Evaluating the effects of fertilization treatments on the nitrogen use efficiency of apple trees using allometric model and 15N tracer

Nitrogen is one of the most essential mineral elements for plant growth. A large amount of nitrogen fertilizer has widely been introduced into the farmland soil every year, to ensure the supply of agricultural products in China. However, the apple Nitrogen Use Efficiency (NUE) is still at a lower level, compared with the developed countries. It is necessary to improve NUE via scientific fertilizer technology, such as the little amount of nitrogen fertilizer, or organic substitution for synthetic fertilizer. Alternatively, the 15N tracer technology can be used to accurately predict the whereabouts of nitrogen in plants or soil, thereby effectively evaluating the NUE of plants. Since 15N tracer has been used in the NUE of apple trees, the input cost is relatively high for the difficulty in field micro-zone control. Thus, it is highly demanding to seek a direct and feasible way for evaluating the NUE of apple trees. In this study, a allometric model was applied to estimate the biomass of fruit trees combining with the nitrogen content of each plant part, thereby assessing the NUE of apple trees. A simpler yet more effective approach was proposed to evaluate the efficiency after comparing the allometric model and the 15N tracer technique. Five-year-old ‘Yanchanghong’ apple trees (selected from ‘Nagafu 2’) were used as research materials. An allometric model and 15N tracer technology were selected to carry out experiments. Four fertilizer treatments were carried out: no nitrogen fertilizer (CK); conventional high nitrogen (N800); optimized nitrogen reduction (N400); and the combined application of organic and inorganic fertilizer (N200+O200). An analysis was made on the plant biomass, absorption, and utilization of nitrogen, as well as the distribution characteristics of nitrogen in different parts of apple trees. The results showed that: There was no significant difference in the apple yield under different fertilization treatments (31.7-37.3 t/hm2). The current season biomass increments of annual branches, perennial branches, leaves, boles, and roots were 0.26-0.38, 1.56-2.12, 0.22-0.24, 1.52-1.68, and 1.30-1.45 kg/plant, respectively. The replacement strategy of organic fertilizer significantly increased the biomass of annual branches and perennial branches, but there was no significant effect on other organs. The NUE values of apple trees were 13.13%-31.94% and 11.64%-32.40%, respectively, after evaluation using the allometric model and 15N tracer. Specifically, the NUE values evaluated by allometric model and tracer increased by 143.26% and 178.35% under N200+O200 treatment, compared with N800 treatment. A reasonable combined application of organic and inorganic fertilizers effectively improved the utilization of nitrogen fertilizers in apple trees, indicating an obvious reduction in chemical fertilizer with high efficiency. Nevertheless, the varying fertilization treatments presented no significant effects on the nitrogen distribution of various plant parts, where 60.38%-61.02% (allometric model) and 67.71%-73.29% (15N tracer) of nitrogen accumulated in the storage organs of fruit trees. Likewise, there were significant differences in the storage organs of fruit trees, the NUE of fruits and leaves, as well as the nitrogen distribution rates of organs, whereas, the overall NUE of the apple tree was similar when using the allometric model and 15N tracer individually. The root mean square error of NUE was 3.10% in the combined, indicating an average difference of 3.10% in two assessments of NUE. Therefore, it is a convenient and feasible way to estimate the NUE of apple trees using the allometric model. The finding can provide promising data support for future research on the nitrogen utilization of apple trees.