Enrichment of natural 15N abundance during soil N losses under 20 years of continuous cereal cropping

It is generally accepted that the enrichment of natural N-15 abundance in soil over time is reflective of historic N cycling and loss, but this process in cropping soils is not yet clear. In this study, we identified an enrichment gradient of natural N-15 abundance during 20-year chronosequence of cereal cropping on Alfisols in southwest Queensland, Australia, that have no history of fertilisation. We demonstrate that the increase in soil N-15 abundance is explained by isotopic fractionation of N-15 during organic N mineralisation and nitrification, which lead to isotopically heavier ammonium retained in the soil and isotopically lighter soil nitrate taken up and removed by seasonal crops during harvest. Here we present a framework for natural N-15 isotopic fractionation co-occurring with N losses during long-term cultivation. Copyright (C) 2016 Elsevier B.V. All rights reserved.