Tomatoes (Solanum lycopersicum) are heavy nutrient feeding crops and require high amounts of nitrogen to maximize fruit production. The type of nitrogen applied and timing of fertilizer applications are important to reduce losses due to volatilization and leaching. Previous research suggested that nitrogen stable isotopes are a useful fingerprinting system for indicating if a crop has been grown with synthetic or organic nitrogen applications. To study the effects of fertilization systems on nitrogen isotopic patterns, "Better Bush" tomatoes were grown in a 2 year greenhouse experiment to analyze nitrogen isotopic composition, nitrogen content, and fruit yield. Three main soil fertility treatments were evaluated, and the results were compared to those obtained on plants grown in unfertilized soil: conventional inorganic (synthetic Miracle Grow (MG)), organic (bonemeal and bloodmeal (BB), BB with liquid Earth Juice (BBL), BB with 25% vermicompost (VC), BBL with 25% VC, and 25% VC), and mixed (MG with 25% VC). The soil fertilizers, treated and untreated soil, immature and mature leaflets tomato fruit peels, and fruit juices were analyzed for both nitrogen isotope ratios and nitrogen concentrations. Plant δ¹⁵N_air decreased in the order organic treatment-no fertilizer-mixed treatment-conventional treatment. The average δ¹⁵N_air values in leaves, fruit peels, and juice from plants grown with organic treatments ranged from 4.5 to 11.9, 5.4 to 10.1, and 6.1 to 11.1‰, respectively, whereas in the case of the inorganic treatment, the average δ¹⁵N_air values varied between -3.0 and 0.4, -1.1 and 0.4, and -0.9 and 1.9‰, respectively. Plant nitrogen concentrations in tomato decreased in the following order (from highest to lowest): inorganic soil fertility treatment, mixed treatments, and organic and control (no fertilizer) treatment. The average weight %N values in leaves and fruit peels from plants grown with organic treatments ranged from 1.3 to 4.2 and 1.1 to 2.3%, respectively, whereas in the case of the inorganic treatment, the average weight %N values varied between 3.7 and 5 and 1.3 and 2.8%, respectively. Plants grown under organic treatments have higher δ¹⁵N_air, lower weight %N, and are enriched in ¹⁵N compared with the original soil than plants grown with inorganic fertilizer, suggesting that the synthetic nitrogen sources are more readily available for plant uptake than the organic ones. The addition of vermicompost increases both δ¹⁵N_air and weight %N in plants. Tomato fruit yields did not differ between cluster 1 and cluster 2 harvest, however, total tomato fruit yields differed indicating that synthetically fertilized plants produced the highest total yields (g) (P ≤ 0.05). However, all treatments with VC soil applications indicated an increase in the amount of plant nitrogen, fruit yield, soil cation exchange capacity, soil organic matter content, and released soil nitrogen. Nitrogen isotope ratios of tomatoes can be used to distinguish among various soil fertility treatments, therefore fingerprinting the organic fertilizer applications.

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