Tài liệu Soil Organisms and Nitrogen Cycle

rillum, Azotobacter spp. and Clostridium spp. (30 % of all N2 fixed in world) 
Symbiotic N fixers --example is bacteria (Rhizobium) and plant (soybean) (70 % of all N2 fixed in world). Symbiotic nitrogen fixers are associated with plants and provide the plant with nitrogen in exchange for the plant's carbon and a protected home. 
Rhizobium-legume symbiosis 
The gram negative bacteria Rhizobium, Bradyrhizobium and Azorhizobium associate with leguminous plants (members of the bean family), the gram positive bacteria Frankia associate with certain fast growing trees, and cyanobocteria associate with some aquatic ferns. See the information on legumes at LEGUMES
In the Rhizobium-legume symbiotic process, the bacteria infect the roots of the plant and a structure known as a nodule is formed. Once the nodule is established, the differentiated bacteria (they become non-motile bacteroids) living in the infected plant cells, reduce atmospheric nitrogen to ammonia which is excreted to the plant cell and is, in turn, assimilated to organic nitrogen (proteins and amino acids) by the plant. The plant provides the bacteroid with carbon skeletons (photosynthate) which are required by Rhizobium, a strict aerobe, to provide the energy that is needed for nitrogen fixation. How a nodule forms can be seen in this diagram 
This symbiosis is a specific process, a certain species of Rhizobium can only nodulate a certain type of legume, for example: R. etli nodulates beans (Phaseolus), R. meliloti nodulates alfalfa (Medicago). The bacterial enzyme responsible for the reduction of gaseous N2 to ammonia is the nitrogenase enzyme complex which is formed from the joining together of three different polypeptides. Different nitrogenase enzyme systems have been found in different microorganisms. 
Soil N 
When soil nitrogen (NO3- or NH4+) levels are high, the formation of nodules is inhibited. 
Also, anything that impacts the carbohydrate production will effect the amount of N fixed. In order for the nitrogen to be used by succeeding crops, the nodules and plant must be incorporated into the soil, or no nitrogen will be gained. Harvesting the alfalfa for animal feed reduces the chances for a net nitrogen gain, unless the manure is returned to the soil. 
Nitrogen fixation research will undoubtedly make important contributions to agriculture by substituting traditional fertilizer N inputs (which are costly, polluting and time consuming), with a cheap natural biological alternative. Indeed, inoculation of crops with Rhizobium strains (coat the seeds with Rhizobium) can induce nodule formation and N2 fixation in legume crops such as soybean. Future goals of the field include the manipulation of the genome of both bacteria and plants to improve existing symbiosis with legumes, the extension of symbiotic nitrogen fixation to other crops and, eventually, the production of plants able to fix nitrogen themselves. 
Besides the knowledge of the molecular basis of biological nitrogen fixation and the technology to manipulate the genome, a deep understanding of the ecology of nitrogen fixing organisms and of the fate of introduced new genetic information into the soil will be necessary to achieve these goals. For more information about research on Rhizobium go to the Rhizobium Research Laboratory . For information about the history of Nitrogen fixation go to History N-Fixing 
Ammonification - Second step in N cycle 
The biochemical process whereby ammoniacal nitrogen is released from nitrogen-containing organic compounds. Soil bacteria decompose organic nitrogen forms in soil to the ammonium form. This process is referred to as ammonification. The amount of nitrogen released for plant uptake by this process is most directly related to the organic matter content. The initial breakdown of a urea fertilizer may also be termed as an ammonification process 
In the plant, fixed nitrogen that is locked up in the protoplasm (organic nitrogen) of N2 fixing microbes has to be released for other cells. This is done by the process of ammonification with the assistance of deaminating enzymes. 
In the plant=Alanine(an amino acid) + deaminating enzyme --------> ammonia + pyruvic acid, 
or in the soil=RNH2 (Organic N) + heterotrophic (ammonifying) bacteria ---------> NH3 (ammonia) + R. 
In soils NH3 is rapidly converted to NH4+ when hydrogen ions are plentiful (ph< 7.5)
When microbes have too much N for their own requirements they excrete the excess as NH4+ into the soil. This happens mainly when the microbes are degrading crop residues with low C:N ratios . In high pH soils NH4+ (ion in solution) is unstable and changes to NH3 (gas) which can be lost via volatilization. Volatilization is prevalent in soils to which farmyard manures or urea have been added (you can smell the NH3 coming off chicken manure for example) . NH4+ is held by the soils cation exchange capacity (negative charge sites) and thus will not leach, but can be lost when soil erosion occurs. 
