Background and methodology:
The authenticity of organic products currently relies on enforcement of production standards through certification and inspection. As the organic market continues to increase, analytical tests that help maintain the authenticity of products in the organic sector should be considered.
Synthetic nitrogen fertilizers used in conventional farming tend to have stable nitrogen isotope values close to zero, usually between -2 and 2‰. Organic manure is probably the most commonly applied source of nutrients in organic systems, which contains nitrogen isotope values between 10 and 20‰.
This study collected samples of commercially produced organic and conventionally grown tomatoes, lettuce, and carrots and analyzed for their ä15N composition in order to assemble datasets to determine if there are any systematic differences in nitrogen isotope composition due to the method of production.
The tomato and lettuce datasets showed that the type of fertilizer used in organic and conventional systems result in differences in the nitrogen isotope composition of these crops. A mean ä15N value of 8.1‰ was found for the organically grown tomatoes compared with a mean value of -0.1‰ for those grown conventionally.
The organically grown lettuces had a mean value of 7.6‰ compared with a mean value of 2.9‰ for the conventionally grown lettuces. The mean value for organic carrots was not significantly different from the mean value for those grown conventionally.
This study suggested that nitrogen isotope analysis could be used to provide useful information to help detect whether conventionally-grown produce has been substituted for organic produce. However, this test will not provide unequivocal evidence as to whether synthetic fertilizers have been used on the crop. It only should be used to provide supporting evidence when there is a suspicion of mislabeling.
Bateman, A. S., Kelly, S. D. & Woolfe, M. (2007). Nitrogen isotope composition of organically and conventionally grown crops. Journal of Agricultural and Food Chemistry, 55 (7), 2664 - 2670. Available on-line at: http://dx.doi.org/10.1021/jf0627726