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Soil Science Society of America Journal Abstract -

Natural Nitrogen-15 Abundance as an Indicator of Soil Organic Matter Transformations in Native and Cultivated Soils1


This article in SSSAJ

  1. Vol. 48 No. 2, p. 312-315
    Received: Oct 25, 1983
    Accepted: Nov 11, 1983

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  1. H. Tiessen,
  2. R. E. Karamanos,
  3. J. W. B. Stewart and
  4. F. Selles2



The natural N isotope composition of soil organic matter associated with organo-mineral particle size fractions of two cultivated and two native grassland soils was studied. In the native soils, N associated with the coarsest (sand size) fraction showed a low enrichment (+7 per mil 15N enrichment) typical for recent plant materials. A similarly low enrichment was observed in the silt-sized fractions (+9 δa 15N) whereas clays had a higher 15N abundance (>12 δa 15N), characteristic of residual material from microbially-mediated N mineralization. Cultivation initially resulted in the incorporation of a large pulse of low-enrichment plant material that could be detected in the sand and coarse silt fractions, as well as in the fine clays that contain a large portion of the soil's microbial products. The 15N abundance of fine-silt and coarse-clay-associated N appeared relatively unchanged even after prolonged cultivation of 60 yr. Increases in the 15N abundance of the sand and silt fractions were attributed to reduced inputs of plant residues under cultivation. A large drop in the 15N abundance of the fine-clay-sized materials indicated a major change in the nature of microbial N transformations.

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