Effect of Chemical Fertilizer and Compost on Natural N-15 Abundance of Tomato Leaves and Fruits

Organic farming is a good way to reduce environmental pollution. However, no related detection of fertilizer applications, which are extremely important in organic farming, exists. The δ15N value of chemical fertilizer (–3.8 to +2‰) is far lower than compost (>+10‰). Thus, natural 15N abundance (δ15N) in leaves and fruits may be used to reveal the application of N fertilizer. In this experiment, six treatments were laid out: 635 g (C1), 952 g (C2), and 1270 g (C3) of compost were applied as basal fertilizer in each pot. Alternately, 10.7 g (U1), 16 g (U2), and 21.3 g (U3) of urea were applied as basal fertilizer and 10.7 g (U1), 16 g (U2), and 21.3 g (U3) urea were applied as top-dressing, respectively, in each pot. C1 and U1, C2 and U2, and C3 and U3 had equal amounts of nitrogen fertilization. In all three fertilizer levels, the leaves and fruits sampled from the U group had significantly lower values than those sampled from the C group within the same period. The δ15N values of the leaves and fruits from the C group decreased slowly. The δ15N values of leaves sampled at 40 days after transplanting were fairly low. The δ15N values of leaves and fruits from the U group increased initially, due to the small amounts of urea left, which then led the plants to absorb N from the culture substrates. However, the δ15N values of leaves and fruits from the U group decreased rapidly after the top-dressing was applied, where the minimum values occurred during the final stage.