Presentation on theme: "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."— Presentation transcript:
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%, Fixed by soil org.= 19% Rainfall = 8%, Fertilizer = 13%, Sewage = 4%.
Nitrogen Fixation- Conversion of N 2 into NH 3 or R-NH 2 A. 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 N 2 -----> NO 3 -
1.Nitrogen Fixation Conversion of N 2 into NH 3 or R-NH 2 B. 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 interaction http://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 N 2 from the air and converts it into R-NH 2 which resides in Bacteria in Nodule and some is in the form of NH 4 + 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.
Dazzo & Wopereis, 2000 Vance et al., 1980 Infection and nodule formation Rhizobium Dazzo & Wopereis, 2000 Gage and Margolin, 2000 Root hair curling around rhizobia Rhizobia reproduce in infection threads Bacteroids filling a single cell Alfalfa root nodule M. Barnett Michael Russelle - USDA-ARS Plant Science Research Unit
Fixed N Nitrogen fixation is (usually) reduced by external N “Soil” N Total N in the plant N2N2 N2N2 Michael Russelle - USDA-ARS Plant Science Research Unit
Grass Legume Soil N Manure N Grass Legume Fixed N Legumes buffer the N supply and fix what they need from the air Michael Russelle - USDA-ARS Plant Science Research Unit
Grass Legume Soil N Manure N Grass Legume Fixed N We need to fertilize non-legumes and can easily guess wrong Fert N Loss 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) R-NH2 ---> NH3 + H+ ----> NH4+ heterotrophic organ.
B. Fates of NH 4 + = 1) fixed by clay minerals, 2) lost by soil erosion, 3) used by plants (NH 4 + ), 4) volatilization NH 4 + ----> NH 3 High pH Soils > 7.5
3. Nitrification 2 - step process 1. 2NH 4 + + 3O 2 ---> 2NO 2 - + 4H+ + 2H 2 0 + E Nitrosomonas 2. 2NO 2 - + O 2 --> 2NO 3 - + E Nitrobacter Process is acid causing due to release of 4 H+
3. Fates of Nitrate- NO 3 - *Immobilization ---> Plant uptake of NO3- *NO 3 - is not held by soil particles and is easily leached - when ppm NO 3 - 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 + 4NO 3 - --> 6CO 2 + 6H 2 O + 2N 2 (gas) + NO + NO 2 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-
Nitrate in drinking water 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. Risk of Groundwater Contamination by Nitrate USGS, 1998
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.
C:N Ratio of some organic materials 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 Break even point for C:N is 20 to 30 : 1.
N-Cycle Plants need NO 3 - This can be supplied as NO 3 -, NH 4 +, or organic N (R-NH 2 ), The rate at which NO 3 - is available depends on : C:N, temp, O 2, 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 (2 nd ) 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