Nitrogen- and Carbon-stable Isotopes in Organic and Conventional Fertilized Greenhouse Tomatoes

Tomatoes require high amounts of nitrogen to maximize productivity. The type of nitrogen and timing of fertilizer applications are important in tomato production systems to reduce nitrogen losses while optimizing yields. Thus, a greenhouse study was conducted in 2014 to determine the effects of various organic and inorganic nitrogen fertilization treatments on nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotopes found in tomato plant leaves and fruit at two stages of plant growth. ‘Better Boy’ tomatoes were grown in 8-L plastic pots filled with 1:1:1 steamed-sterilized sand:silt loam soil:peat mix. Eight fertility treatments were set up in a randomized complete-block design using 3 replications: no fertility, synthetic Miracle Grow® (MG), Organic bonemeal and bloodmeal (BB), BB with liquid Earthjuice (BBL), BB with 25% vermicompost (VC),  BBL with 25% VC, MG with 25% VC, and no fertility with 25% VC.  The results indicated that for both growth stages, δ¹⁵N differed (P ≤ 0.05) between fertility treatments, while no differences were observed for δ¹³C.  The δ¹⁵N in the tip leafs from three different branches, fruit skins from the first two fruit clusters, and soil samples indicated that the organic treatments with VC had significantly higher δ¹⁵N than the synthetic fertilizer or no added fertility treatments.  This project indicated that greater amounts of δ¹⁵N were found in tomato plants that had VC applications, and this form of nitrogen is thought to provide better fertility management since it is associated with less soil leaching and volatilization.  Also, that by using nitrogen isotopes, organic, and conventional fertilizers can be distinguished in tomato foliage and fruit that are grown in a controlled environment.