Soil Biogeochemical Cycles

1. Soil Biogeochemical Cycles Carbon, Nitrogen, Phosphorus

2. Recall BIOTIC REGULATION • in excerpt from Farm as Natural Habitat book • Partnership between plants and soil biota Litter, root exudates soil biota plants Nutrients, structure, tilth

3. 24/118 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. Plant residues Applied organic materials Respiration Plant removal Erosion LOSSES GAINS Soil organic carbon

10. 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

13. Soil management may help curb greenhouse effect due to carbon dioxide emissions“soil carbon sequestration” 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?

14. 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

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

16. Soil Nitrogen Cycle

17. Atmosphere 78% nitrogen • Not in directly accessible form for organisms • Made usable by fixation • Plants can use nitrogen in 2 forms: • Ammonium ions NH4+ • Nitrate ions NO3- • Most terrestrial N is in soil! • 95-99% in organic compounds • Made usable by mineralization

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

19. A. Nitrogen fixation Non-Biological 1. Atmospheric: lightning • Oxidation of N2N2 -----> NO3- • Rainfall additions from lightning • 2-5 lbs....../acre/year

20. Non-Biological 2. Industrial production of N fertilizer (Haber process) N2 + H2→ NH3

21. Non-Biological 3. Air Pollution fuel combustion from cars & stationary fuel combustion sources (electric utilities and industrial boilers) put NO (nitrous oxide) & NO2 (nitrogen dioxide) in atmosphere may remain in the atmosphere for several days and during this time chemical processes may generate nitric acid, and nitrates and nitrites as particles.

22. Biological (soil organisms) 1. Nonsymbiotic, autotrophic: (use solar energy) a) Some actinomycetes b) Cyanobacter (formerly known as blue-green algae) c) Photosynthetic bacteria Azotobacter (aerobic) & Clostridium (anaerobic) about 5-50 lbs....../acre/year d) Archeae

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

24. Legumes : 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. Rhizobium Alfalfa root nodule Root hair encircling bacteria Bacteria invades host plant root Response of host plant root is to grow a nodule for the bacteria to live in.

26. Bacteria takes N2 from the air and converts it into NH3 which resides in bacteria in nodule Energy-demanding process: N2 + 8H+ + 6e- + nitrogenase→ 2NH3 + H2 NH3 + organic acids → amino acids → proteins

27. Fate of N Fixed by Rhizobium: • used by host plant, 2) leaks out of root to become available to surrounding plants, 3) as roots and nodules are sloughed-off, heterotrophic organisms immobilize the N and it eventually becomes part of the SOM.

28. Legumes buffer the N supply and fix what they need from the air Legume Legume Grass Grass Fixed N Manure N Soil N Michael Russelle - USDA-ARS Plant Science Research Unit

29. We need to fertilize non-legumes and can easily guess wrong Legume Legume Grass Grass Loss Fixed N Fert N Manure N Soil N Michael Russelle - USDA-ARS Plant Science Research Unit

30. 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

31. B. Fates of NH4+ • 1) fixed by clay minerals, • 2) lost by soil erosion, • 3) used by plants (NH4+), • 4) volatilization • NH4+ ----> NH3 High pH Soils > 7.5

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

33. Fates of Nitrate • *Immobilization ---> • Plant uptake of NO3- • *NO3- is not held by soil particles and is easily leached • when ppm NO3- is > 10 ppm the water is considered to be contaminated • * Denitrification - stimulated by anaerobic conditions.

34. 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

35. Through nitrification and denitrification 10 - 20 % of the applied N is lost. Nitrification inhibitors can be applied like N-Serve. This chemical inhibits the growth of nitrosomonas and nitrobacter or slows conversion of NH4+ conversion to NO3-

36. Excess nitrate gets into water supply: Duxbury, 1997, Wm. C. Brown Publishers

37. Nitrate in drinkingwater supplies • Nitrate has been detected in surface- and ground-water supplies in various parts of the state. • Low levels of nitrate can be found in most of the surface waters of the state.

38. In cases where the concentration of nitrate-nitrogen exceeds the maximum contaminant level of 10 mg/L, as set forth by the U.S. EPA - water suppliers are required to issue a nitrate alert to users. The health of infants, the elderly and others, and certain livestock may be affected by the ingestion of high levels of nitrate. USGS, 1998 Risk of Groundwater Contamination by Nitrate

40. Some implications for organic farming/gardening with the soil food web in mind… • Some plants prefer their N as ammonium: • Trees, shrubs, perennials • Some plants prefer their N as nitrate: • Vegetables, annuals, grasses

41. When nematodes and protozoa consume fungi and bacteria, they release N in ammonium form • IF nitrifying bacteria are present…. • N-fixing bacteria quickly convert ammonium to nitrate • (Bacteria produce slime with a pH >7 to make the environment favorable to bacteria) • Fungi produce organic acids to decay OM • Makes environment more favorable to fungi

42. The key is to foster a bacterial-dominated soil or a fungal-dominated soil • Can do this by managing the C:N ratio of compost

43. Inorganic, soluble nitrogen fertilizers are great for getting plants to grow but are detrimental to the soil food web! • Water soluble nitrates are readily available to roots • They don’t attach to humus or clays • Therefore deplete organisms in soil and require constant application

44. Phosphorus Cycle

45. 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

46. 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

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

48. 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

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