Chapter 3 - The cycling of matter in ecosystems

• Biogeochemical cycles: circular pathways involving biological, geological, and chemical processes

• Biogeochemical cycles include:– Hydrologic Cycle/Water Cycle– Carbon Cycle– Phosphorus Cycle– Nitrogen cycle– Sulfur Cycle

Objectives:

• Identify and describe the flow of nutrients in each biogeochemical cycle.

• Explain the impact that humans have on the biogeochemical cycles.

• Be able to diagram each cycle.

What Sustains Life on Earth?

• Solar energy, the cycling of matter, and gravity sustain the earth’s life.

Figure 3-7

Two Secrets of Survival: Energy Flow and Matter Recycle

• An ecosystem survives by a combination of energy flow and matter recycling.

Figure 3-14

The Hydrologic Cycle

The movement of water through the biosphere

Water is essential to life; >50% of the typical mammal’s body weight

Allows essential molecules to move within and between cells, draws nutrients into the leaves of trees, dissolves and removes

toxic materials and performs many other critical biological functions

Water is the primary agent responsible for dissolving and transporting the chemical elements necessary for living organisms

Fig. 3-26, p. 72

PrecipitationPrecipitation

Transpiration

Condensation

Evaporation

Ocean storage

Transpiration from plants

Precipitation to land

Groundwater movement (slow)

Evaporation from land Evaporation

from ocean Precipitation to ocean

Infiltration and Percolation

Rain clouds

RunoffSurface runoff

(rapid)

Surface runoff (rapid)

The Hydrologic Cycle

Effects of Human Activities on Water Cycle

• We alter the water cycle by:– Withdrawing large amounts of freshwater.– Clearing vegetation and eroding soils.– Polluting surface and underground water.– Contributing to climate change.

The Hydrologic CycleHuman Activities and the Hydrologic Cycle

Remember: Earth is a closed system in regards to matter

Humans can alter the hydrologic cycle in many ways:• Harvesting trees reduces evapotranspiration• Paving surfaces reduces infiltration/percolation

Both Increase runoff and decrease groundwater recharge

• Diverting water from one area to another can also impact the regional water cycle

(drinking, irrigation, industrial uses)

The Carbon CycleHow carbon (C) moves through our environment

• Producers pull carbon dioxide (CO2) from the air and use it in photosynthesis.

• Consumers eat producers and return CO2 to the air by respiration.

• Decomposition of dead organisms, plus pressure underground, forms sedimentary rock and fossil fuels. This buried carbon is returned to the air when rocks are uplifted and eroded.

Carbon cycle: other ways to transfer carbon

• Ocean water also absorbs carbon from multiple sources, eventually storing it in sedimentary rock or providing it to aquatic plants.

• Limestone keeps carbon out of circulation– Weathering of exposed limestone releases carbon

• A carbon atom cycles about every six years

The global carbon cycle

The carbon cycle

Human impacts on the carbon cycle

• We have increased CO2 in the atmosphere by burning fossil fuels and deforesting forests.

• Atmospheric CO2 concentrations may be the highest now than in 420,000 years.

• This is driving global warming and climate change.

Human impacts on the carbon cycle

• Human intrusion into the cycle is significant• We are diverting or removing 40% of the

photosynthetic effect of land plants• Burning fossil fuels has increased atmospheric

CO2 by 35%• Deforestation and soil degradation release

significant amounts of CO2 to the atmosphere– Recent reforestation and changed agricultural

practices have improved this somewhat

Summary of Carbon Cycle

• Acid rain: from the atmosphere delivers CO2 dissolved in water vapor to earth.

• Photosynthesis: turns CO2 into plant biomass• Respiration: converts sugars and starches into

energy for organisms and releases CO2.• Decomposition: converts dead plants and

animals into CO2 and other inorganic nutrients.

http://www.youtube.com/watch?v=irwRzQNS-GI

The Phosphorous CycleHow phosphorus (P) flows through our environment

• P is most abundant in rocks. Weathering releases phosphate (PO4

3–) ions from rocks into water.• As rock breaks down, phosphate is released– Replenishes phosphate lost through leaching or runoff

• Plants take up phosphates in water, pass it on to consumers, who return it to the soil when they die.

• Phosphates dissolved in lakes and oceans precipitate, settle, and can become sedimentary rock.

