Nutrient Cycles

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

Two Broad Types of Nutrient Cycles

• Local Cycle – operates within an ecosystem• Nonvolatile elements • No mechanism for long-distance transfer

• Global Cycle – involve exchanges (gaseous) between the atmosphere and the ecosystem• Volatile elements: Water, oxygen, carbon,

and nitrogen• Easy exchange among ecosystems

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

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.

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!

• 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

• Waters in local streams and rivers can have as few nutrients as rain water!

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.

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%

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.

The Ocean and CO2

• The oceans are the second largest storage reservoir in the carbon cycle.– Some is dissolved as CO2 and some reacts

with seawater to form carbonate (CO32-) and

bicarbonate (HCO3-).

– As water warms, more CO2 is squeezed out of the water into the atmosphere (like a hot coke).

• Ions can react with Ca++ to form CaCO3 to build the shells and exoskeletons of marine organisms.– When these organisms die, tiny particles of

their shells and bone settle to the oceans bottom and may be buried for eons.

• May eventually be converted to limestone rock

Atmospheric / Aquatic CO2

Food Web

Photosynthesis RespirationCombustion of wood / fossil

fuels

Limestone Rocks

Carbon Cycle

SedimentationWeathering

Volcanic Action

The Carbon Cycle (Terrestrial)The Carbon Cycle (Terrestrial)

photosynthesisphotosynthesis aerobic aerobic respirationrespiration

Terrestrialrocks

Soil water(dissolved carbon)

Land food websproducers, consumers,

decomposers, detritivores

Atmosphere(mainly carbon dioxide)

Peat,fossil fuels

combustion of combustion of wood (for clearing wood (for clearing

land; or for fuelland; or for fuel

sedimentation

volcanic action

death, burial, compaction death, burial, compaction over geologic timeover geologic timeleaching leaching

runoffrunoff

weatheringweathering

Fig. 4.29, p. 93

The Carbon Cycle (Aquatic)The Carbon Cycle (Aquatic)

diffusion between atmosphere and ocean

Carbon dioxidedissolved in ocean water

Marine food websproducers, consumers,

decomposers, detritivores

Marine sediments, includingformations with fossil fuels

combustion of fossil fuels

incorporation incorporation into sedimentsinto sediments

death, death, sedimentationsedimentation

uplifting uplifting over over

geologic geologic timetime

sedimentationsedimentation

photosynthesisphotosynthesis aerobic aerobic respirationrespiration

Fig. 4.29, p. 92-93

Nitrogen Cycle

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

• Normally in short supply and often limits the rate of primary production.– Why most commercial fertilizers contain

biologically useful compounds such as ammonium nitrate (NH4NO3).

• However, nitrogen is the atmosphere’s most abundant element – 78% of the volume is chemically un-reactive

nitrogen gas N2.

Nitrogen Fixation

• The nitrogen cycle is a global gaseous cycle.

• Atmospheric nitrogen must be ‘fixed’ or combined with H or O to provide compounds that plants can use.– Lightning stimulates production of nitrogen

oxides: N2 + O2 2NO

– Certain bacteria in the soil and aquatic systems can convert nitrogen gas into compounds that can enter food webs.

Nitrogen Cycle

• Nitrogen fixation – specialized bacteria convert gaseous nitrogen to ammonia (N2

+ 3H2 2NH3) which can be used by plants.– Done mostly by cyanobacteria in water and

soil and Rhizobium bacteria living in small nodules of some plants

Nitrification and Assimilation

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

- (toxic to plants) and then to NO3

- (easily taken up by plants).

• Assimilation – A process by which plants roots absorb inorganic ammonia, ammonium ions, and nitrate ions to make nitrogen containing organic molecules (DNA, RNA, Proteins).– Animals then get their nitrogen by eating

plants.

Ammonification• Ammonification – the conversion (by

decomposer bacteria) of nitrogen-rich organic compounds, wastes, cast-off particles, and dead bodies into:– Simpler nitrogen-containing inorganic

compounds such as ammonia (NH3)

– Water-soluble salts containing ammonium ions (NH4

+)

Denitrification

• Other specialized bacteria (mostly anaerobic bacteria in waterlogged soil or in the bottom sediments of a water body) convert NH3 and NH4

+ back to nitrite (NO2- ) and nitrate (NO3

-) ions and then into nitrogen gas (N2) and nitrous oxide gas (N2O) to complete the cycle.

Gaseous N2

Nitrogen Fixation

Ammonia: NH3, NH4+

Food Web

1. Nitrification

Nitrite: NO2-

2. Nitrification

Nitrate: NO3-

Denitrification

Nitrogenous Waste

Ammonification

Nitrogen Cycle

Loss by Leaching

-III Ammonium

NH+4

-II

-I Hydroxylamine NH2OH

0 Nitrogen Gas

N2

+I Nitroxyl

NOH

Nitrous Oxide N20

+II Nitric Oxide NO

+III Nitrite

NO-2 NO-

2

+IV

+V Nitrate

NO-3

Aerobic Anaerobic

Nitrification Denitrification

NIT

RO

GE

N O

XID

AT

ION

ST

AT

E

Biochemical Transition of Nitrogen

Nitrogen Cycle

Phosphorous Cycle

• The phosphorous cycle is a sedimentary cycle.– Circulates through the earth’s crust and living

organisms– Bacteria are less important here than in the

nitrogen cycle– At the temperature’s and pressures normally

found on the earth, phosphorous and its compounds are not gases.

• The phopsphorous is slow, and on a human time scale much phosphorous flows from the land to the sea.

Phosphorous

• Phosphorous is usually found as phosphate salts containing phosphate ions (PO4

3-) in terrestrial rock formations and ocean bottom sediments.

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

• Phosphorous also limits primary producer growth in aquatic ecosystems.

Phosphorous Cycle

Sulfur Cycle

• The sulfur cycle is a gaseous cycle.– Although much of the earth’s sulfur is stored

underground in rocks and minerals.– Sulfur is important in some amino acids

• 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

All values are 1012 g S/yr