Nitrogen is the element that most often limits plant growth in many terrestrial ecosystems. Anthropogenic activity in the form of fertilization has altered the amount and relative abundance of the forms of nitrogen that are available for crops adsorption. The bulk δN15-value of crop biomass is determined by that of the inorganic primary nitrogen sources such as NO3, NH4 and N2 as well as by physiological factors, such as different nitrogen uptake mechanisms, different pathways of assimilation, and recycling of nitrogen in the plant.
The objective of this study was to evaluate the hypothesis that measurements of stable nitrogen isotope can be used in authentication of agricultural products from conventional or organic production. Numerous samples of commercially produced organic and conventionally grown agricultural produce supplied to the markets in British Columbia, Canada have been collected and analyzed for their δ15N composition in order to create a statistically significant datasets which allow establishing the links between stable nitrogen isotope ratio and the method of crop cultivation.
The datasets of stable isotope composition δ15N and C:N content of organic and synthetic nitrogen fertilizers were assembled to be used as a tool in authentication of organic fertilizers available on the North American market.
Methods & Instrumentations
Samples of fertilizers for nitrogen isotope analysis were analyzed without pre-treatment. All liquid samples were vigorously shaken to achieve homogeneity and an aliquot of 0.5 – 2.5 mg of liquid fertilizer was transferred into a tin capsule. The amount of sample or standard analyzed in the procedure was based on the criteria that the range of N must be within 0.02 to 0.2 mg. Each sample capsule contained about 0.5 mg of Chemosorb for retaining volatile ingredients of the sample. All produce samples were freeze-dried first and pre-weighted in a tin capsule. Nitrogen isotope composition was determined using thermal combustion elemental analyzer Costech ECS 4010 from Costech Analytical Instruments Inc. (Valencia, CA) coupled with continuous flow isotope ratio mass spectrometer (EA-IRMS) Thermo Finnigan DeltaPLUS Advantage from ThermoFinnigan Inc. (Bremen, Germany).
Each batch of samples included QA/QC samples: three “working standards” analyzed before and after each batch of samples, a sample duplicate, and a procedural blank. The N-content of fertilizers was determined based on the EA-IRMS analysis of acetanilide used as a calibration standard. Nitrogen isotope data are reported in conventional d-notation in units of per mil (0/00) with respect to atmospheric nitrogen (air) according to equation: δ15Nsample (0/00) = (Rsample – Rstandard) / Rstandard x 1000 where, R = 15N/14N and the standard is atmospheric nitrogen with a 15N/14N ratio of 0.00368 and a δ15N value of 0 0/00. Fertilizer samples were referenced against international reference standards: IAEA-N1 (ammonium sulphate reference material certified by the International Atomic Energy Agency) with a δ15N value of 0.40 0/00, IAEA-N2 with a δ15N value of 20.30 0/00, Sucrose ANU with a δ15N value of -10.47 0/00 and NBS-22 OIL with a δ15N value of -29.73 0/00. Long-term performance of the mass spectrometer was monitored by analysis of secondary reference material in every batch: acetanilide with a C and N contents of 71.09 and 10.36 %, respectively, dorm with a δ15N value of 14.33 0/00, and caffeine with a δ15N value of -0.95 0/00. The long-term standard deviation of the values obtained from measurements of the secondary laboratory standards was 0.20 0/00.
Results & Discussion
The assembled data sets show that the differences between commonly used practices in organic and conventional agricultural systems result in distinguished stable nitrogen isotopic signatures for such produce as broccoli, buttercup, celery, cabbage (green and red), carrot, green onion, red onion, red pepper, spinach, tomatoes and zucchini. The range of δ15N -values obtained on organically grown produce mentioned above was from 4.10 to 25.95 0/00, whereas corresponding products from conventional cultivation had δ15N -values from -1.16 to 2.28 0/00.
The group of produce such as lettuce, lime and oranges from organic field production has though distinguished but widely overlapping ranges of δ15N -values with that of corresponding conventionally grown products: from 2.63 to 15.68 0/00 and from -2.68 to 5.240/00 , respectively. Organically grown avocado, banana, garlic, yellow onion, pineapple and strawberries did not show significant difference in nitrogen stable isotope composition from that of conventionally grown corresponding products. The datasets of stable isotope composition, δ15N and C:N content of organic and conventional nitrogen fertilizers have been also assembled in the study to be used as a tool in authentication process of fertilizers. The measurements of δ15N-values and C:N content were performed on organic and synthetic nitrogen fertilizers available on the North American market.
The δ15N values in the compiled dataset obtained on the synthetic fertilizers or a mix of thereof with organic one fall in the range between -5.9 and 2.78 0/00 ,Table 1. The fertilizers sold on the market as organic revealed a broader range in δ15N values ranging from 4.5 to 26.7 0/00. This group included manures, composts, bloodmeal, bonemeal, hoof and horn, fishmeal. Seaweed based fertilizers, considered as organic, have a range of δ15N values between 0.60 and 5.42 0/00. The developed datasets show that the ratio of the C and N contents in a fertilizer can be of help in the verification process. The compiled data of analyzed fertilizers exhibit that C:N content ratios of organic fertilizers fall in the range between 3:1 to 12:1, while for synthetic or a mix of both the range is from 0 to 3:1. Nitrogen stable isotope analysis of agricultural produce, as well as the isotopic and C:N content characterization of fertilizers used for their cultivation demonstrated a good potential in the authentication of both groups of products.
Figure 1. Dataset of δN15 of organic and conventionally grown produce obtained in the study.
Figure 2. δN15 ranges of synthetic and organic fertilizers analyzed in the study.