15N isotope techniques for estimating effects of urea-N fertilizer application rate on yields and nutrient contents of pakchoi cabbage and asparagus lettuce and nitrogen utilization efficiency

A pot experiment combined with 15N isotope techniques was conducted to evaluate effects of the varying rates of urea-N fertilizer application on yields, quality, and nitrogen use efficiency (NUE) of pakchoi cabbage (Brassica chinensis L.) and asparagus lettuce (Lactuca saiva L.). 15N-labbled urea (5.35 15N atom %) was added to pots with 6.5 kg soil of 0.14, 0.18, 0.21, 0.25, and 0.29 g N kg-1 soil, and applied in two splits: 60 percent as basal dressing in the mixture and 40 percent as topdressing. The fresh yields of two vegetable species increased with the increasing input of urea-N, but there was a significant quadratic relationship between the dose of urea-N fertilizer application and the fresh yields. When the dosage of urea-N fertilizer reached a certain value, nitrate readily accumulated in the two kinds of plants due to the decrease in NR activity; furthermore, there was a linear negative correlation between nitrate content and NR activity. With the increasing input of urea-N, ascorbic acid and soluble sugar initially increased, declined after a while, and crude fiber rapidly decreased too. Total absorbed N (TAN), N derived from fertilizer (Ndff), and N derived from soil (Ndfs) increased, and the ratio of Ndff and TAN also increased, but the ratio of Ndfs and TAN as well as NUE of urea-N fertilizer decreased with the increasing input of urea-N. These results suggested that the increasing application of labeled N fertilizer led to the increase in unlabeled N (namely, Ndfs) presumably due to “added nitrogen interaction” (ANI), the decease in NUE of urea-N fertilizer may be due to excess fertilization beyond the levels of plant requirements and the ANI, and the decrease in the two vegetable yields with the increasing addition of urea-N possibly because the excess accumulation of nitrate reached a toxic level.