Effect of different horizontal fertilizer placements on the characteristics of absorption, distribution and utilization of ¹⁵N by Gala/Malus Hupehensi

【Objectives】High nitrogen dosage and low use efficiency become one of important reasons of restricting sustainable development of apple industry in China. Unclear fertilizer placement is one of the main reasons of low fertilizer utilization rate in the production. Therefore, an experiment was carried out to figure out reasonable fertilizer placement, improve fertilizer utilization ratio and reduce waste by studying differences of different level fertilization effect. 【Methods】The experiment was carried out to study the effects of different fertilizer placements on the characteristics of notrogen absorption, distribution and utilization of fifteen year old apple trees (Gala/Malus hupehensis) using 15N urea tracer technique. According to the scope of canopy projection, we set three different fertilizer placements, including 1/3 of canopy projection, referred to as inner fertilizer treatment (I), 2/3 of canopy projection (middle fertilizer treatment, M) and about 20 cm inside to the edge of canopy projection (outer fertilizer treatment, O). The fertilizing method was digging circular furrows on the three placements we had set and backfilling soil after the fertilization. The depth of the furrows was about 25 cm. Samples at the key phonological phases of apple trees were collected and separated at the end of the experiment. 【Results】The absorbed 15N by roots transfers to new vegetative organs prior at the new shoot growing stage and fruit rapid-swelling stage under the different horizontal fertilization placements. The Ndff values of fruits are the highest among the organs at the fruit maturity stage under the three fertilization treatments. The Ndff values of the same organ are different under different fertilization treatments, and the Ndff values of the organs under the inner treatment are significant higher than those under the outer and middle fertilization treatments, especially for roots and fruits. The Ndff values of roots under the three treatments show different variation tendency as the phonological phase changing, and these of the inner fertilization are greater than those of the other treatments. The Ndff values of roots under the M and O treatments present a trend of upward after a fall first. But in the treatment I, the Ndff value of root is increasing until reaches the highest level at the fruit maturity stage. The Ndff values of fruits under the three treatments all present an increasing tendency as the phonological phase changing and reach the maxima at the fruit maturity stage, and the Ndff values of fruits in M and I are respectively 1.43 and 1.42 times of that under the treatment O. The result of the Ndff values of fruit at the new shoot growing stage and fruit rapid-swelling stage is IOM. The 15N distributions in the organs at different phonological phases are significantly different and are not significantly different among different treatments. At the fruit maturity stage, the trend of the nitrogen distribution in different organs under different fertilization treatments is consistent, storage organs＞vegetative organs＞reproductive organs, and the values of NUE under different treatments are I＞M＞O. The NUE of trees under the I treatment is 29.25%, which is significantly higher than those under the other two treatments (19.33% and 19.04%). 【Conclusions】Fertilization placement significantly affects plant organs' ability of absorbing fertilizer, especially roots. Different fertilizer placements show little effects on the nitrogen allocation rate but obviously affect the nitrogen utilization efficiency. Combining previous research results and the results of this experiment, it suggests that for ridging planted orchard, fertilizing at 1/3 of the canopy projection could improve the utilization rate of nitrogen fertilizer.