1. Soil Biogeochemical Cycles
Carbon, Nitrogen, Phosphorus

2. 24/118 required by organisms
Macronutrients: C,H,N,O,P,S
Micronutrients

3. BIOGEOCHEMICAL CYCLES
The complete pathway that a chemical element takes through the biosphere, hydrosphere, atmosphere and lithosphere.

4. Soil Carbon Cycle

5. CARBON CYCLE
atmosphere
photosynthesis
respiration
biosphere

7. Soil organic carbon Plant residues Applied organic materials GAINS
LOSSES
Respiration
Plant removal
Erosion

8. Pools (compartments) of soil organic matter:
(categorized by susceptibility to microbial respiration)
1. Active/Fast
C:N 15:1 – 30:1
1-2 years
readily accessible to microbes; most of mineralizable N
10 – 20% of total
2. Slow
C:N 10:1 – 25:1
yrs
food for autochthonous microbes ; some mineralizable N
3. Passive
C:N 7:1 – 10:1
yrs
colloidal; good for nutrient and water-holding
60 -90% of total

11. pre-Industrial Revolution: 280 ppm CO2
Soil management may help curb greenhouse effect due to carbon dioxide emissions
pre-Industrial Revolution: 280 ppm CO2
post: 370 ppm
0.5% increase per year
Causes:
1. Fossil fuel burning
2. Net loss of soil organic matter
By changing balance between gains and losses, may limit loss of OM…how?

12. How?
1. Restore passive fraction in soils that are degraded. -sequesters carbon for long time 2. Switch to no-till practices 3. Convert to perennial vegetation

13. Cornfield in warm, temperate climate
Net loss of carbon

15. Soil Nitrogen Cycle

16. Not in directly accessible form for organisms
Atmosphere 78% nitrogen
Not in directly accessible form for organisms
Made usable by fixation
Most terrestrial N is in the soil !
95-99% in organic compounds
Made usable by mineralization

17. Let’s look at all components and processes in nitrogen cycle…..

18. A. Nitrogen fixation 1. Atmospheric: lightning 2. Industrial
Oxidation of N2
2. Industrial
production of N fertilizer
N2 + H2 → NH3
3. Biological (soil organisms)
(industrial fixes 85% as much N as organisms)

20. Biological fixation (soil organisms)
Immobilization: microbes convert N2 to
N-containing organic compounds
Nitrogenase

21. 2 groups of N-fixing microorganisms
Nonsymbiotic, autotrophic:
(use solar energy)
Some actinomycetes
Cyanobacter (formerly known as blue-green algae)
Photosynthetic bacteria

22. B. Symbiotic, in association with legume plants
(plants supply energy from photosynthesis)
Rhyzobium
Infect root hairs and
root nodules of legumes

24. Symbiosis: mutualistic: plants provide energy, bacteria provide ammonia for synthesis of tissue
Energy-demanding process:
N2 + 8H+ + 6e- + nitrogenase → 2NH3 + H2
NH3 + organic acids → amino acids → proteins

26. B. Mineralization (ammonification)
Heterotrophic microorganisms
Decomposition
Organic N compounds broken down to ammonia; energy released for microorganisms to use
Organic N + O2→CO2 + H2O +NH3 + energy

27. C. Nitrification
Oxidizes ammonia to nitrate; 2 step oxidation process:
1. Nitrosomonas:
NH3→NO2- (nitrite) + energy
2. Nitrobacter:
NO2-→NO3- (nitrate) + energy

28. D. Denitrification Completes N cycle by returning N2 to atmosphere
(prevents N added as fertilizer from being “locked” in roots and soil)
Requires energy; Reduction of nitrate/nitrite
NO2 or NO3 + energy→N2 + O2 (many steps)
Denitrifying bacteria and fungi in anaerobic conditions

31. NITROGEN applied to soil as fertilizer

32. P is part of many essential molecules of life, including DNA
Phosphorus Cycle
P is part of many essential molecules of life, including DNA

34. Phosphorus Cycle P often limiting factor for plants because :
1. low in parent materials
2. Most P tends to get locked up in minerals, stable oxides or organic compounds: unavailable!
Microbes need P
After N, P is most abundant nutrient in microbial tissue.
Competition with plants for P

35. Differs from N cycle 1. P cycle has no gaseous component
2. Whereas N readily goes into solution as stable, plant-available nitrate,
P reacts quickly with other ions and converts to unavailable forms.

36. Available P P in solution that plants can use: H2PO4- or HPO4-2 ion
(availability is greatest at neutral pH)
If there is enough P, microbes mineralize & release P to soil solution.

37. Mineralization
This slow release of P to plants during growing season reduces need for P fertilizer.
The mycorrhizal fungi and the bacteria who do this get exudates from plants roots in exchange.

38. If there is not enough P
The microbes will immobilize it (in their bodies) as organic P.

39. Adsorption and Desorption
P becomes bound to soil particles (clay, Fe-Al oxides: UNAVAILABLE
Release of adsorbed P to soil solution

40. Role of mycorrhizae in P cycle:
Can infect several plants:
Hyphae connect plants ; conduits for nutrients
Fungi get E from plant ‘s photosynthesis.

43. Soil phosphorus cycle in a grazing system

44. Phosphate crisis

46. Some important points about these cycles…

47. Arbuscular mycorrhizae and N cycle
Involve 2/3 of plant species.
Unlike most fungi, the AM fungi get their supply of sugars for energy and growth from their plant partner and not from the decomposition of organic matter, but they obtain large amounts of nitrogen from
decomposing organic matter.
The fungus itself is much richer in N than plant roots,
calculations suggest that there is as much nitrogen in AM fungi globally as in roots.
Since fungal hyphae (the threads of which the fungus is composed) are much shorter-lived than roots, this finding has implications for the speed with which nitrogen cycles in ecosystems.

48. Biotic regulation vs. synthetic fetilizers
Biota capture and store soil nutrients and return them to plants when they need them.
When plants need nutrients, they stimulate soil biota to release the nutrients.
In biotic regulation, nutrients are held in resistant forms, not readily lost from soil.
Synthetic fertilizers cause physiological changes in plants that make them withhold energy from soil biota.