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

Abstract

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 15 N enrichment) typical for recent plant materials. A similarly low enrichment was observed in the silt‐sized fractions (+9 δ a 15 N) whereas clays had a higher 15 N abundance (>12 δ a 15 N), 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 15 N abundance of fine‐silt and coarse‐clay‐associated N appeared relatively unchanged even after prolonged cultivation of 60 yr. Increases in the 15 N abundance of the sand and silt fractions were attributed to reduced inputs of plant residues under cultivation. A large drop in the 15 N abundance of the fine‐clay‐sized materials indicated a major change in the nature of microbial N transformations.