Abstract: The variation of δ15N of soil,tomato leaves and tomato fruit under different fertilizers were studied with pot experiments.8 treatments were laid out:C1 (compost,9.5g N),CU1 (compost,4.75g N;urea,4.75g N),U1 (urea,9.5g N),C2 (compost,19g N),CU2 (compost,9.5g N;urea,9.5g N),U2 (urea,19g N) and C3 (compost,29g N),CK (no fertilizer).The results showed that (1) the δ15N of soil,tomato leaves and tomato fruit increased with the application rates of compost,whereas decreased by urea.The ranges of δ15N of leaves and fruits sampled from the compost treatments were between 6.02‰~12.75‰ and 4.69‰~8.24‰ respectively,while the corresponding ranges of the urea treatments were between 2.83‰~5.53‰ and 2.66‰~4.50‰.(2) The ranking results of δ15N of different parts were old leaf> new leaf> new stem> fruit> old stem> lateral root> taproot,it indicates that nitrogen was gradually took into tomato plant with the process of absorption,transportation and distribution of nitrogen.(3) The threshold of δ15N of tomato fruit is 5‰ for compost treatments and urea treatments.

Key words: Isotopic fractionation, δ15N, Tomato, Organic food