, and cyanobacteria) to ammonia (N2

+ 3H2 2NH3) which can be used by plants.

• Nitrification - Two-step process in which ammonia is converted first to NO2

- (by Nitrosomonas) and then to NO3-

(by Nitrobacter).

• Denitrification – conversion of nitrate ions (by Pseudomonas or other anaerobic bacteria in waterlogged soil or in the bottom sediments of a water body) into nitrogen gas (N2) and nitrous oxide gas (N2O)

• Ammonification – the conversion (by decomposer heterotrophic bacteria) of nitrogen-rich organic compounds, wastes, cast-off particles, and dead bodies into available ammonia (which can be used by plants).

Gaseous N2

Nitrogen Fixation

Ammonia: NH3, NH4+

Food Web

1. Nitrification

Nitrite: NO2-

2. Nitrification

Nitrate: NO3-

Denitrification

Nitrogenous Waste

Ammonification

Ecosystem Nitrogen Cycle

Loss by Leaching

Provides Energy

Nitrate

Proteins

Requires Energy

Energy and the Nitrogen Cycle

Nitrogen Cycle

Phosphorous Cycle• The phopsphorous cycle is slow, and on a human

time scale most phosphorous flows from the land to the sea.– Circulates through the earth’s crust, water, and living

organisms as phosphate (PO4)– Bacteria are less important here than in the nitrogen

cycle

• Guano (bird poop), mined sediments, and ‘uphill’ movement of wastewater are the main ways phosphorous is cycled in our lifetime

• Geologic process (mountain formations / uplifting of ocean sediments) cycle phosphorus in geologic time

Food web

Soil

River Flow

Ocean Water

Food web

Sediments

Guano

Mining

Phosphorous Cycle

Geologic Uplifting

Phosphorous is Important

• Most soils contain very little phosphorous; therefore, it is often the limiting factor for plant growth on land unless added as fertilizer.

• Phosphorous also limits primary producer growth in freshwater aquatic ecosystems.

Phosphorous Cycle

Sulfur Cycle

• The sulfur cycle is a gaseous cycle.– Sulfate (SO4) is the principal biological form

– Essential for some amino acids– Usually not limiting, but the formation of iron sulfides

converts the insoluble form of phosphorous to a soluble form

• Sulfur enters the atmosphere from several natural sources.– Hydrogen sulfide (H2S) is released by volcanic activity

and by the breakdown of organic matter in swamps, bogs, and tidal flats (you can smell this at low tide in the salt marsh).

– Sulfur dioxide (SO42-) enters from volcanoes.

– Particles of sulfate (SO42-) salts, such as ammonium

sulfate, enter as seas spray.

Sulfur Cycle

Food Web

Organic Matter H2SHeterotrophic

microorganisms

SO4

Anaerobic Sulfur-reducers

Aerobic Sulfide-oxidizers

S

ExcretionSulfur bacteria

Sulfur bacteria

FeS

+Fe3

FeS2

OH SH

Soluble Phosphorous

Black Anaerobic Mud

Very Slow Flux Rate

Rap

id C

ycli

ng

Volcanoes, Sea spray

Sulfur Cycle

Carbon Cycle• Carbon is the basic building block of organic

compounds necessary for life.• The carbon cycle is a global gaseous cycle– Carbon dioxide makes up 0.036% of the troposphere

and is also dissolved in water

• Key component of nature’s thermostat– Too much taken out of the atmosphere, temp’s

decrease– Too much added to atmosphere, temp’s increase

Primary Productivity

CO2

C6H12O6

Photosynthesis Respiration

Solar Energy

Heat Energy

Biomass (g/m2/yr)

O2

Available to Consumers

Chemical Energy (ATP)

NP

P

GP

P

Atmospheric / Aquatic CO2

Food Web

Photosynthesis RespirationCombustion of wood / fossil

fuels

Limestone Rocks

Carbon Cycle

SedimentationWeathering

Volcanic Action

The Recyclers

• Detritus – parts of dead organisms and cast-off fragments and wastes of living organisms

• Detritivores – organisms that feed on detritus (detritus feeders and decomposers).– Detritus feeders – extract nutrients from partially

decomposed organic matter in leaf litter, plant detritus, and animal dung (crabs, carpenter ants, termites, earthworms).

– Decomposers (certain types of bacteria and fungi) are very important in recycling nutrients in an ecosystem

Detritus Feeders and Decomposers

Without detritus feeders and decomposers, the lack of nutrients would quickly stop primary production!

