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

ABSTRACT

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 _a ¹⁵N) whereas clays had a higher ¹⁵N abundance (>12 _a 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.

¹ Publication no. R321 of the Saskatchewan Inst. of Pedology, Univ. of Saskatchewan, Saskatoon, Sask., Canada S7N 0W0. Research supported by the Natural Science and Engineering Research Council of Canada.

² Postdoctorate Fellow, Assistant Professor (NSERC), Professor, and Graduate Student, respectively.

Received for publication October 25, 1983. Accepted for publication November 11, 1983.

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