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

It is generally accepted that the enrichment of natural 15N 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 15N 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 15N abundance is explained by isotopic fractionation of 15N 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 15N isotopic fractionation co-occurring with N losses during long-term cultivation.