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Lecture 9b Nitrogen Cycle- N2 gas into NO3-

Lecture 9b Nitrogen Cycle- N2 gas into NO3-. Nitrogen in Atmosphere = 79% Problem is getting N 2 into a form that plants can use. Most N in soil used for Agriculture or Sources of N used by plants in cropland= OM = 37%, Manure = 19%,

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Lecture 9b Nitrogen Cycle- N2 gas into NO3-

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  1. Lecture 9bNitrogen Cycle- N2 gas into NO3- • Nitrogen in Atmosphere = 79% • Problem is getting N2 into a form that plants can use. • Most N in soil used for Agriculture or Sources of • N used by plants in cropland= • OM = 37%, • Manure = 19%, • Fixed by soil org.= 19% Rainfall = 8%, • Fertilizer = 13%, • Sewage = 4%.

  2. Nitrogen Fixation-Conversion of N2 into NH3 or R-NH2 • . Non-Biological Fixation • -Air Pollution -The main oxides of nitrogen present in the atmosphere are nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide – the result of fuel combustion from motor vehicle exhaust and stationary fuel combustion sources like electric utilities and industrial boilers--oxides of nitrogen may remain in the atmosphere for several days and during this time chemical processes may generate nitric acid, and nitrates and nitrites as particles. • - Rainfall additions from electrical discharge (lightning) 2-5 lbs....../acre/year • N2 -----> NO3-

  3. N2 Denitrification N fixation immobilization R-NH2 NO3- Plants mineralization Nitrification Ammonification NH4+

  4. Nitrogen FixationConversion of N2 into NH3 or R-NH2 • . Biological Fixation • 1. Non-Symbiotic (independent organism) - Azotobacter - aerobic & Clostridium - anaerobic about 5-50 lbs....../acre/year • 2. Symbiotic- mutually beneficial for host organism and bacteria - complex plant - bacteria interactionhttp://www.agron.iastate.edu/~loynachan/mov/

  5. B. Symbiotic N- Fixation • Bacteria = Rhizobia • Plant = Legume - 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.

  6. Symbiotic N Fixation • Bacteria invades host plant root • Response of host plant root is to grow a nodule for the bacteria to live in. • Bacteria takes N2 from the air and converts it into R-NH2 which resides in Bacteria in Nodule and some is in the form of NH4+ • Fate of N Fixed by Rhizobium: • 1) 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.

  7. 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

  8. Michael Russelle - USDA-ARS Plant Science Research Unit Nitrogen fixation is (usually) reduced by external N Fixed N Total N in the plant “Soil” N N2 N2

  9. 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

  10. 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

  11. 2. Ammonification • A. Ammonification in the soil is the conversion of organic N (RNH2) into inorganic ammonia (NH3) heterotrophic organ. • R-NH2 ---> NH3 + H+ ----> NH4+

  12. 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

  13. 3. Nitrification • 2 - step process • 1. 2NH4+ + 3O2 ---> 2NO2- + 4H+ + 2H20 + E • Nitrosomonas 2. 2NO2- + O2 --> 2NO3- + E • Nitrobacter • Process is acid causing due to release of 4 H+

  14. 3. Fates of Nitrate- NO3- • *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.

  15. 4. Denitrification • Involves conversion of NO3- to N2 gas • C6H12O6 + 4NO3- --> 6CO2 + 6H2O + 2N2(gas) + NO + NO2 • Bacteria = anaerobic • 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-

  16. N2 Denitrification N fixation immobilization R-NH2 NO3- Plants Nitrification Ammonification NH4+

  17. Duxbury, 1997, Wm. C. Brown Publishers

  18. 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. • In a recent statewide survey of water wells, a small percentage contained excessive nitrate concentrations.

  19. 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

  20. C:N Ratios • Bacteria require about 5 grams of carbon for each gram of nitrogen assimilated or used C:N in a ratio of 5:1. • Decomposing microorganisms have first priority for any mineralized N. • This use of N by decomposers results in insufficient N for plants. • Eventually period of N starvation is over after all the high C:N material is decomposed.

  21. domestic sewage -5:1 Muni. sewage - 8:1 legume hay -13:1 Mun. Compost 28 : 1 green grass - 35:1 corn stover - 50:1 Straw - 80:1 Sawdust - 400:1 C:N Ratio of some organic materials Break even point for C:N is 20 to 30 : 1.

  22. N-Cycle • Plants need NO3- • This can be supplied as NO3-, NH4+, or organic N (R-NH2), • The rate at which NO3- is available depends on : C:N, temp, O2, water,

  23. Ohio State University Extension Fact Sheet

  24. If you had to dispose of 10 tons of sawdust every month from a local saw mill, what would be your solution?

  25. Soil Inoculants to increase N Fixation • Inoculate soil or seeds with N-fixing bacteria • Introduce bacteria, nematodes, or insects that are predators of pest organisms • Add nitrification inhibitors to reduce bacteria that convert ammonium to nitrate

  26. Composting A biological process that breaks down organic material (such as grass clippings and leaves) into more stable molecules

  27. Stages of Composing Process • Mesophilic stage 1 • Brief • Temperature rises to 40 degrees C • Sugars and readily available microbial food sources are rapidly metabolized

  28. Stages of Composing Process • Thermophilic stage • 50 to 70 degrees C • Easily decomposed compounds are used up and humus-like compounds are formed • Frequent mixing essential to maintain oxygen levels and assure even heating of all material- • If too hot may kill organisms in the pile

  29. Stages of Composing Process • Mesophilic (2nd) • Curing stage • Temperatures fall back to ambient • Material recolonized by mesophilic organisms

  30. Benefits to Composting • Safe storage • Easier handling • Volume reduced 30 to 50% • Material more uniform • Nitrogen competition avoidance • No nitrate depression • Nitrogen stabilization • N in organic form

  31. Benefits to Composting • Partial sterilization • thermophilic stage kills most weed seeds and pathogenic organisms in • Detoxification • Most organic compounds are destroyed • Disease suppression • Compost suppresses soil borne diseases by encouraging microbial antagonisms

  32. The End

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