Nitrogen (N) is a mineral nutrient required by plants in greatest amounts and its availability is limiting factor for plant growth in natural as well as agricultural environments (Kraiser et al, 2011). Most of soil nitrogen comes from the atmosphere, where di-nitrogen (N2) gas is the most predominant gas occupying about 79% of atmospheric air. Only a few organisms, including plants, are able to utilize molecular N2 as nitrogen source while most organisms utilize N combined with other elements such as oxygen and hydrogen (Robertson and Goffman, 2007), Atmospheric has to be fixed into the soil by the activity of some microorganisms, a process known as biological N2 fixation (Robertson and Vitousek, 2009). Biological N2 fixation in agricultural soils is carried by both parasitic and free living microorganisms. Free living microbes obtain their energy from organic matter while parasitic microbes are symbionts living in the roots of legumes and obtaining energy from the host plant (Herridge et al, 2008). Some of the soil nitrogen comes from N fertilizer application (Robertson and Vitousek, 2009), as well as return of ammonia and nitrate in rain water (Schulten and Schnitzer, 1998). N fertilizer application introduces N into the soil in the form that is readily available for plant uptake. Soil organic matter serves as the storage and supplier of N to plant roots and soil microorganisms and most of total soil N is associated with soil organic matter (Schulten and Schnitzer, 1998). Soil organic matter must be decomposed by soil inhabiting microorganisms to release N for uptake by soil microorganisms and plants (Robertson and Vitousek, 2009). Soil N exists as inorganic N forms which are nitrate, ammonium and nitrite, as well as organic N forms which are urea and amino acids, and presence of these different forms in the soil vary depending on the habitat. Plants have evolved multiple strategies for acquiring N, which range from nitrate uptake, to nitrogen fixation and even carnivory. Most plant species are able to absorb and assimilate nitrate, ammonium, urea and amino acids as nitrogen sources, but the response to a particular form of nitrogen differ from species to species (Crawford and Glass, 1998).
Inorganic soil N forms
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Ammonium (NH4+) in the soil is released from complex, insoluble organic N sources (Hodge et al, 200), through a process known as N mineralization. Ammonium-nitrogen is a simpler, soluble that can be taken up by plants and other and soil microbes. The release of ammonium comes as a by-product of consumption of soil organic matter by microorganisms which utilize soil organic matter as source of energy and carbon source for growth. During N mineralization, microorganisms utilize the N rich soil organic matter and absorbing carbon, nitrogen as well as acquiring energy, and during the process any extra N is released into the soil in the form of NH4+. N mineralization is carried out by wide variety of aerobic as well as anaerobic fungi and bacteria. Soil animals also play important role in soil N mineralization because they feed on bacteria and fungi regulating their populations, and create or modify habitats for wide variety of microorganisms (Robertson and Goffman, 2007). NH4+ is involved in the formation of nucleic acids, proteins and other organic compounds, as well as a product of their metabolic break down (Ludewig et al, 2007). NH4+ is appositively charged ion which is held on cation-exchange sites associated with soil organic matter, clay soil particles and variable-charge minerals, hence its less mobile in soil water (Robertson and Goffman, 2007).
NO3- is the major source of N for most plants and its common form of inorganic N in most soils
Nitrate is a signal for the regulation of carbon metabolism through modulating the
expression of genes involved in the biosynthesis of organic acids. Nitrate also functions as a morphogenetic signal governing shoot-root balance (Daniel et al, 1998). NO3- also serves as an important signal for plant growth as plants respond to presence of NO3- by altering their metabolism and by stimulating genes in their NO3- assimilation pathway. These genes encode transporters that take up NO3- from the soil solution and the enzymes involved in the conversion of NO3- to NH4+ within the cell (Crawford and Glass, 1998). NO3- is liberated into the soil through the process of nitrification through which NH3 is oxidised to NO3- through a series of steps catalyzed by different enzymes produced by nitrifying bacteria. Nitrification is carried out by separate groups of bacteria which are ammonia and nitrite oxidizers, respectively. These nitrifiers derive C from CO2 or carbonates, rather than from organic matter and are obligate aerobes. The first step in nitrification is the ionization of NH4+ to NH3 in soil water:
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NH4+(aq) NH3(aq) + H+(aq).
This is followed by production of hydroxylamine catalyzed by the membrane-bound enzyme ammonia mono-oxygenase:
NH3+ + 2H+ + O2 mono-oxygenase NH2OH + H2O
The reaction is irreversibly inhibited by small quantities of acetylene, which inhibits ammonia mono-oxygenase. Hydroxylamine is further oxidized to nitrite by the reaction catalyzed by hydroxylamine reductase:
NH2OH + H2O NH2OH-oxidoreductase NO2- + 4e- + 5 H+
Two of the four electrons released in this reaction are used in the prior NH3 oxidation step; the remaining two are used in electron transport, generating energy for cell growth and metabolism:
2H+ + 1/2O2 + 2e- terminal oxidase H2O
In most soils the nitrite produced by ammonia oxidizers does not accumulate in the soil but is quickly oxidized to nitrate by the nitrite-oxidizing bacteria when they perform nitrite oxidation:
NO2- + H2O nitrite oxido reductase NO3- + 2H+ + 2e-
These reactions are membrane-associated and because nitrite oxidoreductase is a reversible enzyme, the reaction can be reversed to result in nitrate reduction to nitrite (Robertson and Goffman, 2007). NO3- is a negatively charged ion, which is not held by soil particles and remains dissolved in soil solution. NO3- leaches more rapidly in sandy soils than in fine-textured soils because sandy soils have lower water holding capacity than fine-textured soils (Wolkowski et al, 1995). NO3- in soil solution is carried by the bulk flow and it is absorbed into the root epidermal and cortical cytoplasm, a process which requires energy in the form of proton motive force based on cell parameters. Within the root symplasm, NO3- is either reduced to NO2- by the cytoplasmic enzyme nitrate reductase, efflux back across plasma membrane to the apoplasm, influx and storage in the cytoplasm or transport to the xylem for long-distance translocation to the leaves where it is reduced to NO2- or stored in vacuoles. Intermediary compounds formed during the oxidation of hydroxylamine to nitrite can result in the formation of NO, which can escape to the
atmosphere and influence the photochemical production of ozone (O3) and the abundance of hydroxyl (OH) radicals in air, primary oxidants for a number of tropospheric trace gases including methane (Robertson and Goffman, 2007). Excessive irrigation and/or N application rate combined with intense rainfall on excessively drained sandy soils with low water-holding capacity greatly enhances the potential risk of N leaching. Nitrate leaching from agricultural fields is considered to be one of the major contributors to groundwater contamination (Zotarelli et al, 2007).
NO2- produced in the soil during nitrification process during which NH4+ is oxidized to NO2-, then to NO3-.
STATEMENT OF THE PROBLEM
Irrespective of the sandy soils which are highly leachable, the farmers of Seronga are still able to farm without visible N deficiencies.
What are the forms of N in these soils?
H0-There is no difference in N forms in these soils?
H1-The N in these soils occur in many forms.
Prediction-The N found in Seronga soils is in the form of NH4+ which is less leachable form of N in sandy soils.
SIGNIFICANCE OF STUDY
Soil N exists in many forms each of which influence different species differently, so this study intends to identify the N forms in Seronga soils.