The global phosphorus cycle

The phosphorous cycle

Human impacts on the phosphorus cycle

• The most serious intrusion comes from fertilizers• Phosphorus is mined and made into fertilizers,

animal feeds, detergents, etc.• When added to soil, it can stimulate production– Human applications have tripled the amount reaching the

oceans, accelerating the cycle– It can’t be returned to the soil

• Excess phosphorus in water leads to severe pollution– Can cause too many bacteria and fish kills

Summary of Phosphorus Cycle

• Weathering: of phosphate from rocks delivers phosphorus to soil.

• Decomposition: of plant and animal wastes and remains delivers phosphorus to soil.

• Humans make phosphates through mining and chemical weathering.

http://www.youtube.com/watch?v=3iwL24oVpH4

The Nitrogen Cycle• Is a unique cycle– Bacteria in soils, water, and sediments perform many

steps of the cycle– Nitrogen is in high demand by aquatic and terrestrial

plants• Air is the main reservoir of nitrogen (N)– Nonreactive nitrogen: most organisms can not use it

• Reactive nitrogen (Nr): other forms of nitrogen that can be used by organisms

Plants take up nitrogen

• Plants in terrestrial ecosystems (“non-N-fixing producers”)– Take up( N) as ammonium (NH4) and incorporate it into proteins

and nucleic acid compounds– The nitrogen moves through the food chain to decomposers,

releasing nitrogen wastes• Soil bacteria (nitrifying bacteria) convert ammonium to

nitrate to obtain energy– Nitrate is available for plant uptake

• Nitrogen fixation: bacteria and cyanobacteria can use nonreactive N

Means of nitrogen fixation • Bacteria (genus Rhizobium) live in legume root nodules• The legume provides the bacteria a place to live and food– It receives a source of nitrogen in return– Nitrogen enters the food chain from the legumes

• Three other processes “fix” nitrogen– Atmospheric nitrogen fixation: lightning– Industrial fixation: in fertilizer manufacturing– Combustion of fossil fuels: oxidizes nitrogen

• Industrial fixation and fossil fuels release nitrogen oxides, which are converted to nitric acid (acid precipitation)– Denitrification: Microbes use nitrate as a substitute

for oxygen

Nitrogen Cycle Summary

• Nitrification: by bacteria convert NH4 into NO2- and then

into NO3-• Denitrification: Conversion of biological nitrogen

(proteins-No3- and NH4) into atmospheric nitrogen.

• Biological Nitrogen Fixation: bacteria in the roots of legumes fix nitrogen.

• Atmospheric Fixation: lightning converts N2 to NO3- and NH4

• Industrial Fixation: factories make fertilizer from atmospheric nitrogen.

http://www.youtube.com/watch?v=IaGNSmQQmBc

The global nitrogen cycle

The nitrogen cycle

Human impacts on the nitrogen cycle

• Human involvement in the nitrogen cycle is substantial

• Many crops are legumes and draw N from the air– Peas, beans, soybeans, alfalfa– Increases the rate of nitrogen fixation

• Nonleguminous crops (corn, wheat, potatoes) are heavily fertilized with nitrogen from industrial fixation

• We are more than doubling the rate of nitrogen moving from air to land

Serious consequences of fertilization• Nitric acid has destroyed lakes, ponds, and forests• Atmospheric nitrogen oxides adds to ozone

pollution, climate change, and stratospheric ozone depletion

• Abundant nitrates are not incorporated into organisms– They are released into the soil, where they leach calcium

and magnesium• Eutrophication of waterways• Nitrogen cascade: complex of ecological effects as Nr

moves through the environment

Comparing the cycles• Carbon is mainly found in the atmosphere– Directly taken in by plants

• Nitrogen and phosphorus are limiting factors• All three cycles have been sped up by human

actions– Acid rain, greenhouse gases, eutrophication

• Other cycles exist for other elements (e.g., water)– All go on simultaneously– All come together in tissues of living things

The Sulfur Cycle• Most is found in rocks -> released into soil and water

• Plants absorb through roots (Sulfates: SO42-)- cycles through the food web

• Sulfur also has a gaseous component (volcanic eruptions; fossil fuel combustion) as SO2

• Sulfur Dioxide converts to Sulfuric Acid (H2SO4) and comes back to Earth as acid precipitation

• Sulfur-based acid precip has decreased due to Clean Air Act initiatives since 1995, but is still a very relevant environmental concern

Nutrient cycles and energy flow