Abstract
There is a field in Fujimi, Saitama, Japan (unfertilized; UF) where an ordinary amount of crop is produced for marketing without applying any fertilizer or returning above-ground crop residues. We aimed to clarify the source of N in the leaves of tomato cultivated in this field, using the δ15N method. In an adjacent conventional single cropping tomato field (conventionally fertilized; CF), chemical fertilizer and compost (100 and 45 g N m-2, and δ15N of -1.7 and +9.3‰, respectively) had been applied as basal fertilizers. In UF, the δ15N values of soils were +7.2‰ in the 0 - 20 cm surface soil and +7.1‰ in the 20 - 35 cm deep subsoil; the corresponding values in CF were +8.5 and +7.5‰, respectively. Compared with those soils, δ15N values in tomato leaves were much lower, around +3‰ in both UF (+ 3.2 ± 0.4‰) and CF (+ 3.0 ± 1.0‰) fields. These values in tomato in CF, it may be explained by dilution with absorbed chemical fertilizers; but those in UF, without artificial input, cannot be explained by the absorption of the soil N, which has relatively high δ15N values. The results obtained from the UF field suggest that the field had received a substantial amount of N with relatively low δ15N values like atmospheric N from the outside of the field-plant system and that it allowed the ordinary crop production in the UF field.
