Chapter 4

Biogeochemical Cycles

Organisms

Environment

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

Biogeochemical Cycles

• Gaseous Type – a large portion of a given element exists in a gaseous form in the atmosphere.

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

Energy Flows in one direction, but nutrients are more or less cycled within ecosystems

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

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 to ammonia (N2 + 3H2 2NH3) which can be used by plants.

– Biotic: Rhizobium, Azotobacter, cyanobacteria, Rhodospirillium (a purple bacteria), and some Pseudomonas

– Abiotic: Lightning

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

- (by Nitrosomonas) and then to NO3- (by

Nitrobacter).

• Denitrification – conversion of nitrate ions (by some 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

Cycling of Nitrogen

assimilation

Provides Energy

Nitrate

Proteins

Requires Energy

Energy and the 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

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.

• ATP

Food web

Soil

River Flow

Ocean Water

Food web

Sediments

Guano

Mining

Phosphorous Cycle

Geologic Uplifting

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 (This answers Matt’s question about who farted in the salt marsh).

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

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

CO2 Uptake and Release

• Terrestrial producers remove CO2 from the atmosphere and aquatic producers remove CO2 from water via photosynthesis.

• The cells in oxygen-consuming producers, consumers, and decomposers break down the organic compounds and release CO2 back to the atmosphere or water.

• The link between photosynthesis and respiration is a major part of the global carbon cycle

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

Other Links of the Carbon Cycle

• Fossil Fuels – large stores of carbon which are not released as CO2 unless extracted and burned.

– In only a few hundred years, we have extracted and burned fossil fuels that took millions of years to form.

• Limestone (CaCO3) – largest storage for the earth’s carbon is in sedimentary rocks such as limestone.– Carbon reenters the cycle as some of the rock

releases dissolved CO2 back to the atmosphere.

– Geologic processes can bring sediments to the surface and expose carbonate rock to the atmosphere.

Atmospheric / Aquatic CO2

Food Web

Photosynthesis RespirationCombustion of wood / fossil

fuels

Limestone Rocks

Carbon Cycle

SedimentationWeathering

Volcanic Action

~216.9 Tg returned to atmosphere

~212.9 Tg taken from atmosphere

~4 Tg added to atmosphere per year

About 1.3 ppm per year

Each point = monthly average

Average Mean Temperature

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

• More water evaporates from the oceans than return to it as precipitation and vice versa for the land

• Approximately 90% of the rain in the Mississippi valley originates from the sea

• Human activity tends to increase runoff rate, thus reducing the recharge rate of aquifers– Pavement, ditches,

river channelization, deforestation

• Groundwater is the source of about half our drinking water, most irrigation, some significant industrial use– Some significant aquifers may run dry if we don’t let them

recharge

• Increase in global temperature has led to increase in sea level rise.– Glacier melt– Thermal expansion

• River Continuum Concept models how biotic communities adjust to changes in the ‘downhill’ part of the hydrologic cycle.

The Flood Pulse

1959-2005 Atchafalaya River Stage at Butte La Rose USACE Gage ID = 03120

0

1

2

3

4

5

6

7

8

9

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Avg

. M

on

thly

Riv

er S

tag

e (m

)

= Average Stage

= 2005 Stage

September

December

February

April

AprilJuneAugust

September

Floodplain Zones

From Larson et al. 1981; Hall and Lambou 1990

I II III IV V VI

Aquatic Ecosystem

Active Floodplain

Floodplain Upland Transition

Terrestrial Or Upland Ecosystem

Bottomland Hardwood Ecosystem

Floodplain System

Watershed Biogeochemistry

• Watershed – The entire terrestrial and aquatic area that drains into a waterbody.

• Loss of nutrients from ecosystems is usually by runoff

Calcium flow in a forested watershed

Deforestation increases the rate of nutrient loss 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%

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

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

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

Tropical Rain Forest Paradox

• Most tropical rain forests are poor in nutrients – especially oxisol.– as little as 10% of the total nutrients are in an

oxisol soil at any given time.

• 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?

