Natural Nitrogen-15 Abundance as an Indicator of Soil Organic Matter Transformations in Native and Cultivated Soils
1984
Tiessen, H. | Karamanos, R. E. | Stewart, J. W. B. | Selles, F.
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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