Turnover and Residence Times

• Turnover rate – the fraction of the total amount of a nutrient in a compartment that is released (or that enters) in a given period

• Turnover time – the time needed to replace a quantity of a substance equal to its amount in the compartment

• Residence time – the time a nutrient stays in a compartment (similar to turnover time)

Nutrient Cycles in Forests

• Inputs – outputs = storage• Nutrients accumulate in the leaves and

wood over time

Nutrient Storage in Trees is Temperature and Vegetation Type Related

Organic matter (kg/ha) Nitrogen (kg/ha)

Forest Region # Trees Total% Above Ground

Trees Total% Above Ground

Boreal coniferous 3 51,000 226,000 19 116 3,250 4

Boreal deciduous 1 97,000 491,000 20 331 3,780 6

Temperate coniferous 13 307,000 618,000 54 479 7,300 7

Temperate deciduous 14 152,000 389,000 40 442 5,619 8

Mediterranean 1 269,000 326,000 83 745 1,025 73

Average 208,000 468,000 45 429 5,893 7

In cold climates nutrients are tied up in the soil.

Nutrient Turnover Time is Temperature Related

Mean turnover time (yr)

Forest Region #Organic matter

N K Ca Mg P

Boreal coniferous 3 353 230.0 94.0 149.0 455.0 324.0

Boreal deciduous 1 26 27.1 10.0 13.8 14.2 15.2

Temperate coniferous 13 17 17.9 2.2 5.9 12.9 15.3

Temperate deciduous 14 4 5.5 1.3 3.0 3.4 5.8

Mediterranean 1 3 3.6 0.2 3.8 2.2 0.9

All Stands 32 12 34.1 13.0 21.8 61.4 46.0

Turnover time – the time an average atom will remain in the soil before it is recycled into the trees or shrubs

Net Primary Production and Nutrient Cycling

• In general, NPP is closely related to the speed of nutrient cycling.– Tracking the decay of a leaf and the cycling rate of

nutrients provides an indicator of biome productivity.

Mean Residence Time (In Years)*

Biome Organic matter

Nitrogen Phosphorous Potassium Calcium Magnesium NPP

(g C/m2/yr)

Boreal forest

353 230 324 94 149 455 360

Temperate forest

4 5.5 5.8 1.3 3.0 3.4 540

Chaparral 3.8 4.2 3.6 1.4 5.0 2.8 270

Tropical rain forest

0.4 2.0 1.6 0.7 1.5 1.1 900

* Mean residence time is the time for one cycle of decomposition.

Rapid Cycling in the Tropics

• Reasons for rapid cycling in the tropics:– Warm climate– No winter to retard decomposition– An army of decomposers– Abundant mycorrhizal fungi on shallow roots• Fungi that grow symbiotically with plant

roots• Facilitate water and nutrient uptake

The Tropics: A Closed System

• The speed of nutrient cycling in the humid tropics promotes high productivity, even when soils are poor in nutrients.– Nutrients are cycled so quickly there is little

opportunity for them to leak from the system– Waters in local streams and rivers can have as

few nutrients as rain water

• Because there is virtually no loss of nutrients, many tropical forests have virtually closed nutrient cycles.– The opposite would be an open system, in

which nutrients are washed out rapidly

Tropical Rain Forest Paradox

• Most tropical rain forests are poor in nutrients – especially oxisol.

• When the forests are cleared for farmland, the land can only support three or four harvests.

• Well, how can they support the amount of primary production we find in a tropical rain forest?

Standing Biomass

• Standing Biomass - all the plant matter in a given area.

• Nutrients are either found in the soil or in the standing biomass.

• In a temperate forest system, recycling is slow.– Consequently, at any given time, a large

proportion of nutrients are in the soil.– So when the land is cleared, it is fertile and

can support many years of agriculture

Tropical Soils

• In the humid tropics, as little as 10% of the total nutrients are in an oxisol (soil) at any given time.– Hence, when the logging trucks take the trees,

they are carrying the majority of the nutrients!

• An increase in soil acidity often follows timber removal to the point that available phosphorous is transformed to an insoluble form.

Watershed Biogeochemistry

• Watershed – catchment or drainage basin of a river• Streams and rivers are main conduits of nutrient

loss• Vegetation type can influence nutrient loss:

Species Bark Wood Twigs Leaves

Chestnut Oak 1.25 ± 0.17 0.09 ± 0.01 0.68 ± 0.06 0.58 ± 0.07

Flowering Dogwood 2.36 ± 0.26 0.11 ± 0.01 0.80 ± 0.06 1.85 ± 0.11

Rhododendron 0.30 ± 0.10 0.07 ± 0.31 0.99 ± 0.24 1.20 ± 0.29

Mean calcium concentrations (% dry wt) in three plant species.

