Biogeochemical Cycles
Lecture 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 molecules
Cl, 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
Fixation
Mineralization
Fixation
Mineralization
Exchange Pool
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 chain
heterotrophs: feed on dead material
Provide prey in herbivore foodchain
Fragmentation:
1. Microfauna and flora <100um
Protozoans and nematodes
2. Mesofauna 100um 2mm
Mites, potworms, springtails
3. Macrofauna
Millipedes, earthworms, snails,
amphipods & isoods
Decomposition:
Bacteria and fungi – produce extracellular
enzymes
Fungi belong to a separate
kingdom
several groups
produce long, threadlike strands (hyphae)
reproductive structures
may be large and visible
Bacteria: two distinct
kingdoms
Single celled
Microscopic
Various shapes
Many may not be easily
cultured
May 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• Withdrawn after time to determine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
Other factors which may
impact rate of
decomposition?
Decomposition of red
maple leaves more
rapid in warmer, more
humid climatesdde
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 (PO43–) 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
Precipitation
Geologic
uplift
Weathering
of rocks
Runoff
Consumption
Decomposition
Plant
uptake
of PO43–
Plankton Dissolved PO43–
Uptake
Sedimentation
Soil
Leaching
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)
Ammonium
form available
to plants
Ammonia
(gas)
N2
N+N
(NH3)2
NH4
H + energy
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
NO3
• 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
N2 in atmosphere
Assimilation
NO3–
Nitrogen-fixing
bacteria
Decomposers
Ammonification
NH3
Nitrogen-fixing
soil bacteria
Nitrification
NH4+
NO2–
Nitrifying
bacteria
Denitrifying
bacteria
Nitrifying
bacteria
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