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Abstract

Distinguishing organic and conventional products is a major issue of food security and authenticity. Previous studies
successfully used stable isotopes to separate organic and conventional products, but up to now, this approach was not
tested for organic grassland hay and soil. Moreover, isotopic abundances could be a powerful tool to elucidate differences
in ecosystem functioning and driving mechanisms of element cycling in organic and conventional management systems.
Here, we studied the d15N and d13C isotopic composition of soil and hay samples of 21 organic and 34 conventional
grasslands in two German regions. We also used Dd15N (d15N plant - d15N soil) to characterize nitrogen dynamics. In order to
detect temporal trends, isotopic abundances in organic grasslands were related to the time since certification. Furthermore,
discriminant analysis was used to test whether the respective management type can be deduced from observed isotopic
abundances. Isotopic analyses revealed no significant differences in d13C in hay and d15N in both soil and hay between
management types, but showed that d13C abundances were significantly lower in soil of organic compared to conventional
grasslands. Dd15N values implied that management types did not substantially differ in nitrogen cycling. Only d13C in soil
and hay showed significant negative relationships with the time since certification. Thus, our result suggest that organic
grasslands suffered less from drought stress compared to conventional grasslands most likely due to a benefit of higher
plant species richness, as previously shown by manipulative biodiversity experiments. Finally, it was possible to correctly
classify about two third of the samples according to their management using isotopic abundances in soil and hay. However,
as more than half of the organic samples were incorrectly classified, we infer that more research is needed to improve this
approach before it can be efficiently used in practice.