Tropical Soils

• When the logging trucks take the trees in the tropics, they are carrying the majority of the nutrients!

Cycling of Nonessential Elements

• Bioaccumulation - The storage of chemicals in an organism in higher concentrations than are normally found in the environment.

• Fat soluble compounds move across cell membranes and dissolve in fats (lipids).– Tend to stay in the organism and thus accumulate

– If they were soluble in water, then they would flush out

Bioaccumulation of Tributylin (TBT)

• TBT is a chemical found in nautical paint that was found in oysters along the coast of California in the late 1980s’.– Probably caused shell thickening and chamber

malformations

• Some oysters had TBT concentrations 30,000 times higher than in the water.

Biomagnification

• Biomagnification – the accumulation of chemicals in organisms in increasingly higher concentrations at successive trophic levels.

• Consumers at higher trophic levels ingest a significant number of individuals, along with the fat-soluble pollutants stored in their tissue.

• Top carnivores may accumulate poisons in concentrations high enough to prevent their eggs from hatching, cause deformities, or even death.– Concentrations in predators can be a million times

higher in predators than the concentration in the soil or the water

Terrestrial Biomagnification• DDT used to control elm bark beetle

(Dutch elm disease).

Aquatic Biomagnification

• PCB’s dumped into the Great Lakes and move through the food chain

One of the reasons the Brown Pelican became endangered.

Brown Pelican Recovery• The Brown pelican was abundant in LA in 1950. • Texas populations significantly declined between

1957 and 1961. LA’s population was eliminated.– Listed as endangered in the US on October 13,

1970• Primary cause of decline was pesticides: DDT

compounds (DDE and DDD), and PCB’s (dieldrin and endrin).– These chemicals were moved through the food chain– Impaired reproductive success (egg shells became very

thin and would often collapse)

• Populations have since recovered– DDT banned in 1972– Egg shells have shown increasing thickness

Environmental Mercury

• Usually implicated in fish consumption advisories: 1.0 ppm methyl mercury warrants fish consumption advisories in the US.

• Natural Sources:– Volcanoes, soil, under sea vents, mercury-rich

geologic zones, freshwater, oceans, plants, forest fires etc.

• Anthropogenic Sources– Mining and industrial applications, waste

incineration, coal-fired plants, paint, thermometers, etc.

Mercury Chemistry

• Elememental mercury (Hg0) – Most common form of environmental mercury – High vapor pressure, low solubility, does not

combine with inorganic or organic ligands, not available for methylation

• Mercurous Ion (Hg+)– Combines with inorganic compounds only– Can not be methylated

• Mercuric Ion (Hg++)– Combines with inorganic and organic

compounds– Can be methylated

Methylation

• Basically a biological process by microorganisms in both sediment and water

• Influenced by environmental variables that affect both the availability of mercuric ions for methylation and the growth of the methylating microbial populations.– Rates are higher in anoxic environments,

freshwater, and low pH

– Presence of organic matter can stimulate growth of microbial populations, thus enhancing the formation of methylmercury (sounds like a swamp to me!)

Methylmercury Bioaccumulation

• Mercury is accumulated by fish, invertebrates, mammals, and aquatic plants.

• Inorganic mercury is the dominate environmental form of mercury, it is depurated about as fast as it is taken up so it does not accumulate.

• Methylmercury can accumulate quickly but depurates slowly, so it accumulates– Also biomagnifies

• Percentage of methylmercury increases with organism’s age.

Environmental mercury has increased due to anthropogenic causes

http://water.usgs.gov/wid/FS_216-95/FS_216-95.html

Percent of sites in Louisiana that have mercury advisories that include each group of fish. Total sites listed = 16.

Species % Sites

Largemouth Bass 75

Bowfin 69

Crappie 56

FW Drum 50

Catfish 25

Buffalo 19

Sunfish 19

Top predators tend to be listed more often.

How Much Mercury Do You Have?

Dr. Ed Chesney at LUMCON is part of a larger study:

https://webapps.sph.harvard.edu/eer/LRAS/

http://www.louisianasportsman.com/details.php?id=191

http://www.lumcon.edu/