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Verifying the authenticity of organically grown crops using stable nitrogen isotope analysis.
Project Code: Q01076
University of East Anglia
Bateman, A ;
Institute of Food Research Enterprises Ltd
The aim of this project was to develop analytical methods based on stable isotope and multi-element analysis that may make it possible to identify conventionally grown produce fraudulently mislabelled as “organic”.
Proof of concept greenhouse experiments showed that fertiliser type is an important factor influencing the nitrogen isotope composition of the crop under cultivation. Under controlled conditions, applications of synthetic nitrogen fertilisers (commonly used in conventional agriculture) resulted in crops with lower factors, which may affect crop nitrogen isotope composition, were kept the same. For example, for equivalent amounts of nitrogen applied, carrots grown with ammonium nitrate fertiliser had N values between 3‰ and 4‰ lower than those grown using a manure-based fertiliser.
Baseline survey data (in which tomatoes, lettuces, carrots and mushrooms were collected directly from organic and conventional producers in the UK and EU in order to assemble datasets of authentic samples) suggest that fertiliser d15N is influential over crop N even when other factors that may influence crop N are variable, e.g. soil type, weather conditions, type/amount/timing of fertiliser application, the use of legumes etc. Mean N values for conventionally grown tomatoes, lettuces and mushrooms were lower by 8.2‰, 4.7‰ and 4.9‰ respectively than the mean 15N values for the corresponding organically grown crops. The mean value for conventionally grown carrots was not significantly different from the mean value for organic carrots.
The results from the baseline survey clearly show that analysis of crop 15 N is a better indicator of whether crops have been grown with/without synthetic fertilisers for some crops types than others. Crops with high nitrogen requirements, crops grown under protected conditions and particularly crops grown in hydroponic conditions are much more likely to exhibit larger differences in nitrogen isotope values due to the type of fertiliser applied than field-grown crops with relatively low nitrogen requirements.
For all crop types analysed there is overlap between the 15N values of the organically and conventionally grown crops. Of the crop types analysed, 15N is most successful at discriminating between organically and conventionally grown tomatoes. This is likely to be because a large proportion of conventional tomatoes are hydroponically grown and continuously fed nutrient solutions containing dissolved chemically synthetised nitrogen. Under these conditions, many of the factors which might otherwise be influential in determining crop 15N (such as soil type etc.) do not have an effect.
Canonical Discriminant Analysis (CDA), used for statistical analysis, correctly placed tomato samples (treated as an unknowns) into their appropriate group (organic/conventional) in 95.3% of cases based on their nitrogen isotope composition (n=107). Including the multi-element analysis in the CDA resulted in 100% correct classification of all the tomato samples (for the Year 1 collected samples only, n=51).
For lettuces, the CDA was able to correctly assign ~74% of the lettuce samples as organic/conventional when they were treated as ‘unknowns’ based on their nitrogen isotope composition. Including multi-element and anion (nitrate, phosphate, sulphate) data did not appreciably improve the percentage of samples correctly classified. An experiment investigating the possible influence of the use of leguminous plants on the 15N values of subsequently grown crops suggests that the use of clover (and other leguminous crops) by organic growers to increase the nitrogen fertility of soils does complicate the use of nitrogen isotopes to determine whether crops have been grown with or without the use of synthetic nitrogen fertilisers. The experiment indicates that when clover is used in conjunction with manure, this results in lettuces with nitrogen isotope signatures that are isotopically distinct from those grown where ammonium nitrate is used as a supplementary nitrogen source. However, lettuces grown using legumes only for soil nitrogen fertility, were not isotopically distinct from lettuces grown using ammonium nitrate fertiliser. Restrictions on the use of synthetic nitrogen fertilisers are only one of a range of conditions that must be met in order to satisfy requirements for organic food production. Additional obligatory conditions are a period of conversion, registration with an appropriate certifying authority and restrictions on the use of pesticides and other soil improvers and conditioners. More broadly, organic farming includes the principles of i) minimising reliance on external inputs by, for example, building soil fertility using crop rotations that include green manures, ii) maintaining genetic diversity in agricultural systems, iii) protecting wildlife habitats, and iv) seeking to avoid environmental pollution as a result of farming practices. It is therefore important to emphasise that even if crop 15N can be used to distinguish satisfactorily between crops that have been grown with and without the application of synthetic nitrogen fertiliser, this is not the same as being able to distinguish between conventionally and organically grown crops because of the additional statutory requirements that apply to organic farming. In summary, if synthetic nitrogen fertilisers have been applied to a crop then the crop cannot be described as organic. However, if synthetic fertilisers have not been applied to a crop, it does not follow that the crop can be described as organic since the crop may not have been grown in conditions which comply with all the other requirements of organic cultivation. The 15N technique has the potential to detect produce grown using synthetic fertilisers that is fraudulently on sale as organic but not to verify authenticity.
