Interactive effects of N fertilizer source and timing of fertilization leave specific N isotopic signatures in Chinese cabbage and soil

Abstract

Natural 15N abundances ( δ 15N) in plant and soil can be used as a powerful marker to reveal the history of N fertilization. To investigate whether N fertilizer source and timing of fertilization leave specific δ 15N signals in plant tissue and soil inorganic N, Chinese cabbage ( Brassica campestris L. cv. Maeryok), one of the most popular vegetables in Asia, was grown in pots for 60 days with a single or split N applications of organic (composted manure; δ 15N=+16.4‰) or inorganic N (urea; δ 15N=−0.7‰). Seven N treatments were studied: (1) a single basal fertilization with compost or (2) urea; (3) a basal urea application followed by an additional (at 40 days after transplant, same below) compost or (4) urea application; (5) a basal compost application followed by an additional compost or (6) urea application; and (7) no N fertilization. Regardless of the time of N application, δ 15N of cabbage treated with compost was higher (>+9.0‰) than that (< +1.0‰) treated with urea, reflecting the effect of isotopically different N sources. In split N fertilization, only the addition of isotopically different N sources in the middle of the growth period significantly affected the δ 15N of the whole plant. Specific δ 15N signals of basal N inputs were detected in outer cabbage parts formed in the early growth stage, while those of additional N inputs were detected in inner cabbage parts formed in the latter growth stage. We conclude that measurements of temporal variations in δ 15N of plant parts formed in different growth stages could reveal the history of N fertilization.