Biogeochemical CyclesLecture 11

Chapter 23

Nutrients –

Macronutrients: Organism would fail completely C, H, O, N, P, K, Ca, Mg, S

Micronutrients: required as cofactors, components of certain moleculesCl, Fe, Mn, B, Cu,. Mo, Zn, Ni

Availability is influenced by pH …

(See 6.12)

• Life depends on recycling chemical elements• Nutrient circuits in ecosystems involve biotic

and abiotic components and are often called biogeochemical cycles

• Focus on:– Each chemical’s biological importance– Forms in which each chemical is available or used

by organisms– Major reservoirs for each chemical– Key processes driving movement of each chemical

through its cycle

Mineralization

Fixation

Exchange Pool

Fixation

Mineralization

Two food chains:• Grazing food chain

– Herbivore carnivore

• Detritus food chain– Dead matter and

waste from grazing food chain and primary production

– Provides input to grazing food chain

Detrivore food chainheterotrophs: feed on dead material

Provide prey in herbivore foodchain

Fragmentation: 1. Microfauna and flora <100um

Protozoans and nematodes

2. Mesofauna 100um 2mmMites, potworms, springtails

3. MacrofaunaMillipedes, earthworms, snails,

amphipods & isoods

Decomposition:Bacteria and fungi – produce

extracellular enzymes

Fungi belong to a separate kingdomseveral groups produce long, thread-like strands (hyphae)reproductive structures may be large and visible

Bacteria: two distinct kingdoms

Single celledMicroscopicVarious shapesMany may not be easily

culturedMay develop populations

quickly

Study of Decomposition – Litterbag Studies• Weighed sample in mesh bag placed in

soil• Withdrawn after time to determine

remaining dry-weight– Dry weight estimate distorted by biomass of

decomposer

• Gives estimate of decomposition impacted by

– Species– conditions

Decomposition of red maple leaves more

rapid in warmer, more humid climatesdde

Other factors which may impact rate of decomposition?

Two types of biogeochemical cycles based input source to ecosystems

• Sedimentary– Rock and salt solution phases

• Gaseous– Global

• Many cycles hybrid – Exchange pool– Reservoir

Carbon cycle:• Closely tied to energy flux• Major exchange pool: atm CO2 (at

~0.03% )• Uptake via photosynthesis• Immobilized in carbonates of

shells, fossil fuels• Subject to daily + seasonal flux

The Phosphorus Cycle• Phosphorus is a major constituent of nucleic

acids, phospholipids, and ATP• Phosphate (PO4

3–) is the most important inorganic form of phosphorus

• The largest reservoirs are sedimentary rocks of marine origin, the oceans, and organisms

• Phosphate binds with soil particles, and movement is often localized

Fig. 55-14d

Leaching

Consumption

Precipitation

Plantuptakeof PO4

3–

Soil

Sedimentation

Uptake

Plankton

Decomposition

Dissolved PO43–

Runoff

Geologicuplift

Weatheringof rocks

Nitrogen cycle:• N essential to life – amino acids, nucleic acids• Atm. N2 stable, difficult bond to break• Fixation largely biological (ca 90%); agricultural use requires

fossil fuel input

• Fixation of N N– Free living aerobics as Azotobacter, & certain cyanobacter– Lichen symbionants– Mutualists associated with certain plant groups

(Rhizobium spp. on leguminous plants)

N2 N + N (NH3)2 NH4

H + energy

NO3

Ammonia (gas)

Ammonium form available

to plants

Under acidic conditions converts to ammonium

but may be lost to atmosphere

Nitrate produced by soil bacteria from ammonium may also be taken up by plants or mineralized to

N2

• Organic nitrogen is decomposed to NH4+ by

ammonification, and NH4+ is decomposed to

NO3– by nitrification (2 step processes, involving

2 different soil bacteria: Nitrosomonas and Nitrobacter)

• Denitrification converts NO3– back to N2

– Anaerobic, involves Pseudomonas spp

• Soil pH impacts both processes (low pH inhibits)

Fig. 55-14c

Decomposers

N2 in atmosphere

Nitrification

Nitrifyingbacteria

Nitrifyingbacteria

Denitrifyingbacteria

Assimilation

NH3 NH4 NO2

NO3

+ –

–

Ammonification

Nitrogen-fixingsoil bacteria

Nitrogen-fixingbacteria

Human activities now dominate most chemical cycles on Earth

• As the human population has grown, our activities have disrupted the trophic structure, energy flow, and chemical cycling of many ecosystems

Nutrient Enrichment

• In addition to transporting nutrients from one location to another, humans have added new materials, some of them toxins, to ecosystems

Agriculture and Nitrogen Cycling

• The quality of soil varies with the amount of organic material it contains

• Agriculture removes from ecosystems nutrients that would ordinarily be cycled back into the soil

• Nitrogen is the main nutrient lost through agriculture; thus, agriculture greatly affects the nitrogen cycle

• Industrially produced fertilizer is typically used to replace lost nitrogen, but effects on an ecosystem can be harmful

Fig. 55-17

Contamination of Aquatic Ecosystems

• Critical load for a nutrient is the amount that plants can absorb without damaging the ecosystem

• When excess nutrients are added to an ecosystem, the critical load is exceeded

• Remaining nutrients can contaminate groundwater as well as freshwater and marine ecosystems

• Sewage runoff causes cultural eutrophication, excessive algal growth that can greatly harm freshwater ecosystems

Fig. 55-18

Winter Summer