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Organic N

Organic-N includes all substances in which N is bonded to C. It occurs in both soluble and particulate forms. The largest fraction is made up of amino acids and peptides and is often called amino N. Particulate organic-N includes small organisms (algae, bacteria, ...), both living and dead, and fragments of organisms. Soluble organic N is from wastes excreted by organisms or from the degradation of particulate organic-N.

Organic-N concentrations in natural waters, soils, and sediments are operationally defined. In the Kjeldahl method, a sample of water, soil, or sediment is heated with H2SO4 and a catalyst and N from amino acids is converted to NH4+. Total Kjeldahl N (TKN) includes N from amino acids and any NH4+. Organic-N is calculated by subtracting NH4+ (determined separately) from TKN. In another method, a water sample is oxidized using various combinations of potassium persulfate, heat, and ultraviolet light and all N is converted to NO3-. Organic-N is calculated by subtracting NO3- (determined separately) from the NO3- in the oxidized sample. Concentrations of individual amino acids can be determined chromatographically.

Typically, most N in soils and surficial sediments occurs in organic form. The amount of organic-N in soils and sediments is influenced by climate - all else being equal, increasing with moisture and decreasing with temperature in the United States. It is also influenced by vegetation. In Illinois equivalent soils developed under prairie had twice the organic-N of soils developed under forest. It is also influenced by topography. Soils such as the more upland prairie of the Morrow plots are estimated to originally have had 0.3 percent by weight N whereas soils developed under prairie wetlands originally had 2.2 percent N. The amount of N is also influenced by the particle size of the soil and sediment (more accumulating in fine-grained material) and the amount of mineral nutrients (especially phosphorus) in the soil and sediment. It is also influenced by the age of the land surface - older surfaces generally being lower in C, N, and mineral nutrients. Agricultural practices have reduced the N content in the plow layer of cornbelt soils by about 40 percent, on average, from their estimated virgin condition.

Organic-N sometimes makes up a significant fraction of soluble and particulate N in natural waters. See Examples of Organic-N Measurements.

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