Lecture: Chapter 23 - Biogeochemical Cycles Flashcards
Biogeochemistry
The study of the physical, chemical and biological factors that influence the movement and transformation of elements.
Nutrient Cycles
There are about 100 elements
Approximately 40 are required by organisms
They cycle in the biosphere
The most important chemical elements
Carbon
Nitrogen
Phosporus
Sulfur
Biogeochemical cycles
Inorganic (nonliving) - Biotic (living)
Elemetns are mostly moved in aqueous solution
Each cycle has a reservoir (atmosphere, ocean, sediments, rock, etc)
Some cycles are more perfect than others. Perfect is in the direction of very rapid transfer of the element to the environment followed by very rapid removal by organisms.
How do nutrients enter ecosystems?
Through the chemical breakdown of minerals in rocks or through fixation of atmospheric gases
Two basic biogeochemical cycles
Sedimentary / Solid (2.8% of living matter)
26 elements
The solid cycles are less “perfect”
Gaseous (97.2% of living matter)
4 elements : Carbon, hydrogen, oxygen, nitrogen
CHON
Solid cycle
All nutrients are ultimately derived from abiotic sources; minerals in rocks and gases in the atmosphere
Weathering
Elements are released from rock minerals by weathering physical and chemical breakdown of rocks
Atmosphere
The atmosphere si the ultimate source of carbon and nitrogen for ecosytems
- Carbon is taken up as CO2 by autrophs through photosynthesis and fixed into organic compounds
- The atmosphere is 78% nitrogen, in the form of N2, which cannot be used by most organisms because of the energy required to break the triple covalent bond
Nitrogen fixation
The process of converting N2 to a biologically useful form
Pools
Total amount of a nutrient in a component of the ecosystem
Mean residence time (turnover rate)
Amount of time on average that a molecule spends in the pool
Where do freshwater and marine ecosytems receive nutrient inputs
Freshwater and marine ecosytems recieve nutrient inputs from terrestrial ecosytems
Nutrients in aquatic ecosystems: Lakes
receive nutrients from stream water, atmospheric deposition, fixation and terrestrial litter.
- phosphorus is usually the limiting nutrient in lakes
- Detritus is decomposed int he water column and sediments, providing internal nutrient input
- Nutrients are progressively lost as detritus is deposited in the lake sediments
Carbon cycle
Closely linked to energy flow
the largest carbon pool is the ocean
Net Ecosystem Production
Production available for accumulation of C/energy witihin the ecosystem
NEP = GPP - (RA+RH+L+D)
Rate of Carbon cycle through the ecosytem determined by
- Rate of primary productivity
- Rate of decomposition
Both of these processes are influenced by climate
- temperature
- precipitation
Fastest cycling is in the hot and humid tropics
Nitrogen cycle: an atmospheric cycle
The largest nitrogen pool is the atmosphere
The nitrogen cycle is a relatively perfect, self-regulating cycle with a large gaseous reservoir – the atmosphere, which is 78% N
Bacteria are responsible for key steps in the N cycle
The average resident time of fixed N in the biosphere is thought to be about 625 years while for P it is thousands of years.
Nitrification
NH3 and NH4+ are converted to NO3- by chemoautotrophic bacteria, in aerobic conditions. It occurs mainly in terrestrial systems. NO3- tends to be more available to plants than other forms of nitrogen
Denitrification
Some bacteria use NO3- as an electron acceptor, converting it into N2 and N20, in anoxic conditions
Routes of N uptake
Uptake of N2 from atmosphere (very costly energetically)
- N fixtaion: high energy fixation, biological
Uptake of mineral nitrogen from soil
- Ammonium (NH4+) : released by decomposition, created by deposition, affected by cation exchanged capacity
- Nitrate (NO3): deposition, nitrification
Factors affecting N release
Decomposition rate influenced by
- water
- temperature
- litter quality
- soil biota
Human activity in N release
Human activity greatly increased the amount of N entering the biosphere through agricultural and industrial practices
Sources of nitrate: nitrogen deposition
NOx emitted by fossil fuel burning
Some falls as dry deposition
Falls as wet acidic precipitation
Nitrogen loss
Leaching
- nitrate is primary form that is lost from ecosystems
- Does not adhere to soils
Fire - volitilization of organic nitrogen
Dentrificiation
- Transformation of nitrate to N2
Requires: lack of oxygen, nitrate
Phosophorus cycle
Solid chemical, essential nutrient that moves in sedimentary cycle, which is less perfect than gas cycles.
- vital components of cells in organisms. Biological uses of phosphate: nucleic acids
- P almost always present as phosphate PO4
- Rock is the only source
