Soil Biogeochemical Cycles

1. Soil Biogeochemical Cycles Carbon, Nitrogen, Phosphorus

2. Refer to BIOTIC REGULATION in Farm as Natural Habitat book, pp 156-7

3. 24/103 required by organisms Macronutrients: C,H,N,O,P,S Micronutrients

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

5. Elements transferred between compartments (pools) Active: accessible to living things Storage: inaccessible

6. Soil Carbon Cycle

7. CARBON CYCLE atmosphere photosynthesis respiration biosphere

9. Gains? Soil Organic Carbon Losses?

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

11. Pools (compartments) of soil organic matter: (categorized by susceptibility to microbial respiration) 1. Active 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 15-100 yrs food for autochthonous microbes ; some mineralizable N 3. Passive C:N 7:1 – 10:1 500-5000 yrs colloidal; good for nutrient and water-holding 60 -90% of total

14. 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?

15. 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

16. Cornfield in warm, temperate climate Net loss of carbon!!

17. Soil Nitrogen Cycle

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

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

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

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

22. 2 groups of N-fixing microorganisms • Nonsymbiotic, autotrophic: (use solar energy) Cyanobacter (formerly known as blue-green algae) in anaerobic; Azotobacter in aerobic 5-50 lbs....../acre/year

23. B.Symbiotic, in association with legume plants (plants supply energy from photosynthesis) 1. Rhyzobium 2. Bradyrhizobium Infect root hairs and root nodules of legumes

24. peas, clover, alfalfa, cowpeas, peanuts, beans, soybeans • Alfalfa - 200 lbs....../acre/year • Soybeans - 100 lbs......./acre/year • Beans - 40 lbs...../acre/year • * Green manure is live plant material added to soil to increase N content and SOM.

25. 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. Infection and nodule formation Rhizobium Alfalfa root nodule Dazzo & Wopereis, 2000 Root hair curling around rhizobia Rhizobia reproduce in infection threads M. Barnett Bacteroids filling a single cell Dazzo & Wopereis, 2000 Michael Russelle - USDA-ARS Plant Science Research Unit Gage and Margolin, 2000 Vance et al., 1980

27. B. Mineralization (ammonification) Heterotrophic microorganisms Decomposition Organic N compounds broken down to ammonia; energy released for microorganisms to use

28. ammonification Organic N + O2→CO2 + H2O +NH3 + energy

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

30. 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

33. Phosphorus Cycle

34. Phosphorous Cycle • P often limiting factor for plants: • low in parent materials • inclination to form low-soluble inorganic compounds • After N, P is most abundant nutrient in microbial tissue

35. Differs from N cycle 1. No gaseous component 2. N goes into solution as nitrate • Stable, plant-available But P reacts quickly with other ions and converts to unavailable forms

36. Available P in soil solution: • as H2PO4- or HPO4-2 ion • Microbes constantly consume and release P to soil solution

37. Unavailable forms of P depend on soil pH: • High pH: calcium phosphate CaHPO4 • Stable in high pH • Soluble in low pH • E.g., rhizosphere, so plants can get it • Can be transformed to less-soluble Ca-P form (apatite) • Low pH: iron and aluminum phosphates • Highly stable • Slightly soluble in low pH

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