3.4. Effects of additional N fertilization on plant d15N
variations
Additional fertilization with isotopically different N
sources caused significant alteration of plant d15N, but
not when isotopically similar additional N sources were
applied (Fig. 1). For plants receiving compost as basal N
input, additional urea application (C2/U1) decreased plant
d15N from +10.7 at 40 DAT to +3.4% at 60 DAT
(LSD ¼ 1.4), and the magnitude of decrease was much
greater than that from 20 to 40 DAT; however, additional
compost application (C2/C2) did not change plant-d15N. In
contrast, for plants receiving urea as basal N input, plant
d15N was significantly increased by additional compost
application (U1/C2) from 1.0 at 40 DAT to +2.7% at 60
DAT (LSD ¼ 2.3), but changed little with further addition
of urea (U1/U1). This phenomenon was due mainly to
isotopic signatures of additional N applied, and the extent
of influence caused by isotopic composition of additional
N sources was further modified by the availability of each
N source to cabbage.