Normal Nutrient Loss• Rain runoff is the major vector of nutrient

loss from most ecosystems

Precipitation (mg/L) Streamwater (mg/L)

Calcium 0.21 1.58

Magnesium 0.06 0.39

Potassium 0.09 0.23

Sodium 0.12 0.92

Aluminum ---a 0.24

Ammonium 0.22 0.05

Sulfate 3.10 6.40

Nitrate 1.31 1.14

Chloride 0.42 0.64

Bicarbonate --- a 1.90

Dissolved silica --- a 4.61b

a Not determined, but very low; b Watershed 4 only

Deforestation Can Increase Loss of Nutrients From Areas Due to Runoff

Note Scale Change

Stream Nitrite Concentration

Other stream nutrient increase two years after the deforestation:

Calcium 417%, Magnesium 408%, Potassium 1,558%, Sodium 177%

Riparian Buffer Zone• Areas of trees, shrubs and other vegetation,

that are adjacent to a body of water, that are managed for several purposes: – to maintain the integrity of stream channels and

shorelines;– to reduce the impact of upland sources of

pollution by trapping, filtering, and converting sediments, nutrients and other chemicals;

– to supply food, cover and thermal protection to fish and other wildlife.

• The main purpose of a riparian buffer is to help control non-point source pollution.

Three Zone Riparian Buffer

Other Methods to Control Erosion

• Silt Fence / hay bales– Allows water to pool so that sediment is

dropped.

What is Soil?

• Complex mixture of eroded rock, mineral nutrients, decaying organic matter, water, air, and billions of living organisms (mostly decomposers)

• Soil is created by1) Weathering of rock2) Deposit of sediments by erosion3) Decomposition of organic matter in dead

animals

Soil Horizons (Profiles)O horizon - Consists mostly of freshly fallen and partially decomposed leaves, twigs, animal wastes, fungi, and other organic materials.

A horizon - A porous mixture of partially decomposed organic matter (humus) and some inorganic mineral particles.

Humus is a sticky, brown residue of partially decomposed organic material.•B Horizon (sub-soil) and C horizon (parent material) - Contain most of a soil’s inorganic matter. Mostly broken-down rock consisting of varying mixtures of sand, silt, clay, and gravel.

O horizonO horizonLeaf litterLeaf litter

A horizonA horizonTopsoilTopsoil

B horizonB horizonSubsoilSubsoil

C horizonC horizonParentParent

materialmaterial

Mature soilMature soil

Young soilYoung soil

RegolithRegolithRegolithRegolith

BedrockBedrockBedrockBedrock

Immature soilImmature soil

Soil Horizons (Profiles)

Life in Soil

• The two top layers of most well-developed soils teem with bacteria, fungi, earthworms, and small insects that interact in complex food webs and nutrient cycles.

Soil Texture

• Clay – very fine particles

• Silt – fine particles

• Sand – medium-size particles

• Gravel – Coarse to very coarse particles

Loam – roughly equal mixtures of clay, sand, silt, and humus

Soil Texture

Topsoil – Renewable Resource?

• Is regenerated by renewable resources, but it takes 200 - 1,000 years to produce about an inch of topsoil in tropical and temperate climates– Rate depends on climate and soil type

• If erosion exceeds regeneration, then the resource is not renewable

Soil erosion – movement of soil components, especially surface litter and top soil, from one place to another.

- Typically caused by flowing water and wind

Any activity that destroys plant cover makes soil vulnerable to erosion (e.g., farming, logging, construction, over-grazing by livestock, off-road vehicles, and deliberate burning of vegetation).

Moving Water Causes Most Soil Erosion

• Sheet Erosion – fairly uniform sheets of soils are removed as surface water flows over a slope or across a field in a wide flow.

• Rill Erosion – occurs when surface water forms fast-flowing rivulets that cuts small channels in the soil.

• Gully Erosion – occurs when rivulets of fast-flowing water join each other and with each succeeding rain cut the channels wider and deeper until they become ditches or gullies.

Harmful Effects of Soil Erosion

1) Loss of soil fertility and its ability to hold water

2) Runoff of sediment that pollutes water, kills fish and shellfish, and clogs irrigation ditches, boat channels, reservoirs, and lakes.