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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 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%.
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-
N2 Denitrification N fixation immobilization R-NH2 NO3- Plants mineralization Nitrification Ammonification NH4+
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/
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.
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.
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
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
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
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
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+
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
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+
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.
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-
N2 Denitrification N fixation immobilization R-NH2 NO3- Plants Nitrification Ammonification NH4+
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.
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
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.
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.
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,
If you had to dispose of 10 tons of sawdust every month from a local saw mill, what would be your solution?
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
Composting A biological process that breaks down organic material (such as grass clippings and leaves) into more stable molecules
Stages of Composing Process • Mesophilic stage 1 • Brief • Temperature rises to 40 degrees C • Sugars and readily available microbial food sources are rapidly metabolized
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
Stages of Composing Process • Mesophilic (2nd) • Curing stage • Temperatures fall back to ambient • Material recolonized by mesophilic organisms
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
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
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