1. Detritus Food Chains and Biogeochemical Cycles Lecture 8 Chapters 21 and 22

2. 1. Much recycling of materials occurs within organisms
Example: water/nutrients withdrawn from senescent leaf tissues of plants  roots
Dead matter broken down via detritral food chain
Decomposition: multistep process leading to mineralization
mineralization: conversion of organic nutrients to mineral form
Terms:
Fixation = incorporation to organic molecule
Mineralization = reduction of organic molecule returning component elements to inorganic associations

3. 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

4. Fixation
Mineralization
Exchange Pool

5. Two food chains: Grazing food chain Detritus food chain
Herbivore  carnivore
Detritus food chain
Dead matter and waste from grazing food chain and primary production
Provides input to grazing food chain

6. Microfauna and flora <100um Mesofauna 100um 2mm Macrofauna
Detrivore food chain
heterotrophs: feed on dead material
Provide prey in herbivore foodchain
Fragmentation:
Microfauna and flora <100um
Protozoans and nematodes
Mesofauna 100um 2mm
Mites, potworms, springtails
Macrofauna
Millipedes, earthworms, snails, amphipods & isoods
Decomposition:
Bacteria and fungi – produce extracellular enzymes

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

9. Bacteria: two distinct kingdoms Single celled Microscopic Various shapes Many may not be easily cultured May develop populations quickly

10. 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Withdrawn after time to determine remaining dry-weight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

11. Other factors which may impact rate of decomposition?
Decomposition of red maple leaves more rapid in warmer, more humid climatesdde

12. Decomposition of different species and materials vary
Simple sugars  bacteria and fungi
polymers (as cellulose)  mainly fungi, some bacteria
Lignins  only certain fungi

13. Cellulose goes second, degraded by bacteria and fungi
Lignin – the stuff of wood, slow to degrade and degredation rate levels off as it remains
Cellulose goes second, degraded by bacteria and fungi
Proteins, solub. Carbohydrates easily degraded

14. Fig
EXPERIMENT
Ecosystem type
Arctic
Subarctic
Boreal
Temperate A
Grassland G
Mountain M
B,C D P T
H,I S
E,F U N L O J K Q R
RESULTS 80
Figure How does temperature affect litter decomposition in an ecosystem? 70 60 R U O 50 K Q T
Percent of mass lost J 40 P D S N 30 F C I M L 20 A H B E G 10
–15
–10 –5 5 10 15
Mean annual temperature (ºC)

15. The Rhizoshpere and Decomposition
~40% photosynthetic dry matter fuels microbial growth in rhizosphere
Fuels microbial growth which eventually releases nutrients  increased availability to plants … via mineralization
Soil Microbial Loop

16. Case Study: Nutrient Cycling in the Hubbard Brook Experimental Forest
Vegetation strongly regulates nutrient cycling
Research projects monitor ecosystem dynamics over long periods
The Hubbard Brook Experimental Forest has been used to study nutrient cycling in a forest ecosystem since 1963

17. (a) Concrete dam and weir
Fig a
Figure 55.16a Nutrient cycling in the Hubbard Brook Experimental Forest: an example of long-term ecological research
(a) Concrete dam and weir

18. Nitrate concentration in runoff
Fig c 80
Deforested 60 40 20
Nitrate concentration in runoff
(mg/L) 4
Completion of
tree cutting 3
Control 2 1
Figure 55.16c Nutrient cycling in the Hubbard Brook Experimental Forest: an example of long-term ecological research
1965
1966
1967
1968
(c) Nitrogen in runoff from watersheds

19. Decomposition in Aquatic systems Impacted by environment Photosynthetic processes at surface Detritus falls to benthic zone Low oxygen, cool temperatures Stratification of water occurs through summer spring/fall turnover events  mixing of water column

20. Export of resources  loss of nutrients Example: 1. logging
Logs = nutrients removed from forest
Increased nutrient water flow  nutrient loss via streams
Stream salmon fishery
Salmon represent nutrient transfer mechanism from sea to terrestrial ecosystem
Fire
Alters mineralization rate/processes
Subsequent leaching/runoff  nutrient loss

21. Import of nutrients/alteration of normal cycling Possible sources
Runoff from adjacent ecosystems
Agricultural
Municipal/industrial
Impacts
Toxins – some may accumulate higher levels food chain via bio-accumulation or bio-magnification
Eutrophication – nutrient enrichment in lake/pond  biological activity  stimulation of detrital food chain  anoxia
Global/environmental – C,N and global climate

22. Two types of biogeochemical cycles based input source to ecosystems
Sedimentary
Rock and salt solution phases
Include S, P
Gaseous
Global
Include C, N, O
Many cycles hybrid
Exchange pool
Reservoir

23. 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

24. 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

25. Precipitation Geologic Weathering uplift of rocks Runoff Consumption
Fig d
Precipitation
Geologic
uplift
Weathering
of rocks
Runoff
Consumption
Decomposition
Plant
uptake
of PO43–
Plankton
Dissolved PO43–
Soil
Uptake
Leaching
Figure Nutrient cycles
Sedimentation

26. 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

27. Ammonium form available to plants
Fixation of N N
Free living aerobics as Azotobacter, & certain cyanobacter
Lichen symbionants
Mutualists associated with certain plant groups
N N + N (NH3) NH4
H + energy
NO3
Ammonium form available to plants
Ammonia (gas)
Nitrate produced by soil bacteria from ammonium may also be taken up by plants or mineralized to N2
Under acidic conditions converts to ammonium but may be lost to atmosphere

28. Organic nitrogen is decomposed to NH4+ by ammonification, and NH4+ is decomposed to NO3– by nitrification
Denitrification converts NO3– back to N2

29. NO3 – NH3 NH4 + NO2 – N2 in atmosphere Assimilation Denitrifying
Fig c
N2 in atmosphere
Assimilation
Denitrifying
bacteria
NO3 –
Nitrogen-fixing
bacteria
Decomposers
Nitrifying
bacteria
Figure Nutrient cycles
Ammonification
Nitrification
NH3
NH4 +
NO2 –
Nitrogen-fixing
soil bacteria
Nitrifying
bacteria

30. 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

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

32. 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

33. Fig
Figure Fertilization of a corn crop

34. 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

35. Fig
Figure The dead zone arising from nitrogen pollution in the Mississippi basin
Winter
Summer