Nitrogen isotope analysis of certain off-the-shelf crops could provide useful ‘intelligence’ as to whether synthetic nitrogen fertilisers have been applied to certain crop types. The observed overlap between the 15N values of organically and conventionally grown crops means that 15N analysis of a crop and reference against the authentic organic and conventional databases would not allow an assertion to be made that a crop has definitely been grown using synthetic fertilisers. The type of statement that could be made would be, “it is unlikely highly unlikely that a tomato with this nitrogen isotope composition would have been grown without the use of synthetic nitrogen fertilisers”.
Multi-element and anion analysis has identified statistically significant differences between the mean values for some of the measured anions and elements and possible explanations have been put forward for the observed differences. However, the datasets analysed are not large and analysis of further samples, after the development of hypotheses for the expected differences between organic and conventional samples, is essential
Central to the nitrogen isotope approach is that a valid appraisal of off-the-shelf products is dependent on having a database of authentic samples for comparison. Simpkins and Harrison (1995)comment that, “Trade in foodstuffs is global so there is a need for data on authentic products from many different parts of the world. The exchange of reliable database information should increase…”. Similarly, Rossman (2001) asserts that, “the establishment of a relevant database for statistical evaluation…should take into considerations important factors, e.g. regional origin, plant species, variety, season and climate”. From this perspective there are certainly some limitations in the work presented here since the organic datasets are dominated by samples of UK origin with a few EU samples. Rossman 2 goes on to say that, “[a] second prerequisite is that a sufficient number of laboratories are able to apply the method with good accuracy and precision”. This is one of the strengths of the nitrogen isotope approach. Preparation of samples and the nitrogen isotope analysis is relatively straightforward and inexpensive, suitable for daily routine processing with a high throughput of samples. Accuracy and precision of samples analysed for their nitrogen isotope composition is routinely monitored in stable isotope laboratories using widely available International Atomic Energy Agency (IAEA) standards. This means that extending the authentic databases to include samples from a wider geographical spread would be relatively straightforward and is likely to occur over the next few years with interest in this type of approach from research groups in Germany, France, New Zealand, Japan and Korea.
Ideally, an end-product test such as the nitrogen isotope approach would always correctly determine whether a crop has been grown with or without the use of synthetic fertilisers. However, the consequences of such a test incorrectly assigning a sample of unknown origin must be carefully taken into account. There are two possible ways in which an unknown sample maybe mis-assigned. Firstly, there is the situation in which a test indicates that synthetic fertiliser has not been used on a crop being sold as organic when in reality, it has. This is a ‘false negative’ – in this instance the test does not flag up fraud when in fact it is taking place. With a test balanced in favour of false negatives, there is the danger of unwarranted complacency that there is no fraud taking place. More serious is the issue of ‘false positives’. In this situation, the test indicates that a synthetic fertiliser has been used when in fact it hasn’t. This interpretation could lead to false accusations against legitimate producers, wholesalers or packing houses with the potential to undermine trust in the organic sector if such claims were openly made (even if subsequently retracted).
We strongly advocate that end-product tests such as the nitrogen isotope approach cannot and should not be thought of as a replacement to organic certification and inspection schemes and that interpretation of analyses should be made carefully and sensitively.
End-product testing is controversial. The UK Soil Association’s point of view is that, “Most organic standards, both nationally and internationally, are based on a definition of the production process rather than the end product…Any shift towards monitoring the product rather than the process would undermine the integrity of the farming system” (Soil Association, 1998). However, the position of the Organic Trade Association (a business association for the organic industry in the USA) is that, “…residue testing has a place in organic certification. It is a valuable tool for detecting fraud; a deterrent for farmers who might be tempted to commit fraud…[and]…a means to assure those who purchase organic food...”. Although the OTA’s comments relate specifically to pesticide residue testing, the contrasting comments of the Soil Association and the OTA illustrate the controversy that surrounds this kind of end-product testing.
However controversial this type of testing may be, there is a clear and undeniable incentive for dishonest traders to describe conventional produce as organic for financial gain. This has been highlighted recently by a number of high profile prosecutions and investigations 3. Consequently, the existence and careful use of end product testing to provide intelligence, in conjunction with paper traceability investigations, can have an important role as a disincentive to dishonest traders, in an industry that relies heavily on consumer’s trust of the integrity of the organic system.
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