Multi-elemental and Isotopic Analysis of Organic and Conventional Crops using Atomic Spectroscopy and Chemometrics

Summary

The organic sector is rapidly expanding worldwide. The global area of organic agricultural land has more than tripled the past decade while the number of organic producers has increased by 800%. This development is greatly driven by political initiatives towards more environmentally friendly food production systems as well as an intensive promotion of organic produce, which has boosted the consumer demand. Consumers are willingly paying premium prices for the organic products due to the general perception that organic products are healthier, safer and environmentally friendlier than the conventional counterparts. This perception is largely based on the prohibition of pesticides and synthetic fertilizers in organic plant production. These agricultural practice differences are expected to be reflected in the chemical composition of plants, which has lead to numerous comparative studies of organic and conventional plant products. However, the large diversity of organic and conventional production methods in combination with the natural variation caused by e.g. plant species, cultivar, soil type and climate, complicate direct comparisons of organic and conventional produce. So far, only a few systematic differences of organic and conventional plants, such as a lower content of pesticide residues in organic products, have been confirmed.
The objective of this PhD project was to compare the chemical composition of organic and conventional plant products based on selected atomic spectroscopy based techniques. It was hypothesized that the fertilizer management strategies in organic and conventional agriculture would cause systematic and measurable differences. The studies conducted were based on comprehensive and rigorously controlled long-term field trials comprising wheat, barley, faba bean and potato cultivated at three geographically different locations using one conventional and two organic management practices. All plant products were harvested in two succeeding years and their multi-elemental and isotopic composition was recorded. A novel method based on inductively coupled - plasma mass spectrometry (ICP-MS) was developed for semi-quantitative analysis of more than 70 elements of the periodic table. When applying this method to the field grown plants, chemical signatures of the geographical origin as well as the cultivation form were found. This information was only extractable by chemometric data analysis as no single element differed systematically between organic and conventional plants. Subsequently, the versatility was increased by inclusion of stable isotopes of the elements hydrogen, carbon, nitrogen, oxygen, magnesium and sulphur. This was conducted by isotope ratio - mass spectrometry (IRMS) and multicollector-ICP-MS (MC-ICP-MS). It was shown that isotope values of hydrogen (δ2H) and nitrogen (δ15N) are promising markers for organic plant production as δ2H values discriminated organic and conventional wheat and barley while δ15N values efficiently revealed use of animal manure in non-legumes. In addition, compound-specific isotope analysis of nitrogen and oxygen in nitrate (δ15NNO3- and δ18ONO3-) was used for the first time in plants, which allowed discrimination of organic and conventional potato even when organic crops relied on two different nutrient sources, viz. animal manure or green manures.
It was concluded that multi-elemental and isotopic analysis enabled discrimination of organic and conventional plants. However, discrimination was only feasible by elemental fingerprints when combined with chemometrics. In contrast, specific markers of organic plant products were found when focusing on stable isotopes. The analytical methods evaluated here have the potential to discriminate, and thereby authenticate, organic plant products. It is expected that the developed methods can support and improve the existing certification and inspection procedures of the organic sector. However, none of the developed techniques can ensure compliance with all regulations of organic plant production. It is accordingly recommended that multi-elemental and stable isotope analyses are combined with pesticide residue analysis. In addition, multi-elemental and stable isotope databases covering different plant species, geographical origins, fertilization strategies etc. should be generated to enable classification of plant products of unknown geographical and agricultural origin.