Fertilizer nitrogen isotope signatures

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Abstract
There has been considerable recent interest in the potential application of nitrogen isotope analysis in discriminating between organically and conventionally grown crops. A prerequisite of this approach is that there is a difference in the nitrogen isotope compositions of the fertilizers used in organic and conventional agriculture. We report new measurements of δ15N values for synthetic nitrogen fertilizers and present a compilation of the new data with existing literature nitrogen isotope data. Nitrogen isotope values for fertilizers that may be permitted in organic cultivation systems are also reported (manures, composts, bloodmeal, bonemeal, hoof and horn, fishmeal and seaweed based fertilizers). The δ15N values of the synthetic fertilizers in the compiled dataset fall within a narrow range close to 0‰ with 80% of samples lying between−2 and 2‰ and 98.5% of the data having δ15N values of less than 4‰ (mean=0.2‰ n=153). The fertilizers that may be permitted in organic systems have a higher mean δ15N value of 8.5‰ and exhibit a broader range in δ15N values from 0.6 to 36.7‰ (n=83). The possible application of the nitrogen isotope approach in discriminating between organically and conventionally grown crops is discussed in light of the fertilizer data presented here and with regard to other factors that are also important in determining crop nitrogen isotope values.