Biogeochemical Cycles

Vivek Srivasatava

Energy/Matter Flow• Energy and matter flow through the biosphere

• Sun >> autotroph >> heterotroph >> fossil fuel storage >> human fuel consumption

• Biosphere < > lithosphere < > atmosphere < > hydrosphere

• Elements cycle through both the biological and geological world, hence biogeochemical cycles

Elemental Cycles• H, O, and C make up > 99 % of the Earth’s

biomass

• N, Ca, K, Mg, S, and P are significant nutrients

• Cycling of C, O, N, P, and S are discussed in this chapter

Concepts in Biogeo. Cycles• Reservoir - where material or mass is stored

• Flux: rate of flow of material

• Steady state: inflow = outflow

• Dynamic state: fluxes are reservoirs are changing with time

• Residence time: length of time a chemical stays in a reservoir

• Feedback: positive and negative

Reservoirs of Carbon• Carbon is found in all four spheres

• Biosphere - organic matter

• Atmosphere - CO2, CH4

• Hydrosphere - H2CO3 ,HCO3 - , CO3 =

• Lithosphere - CaCO3 , coal, oil, and gas

• Processes: photosynthesis, formation of sediments, weathering, combustion, plate tectonics

Carbon Cycle

Human Interference• Human-induced processes

– Extraction and combustion of fossil fuels (speeds up the medium-term cycling)

– Cement manufacturing– Deforestation (biomass burning)

• All of these processes release CO2 into the atmosphere and affect the natural cycling of carbon

Oxygen Cycle• Essential for aerobic life

• Closely linked to carbon cycle

• Very large reservoir (20% of gas in atm.), not susceptible to human interference

• Also, not a greenhouse gas

• Reservoirs: atmosphere, surface organic material (biosphere), and buried organic matter (lithosphere)

Nitrogen Cycle• Essential to life - important in forming amino acids

>> proteins

• Most abundant in the atmosphere (79%)

• Flows continuously through the spheres

• Reservoirs: atmosphere and biosphere (soil)

• Processes: Nitrogen fixation/ nitrification and denitrification

• Atmosphere => soil => plants => atmosphere (fig. 5.7)

Nitrogen Cycle• Nitrification/ nitrogen fixation: converts N2 to

forms usable by plants (NH3, and NO3-)

• Denitrification: is the conversion of NO3- back

to N2 in the atmosphere or in gases in the soil

• Symbiotic relationship: bacteria supply the plant with usable nitrogen and feed off the sugars and starches made by the plant

Nitrogen Cycle• Human activities account for >50% of

nitrogen fixation (fertilizers, cultivation of nitrogen fixing plants)

• Denitrification - done mainly by bacteria not by humans.

• Despite the huge size of the atmospheric reservoir of nitrogen, human activites profoundly affect the nitrogen cycle

Nitrogen Cycle

Eutrophication• Consequence of excess nutrients (nitrates, phosphates)

entering bodies of water• Produces algal blooms (over productivity)• When these algae die they settle at the bottom of the

water column• Decomposition process consumes oxygen and depletes

it from water• This destroys the organisms that need oxygen (fish)-

Lions Lake, Warrensburg, source of nutrient - goose droppings

Phosphorus CyclePhosphorus Cycle(Phosphorus is required for the

manufactureof ATP and all nucleic acids)

1. Reservoir – erosion transfers phosphorus to water and soil; sediments and rocks that accumulate on ocean floors return to the surface as a result of uplifting by geological processes

2. Assimilation – plants absorb inorganic PO4

3- (phosphate) from soils; animals

obtain organic phosphorus when they plants and other animals

3. Release – plants and animals release phosphorus when they decompose; animals excrete phosphorus in their waste products

Phosphorus Cycle• Long-term cycle: Burial of phosphate in

sediments => uplift => weathering => phosphate in soil or ocean

• Residence time ~108 yrs

• Extraction (mining) short-circuits the long-term cycle

Phosphorus Cycle• Reservoirs: Hydrosphere (as phosphate

ion), lithosphere (phosphate minerals), and biosphere (bones, teeth, shells)

• Short-term cycle: PO43- in soil or ocean =>

assimilation by plants => consumption by animals => decay or excretion => recycled to soil or ocean, residence time 100s of yrs

Phosphorus Cycle• Atmosphere is not a source

• As with nitrate, phosphate is an important nutrient in coastal upwelling zones

• As with nitrate, humans are doubling the rate of transport of phosphate into the environment through the application of fertilizers

Nitrogen Cycle

Phosphorus Cycle