Nitrification- Third step in the N Cycle 
Nitrification is the conversion of NH4+ to NO3- . This aerobic reaction is carried out by autotrophic bacteria. Maximum nitrification rates occur at neutral pH and high temperatures (factors that favor the bacteria involved in this process - Nitrosomonas and Nitrobacter). See Nitrification Diagram 
Denitrification - Fourth and last step of N Cycle 
Involves conversion of NO3- to N2 gas in the presence of low oxygen levels. 
C6H12O6 + 4NO3- --> 6CO2 + 6H2O + 2N2(gas) + NO + NO2 
Bacterial denitrification is the microbial reduction of NO3- to NO2- or N. For example Pseudomonas Use NO3- instead of O2 as a terminal electron acceptor. 
Denitrification is accelerated under anaerobic (flooded or compacted) conditions and high nitrogen inputs. Denitrification results in environmental pollution (destroys ozone) and also contributes to global warming since nitrous oxides do have a minor effect as a greenhouse gas. 
Through nitrification and denitrification 10 - 20 % of the applied N is lost. 
Nitrogen Management 
N Gains and Losses 
N Gains=N2 fixation, rainwater (dissolved NH4+ and NO3-), animal manure, and plant residues (indirect). 
N Losses=crop removal, leaching, gaseous losses (volatilization), soil water erosion and runoff . 
Do these inputs provide more N than what is lost or is the soil N sustainable ? In some cases excess N is lost from leaching or erosion due to inadequate crop cover in winter, or N is lost from improper application of animal manure as NH3. Generally there is insufficient N in the soil for maximum crop yields, and N fertilizer is needed. Using N fertilizers efficiently is an important for crop management and environmental protection. 
The use of N fixing plants (i.e. beans) can reduce the use of N fertilizers, however not all management plans will allow for this. The need to optimize gains (enhance N fixation) and minimize losses by reducing NO3- leaching will aid all aspects of nitrogen management and environmental protection. Understanding the N Cycle is important in managing the nitrogen needs of the future. See Nitrogen Cycle for a review of this process.
Tuesday, July 6, 1999 
A North Carolina Sea Grant study released June 30 blames increased atmospheric nitrogen pollution for increased harmful algae bloom activity in the North Atlantic Ocean Basin. "The study is significant because it reconfirms that atmospheric nitrogen has been found to be a regional and global source of pollution," said Hans Paerl, author of the report, which appeared in the June issue of the peer-reviewed Royal Swedish Academy of Sciences' journal Ambio. 
He was one of the first environmental scientists to identify atmospheric nitrogen as a possible pollutant. "We also found a strong spatial linkage between water in areas with high amounts of atmospheric nitrogen and in places where there has been documented increases in harmful algae blooms. This is critical as we are only beginning to understand the importance of links between human-induced pollution of coastal oceans and harmful algae bloom expansion." Atmospheric nitrogen has grown significantly over the past three decades, said Paerl. 
In the study, scientists found that atmospheric nitrogen accounted for 46 to 57 percent of the total externally supplied or new nitrogen deposited in the nitrogen-sensitive North Atlantic Ocean Basin. The increase can be attributed to growing agricultural, urban and industrial emissions of nitrogen oxides, ammonia and possibly organic nitrogen, Paerl said. Algae blooms deplete the ocean's oxygen, which can kill ocean-dwelling organisms and disrupt the entire food chain. In North Carolina, researchers found that increased atmospheric nitrogen in coastal waters reflects changing land use and human activities. 
One of the most prominent land-use changes since the late 1970s has been the rapidly growing swine and poultry industry in the mid-Atlantic coastal plain, said Paerl. The North Carolina Department of Environment and Natural Resources Division of Air Quality estimates that animal operations account for at least half the atmospheric nitrogen emissions in the eastern part of the state, Paerl said. He is investigating how far these emissions travel over the coastal zone. Atmospheric transport modeling efforts indicate that some portion of these emissions will travel far enough to be deposited in the North Atlantic, but how much and what distance are still unknown. "We are starting to acquire a sizeable data set on where sources of nutrients are accumulating in our coastal waters. We are also looking at studying blooms, improving our ability to detect them and characterize them in terms of their harmful effects," Paerl said. Source: Environmental News Network, 1999 
Lab 9 Chapters 
Chapter 1 
Lab Units 
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