Incorporation of 15N‐Tagged Mineral Nitrogen into Stable Forms of Soil Organic Nitrogen

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Abstract
Transformation of 15N‐tagged mineral N into organic forms was accelerated by growing a series of oat crops (Avena sativa L.) in the greenhouse, with each crop growing to sufficient maturity for the seed to be used for the next crop. The plant material was returned to the soil, except for triplicate pots which were removed for analysis at each cropping period. Availability ratios (fraction of plant N that is 15N divided by the fraction of soil N that is initially 15N for each cropping period) declined from an initial value of 10.4 for the Chester surface soil to approximately 2 after 11 crops.During long‐term mineralization tests with soil samples taken after two, five, and eight cropping periods, availability ratios were also stable, or soon became stable, at approximately 2. When similar soil samples were extracted after autoclaving in 0.01M CaCl2 or subjected to acid hydrolysis, however, the “extractability ratios” approached unity. It is hypothesized that about half of the indigenous N in the Chester soil is biologically unavailable because of inaccessibility to microorganisms of organic N compounds or complexes incorporated within microaggregates.

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