Effects of nitrogen fertilization on characteristics of distributionand utilization of ¹⁵N and¹³C of Gala seedlings

【Objectives】 Nitrogen fertilizer has stimulative effect on fruit tree growth, but the effect is different. Studying the allocation of nitrogen nutrition and photosynthetic products has important significance on guiding rational use of nitrogen fertilizer. This experiment studied characteristics of distribution and utilization of15N and13C in potted Gala seedlings under different nitrogen levels in order to provide the basis for rational fertilization.【Methods】 Two-year Gala(Malus domesticca)/Malus robusta(Malus micromalus Makino) seedlings were treated with three nitrogen levels: low, middle and high nitrogen, and 0.3 g(15NH2)2CO was used evenly as a marker. The13CO2 was used as a gas marker at the early vigorous growing period, vigorous growing period and slow growth period of shoots. After 72 hours, the whole plants were divided into leaves, shoots and roots for analysis. The total nitrogen content was detected by the micro-kjeldahl method. The15N abundance was detected by ZHT-03 mass spectrometer. The13C abundance was detected by DELTA V Advantage isotope ratio mass spectrometer. 【Results】 The middle and high nitrogen levels improve the dry matter accumulation of leaves, stems and the whole plant in different degrees. There are no significant differences in plant dry biomasses between the middle and high nitrogen treatments at the early vigorous growing period and slow growth period of shoots, nor the dry biomass of leaves between the middle and high nitrogen treatments at the vigorous growing period. After the vigorous growing period, the shoot lengths are in order of middle highlow N treament, and the differences among them are quite remarkable. The middle nitrogen level is beneficial to shoot growth. Under low nitrogen treatment in early vigorous growing period, as high as 50% of the15N is distributed in leaves, which is 13 percentage points higher than those of the middle and high N treatments, indicating the less transform of nitrogen from leaves to other organs in low nitrogen treatment. The ratios of the15N distributed in leaves are not statistically significant between the middle and high N treatments, so the middle nitrogen supply can meet the demand of plant to nitrogen in this study. The13C assimilation capacity of the middle nitrogen treatment is the highest at the vigorous growing period and slow growth period of shoots. The order of the13C distribution ratios in roots under different N levels is middlehigh low at the early vigorous growing period, which shows that the middle level of nitrogen treatment is beneficial to carbon assimilate distribution. The15N utilization rates decrease with the nitrogen level increase, and the high nitrogen treatment does not significantly affect the distribution of assimilated carbon.【Conclusions】 Different nitrogen levels have no obviously impact on the distribution of assimilated carbon. The middle level of nitrogen fertilization can satisfy the N nutrition requirement, and benefit shoots growth and tree vigour.