Effect of organic and inorganic fertigation on yields, δ¹⁵N values, and δ¹³C values of tomato (Lycopersicon esculentum Mill. cv. Saturn)

Abstract

We examined the effects of fertilizer application, especially the effects of fertigation and types of fertilizer (inorganic and organic) on yields and δ¹⁵N and δ¹³C values of tomato (Lycopersicon esculentum Mill. cv. Saturn). Fertigation is a method in which an appropriate diluted liquid fertilizer is applied to the plants each time they are drip-irrigated. We developed a method of organic fertigation using corn steep liquor (CSL) as the liquid fertilizer, because it is an industrial byproduct of cornstarch manufacture and can be used very effectively. We compared fruit yield, mineral content, δ¹⁵N value, and δ¹³C value of tomatoes grown under three different fertilizer treatments, basal dressing: basal dressing with granular chemical fertilizer; inorganic fertigation: fertigation with liquid chemical fertilizer; and organic fertigation: fertigaion with CSL. Mineral contents of tomatoes grown with basal dressing were generally lower than those grown under either fertigation treatment. These results indicated that yields and mineral contents were influenced more by the method of fertilizer application than by whether the fertilizers were inorganic or organic. There were, however, significant differences in the δ¹⁵N values of tomato fruits grown under different types of fertilizer applications, especially between inorganic and organic fertilizers. The δ¹⁵N value of the chemical fertilizer used for basal dressing was 0.81 ± 0.45{‰}, that of the chemical fertilizer for fertigation was 0.00 ± 0.04{‰}, and that of CSL was 8.50 ± 0.71{‰}. The δ¹⁵N values of the soils reflected the δ¹⁵N values of the fertilizers. Moreover, the δ¹⁵N values of the fruits corresponded to the δ¹⁵N values of the applied fertilizers. The δ¹⁵N values were 3.18 ± 1.34{‰} in the fruits grown with a basal dressing of chemical fertilizer, 0.30 ± 0.61‰ in those grown under inorganic fertigation, and 7.09 ± 0.68‰ in those grown under organic fertigation. On the other hand, although the δ¹³C values in the soil also reflected the δ¹³C values of the applied fertilizers, there was no significant difference in the δ¹³C values of fruits among the different treatments. In conclusion, because the δ¹⁵N values of fertilizers correlated well with those of the fruits, it may be possible to use δ¹⁵N values as an indicator of organic products.