Nitrogen Isotope Relationships between Crops and Fertilizer:  Implications for Using Nitrogen Isotope Analysis as an Indicator of Agricultural Regime

The effect of fertilizer type, amount of fertilizer applied, growing medium, and water type on the nitrogen-15 content of carrots, tomatoes, and lettuces has been investigated. Crops grown using synthetic nitrogen fertilizer were isotopically lighter than those grown using pelleted chicken manure. For example, for equivalent amounts of nitrogen applied, carrots grown with ammonium nitrate fertilizer had δ15N values between 3‰ and 4‰ lower than those grown using pelleted chicken manure. Plants grown in peat-based compost were generally found to be isotopically lighter than those grown in composted bark based compost. Results suggest that nitrate content and the δ15N of the nitrate in irrigation water may also influence crop δ15N. Wider implications of using crop δ15N more generally as an indicator of whether synthetic nitrogen fertilizers have been applied to a crop and the possible application and limitations of using crop δ15N as an indicator of agricultural regime (organic/conventional) are discussed. Keywords: Nitrogen; isotope; fertilizer; organic; conventional; δ15N