Effects of nitrogen application level and sand mulching on 15N-urea absorption, allocation and utilization in apple trees

Abstract: In order to explore the effect of groundcover management and nitrogen application level on 15N-urea absorption, allocation and utilization of apple trees in Loess Plateau, and establish scientific basis for realizing reasonable nitrogen application, enhance utilization of nitrogen in semi-arid area, this research conducted a series of field experiments at apple breeding base in Jingning County, Gausu Province. Three-year-old 'Fuji' apple trees were used as materials. A split plot design was applied for this experiment, the main factors included conventional tillage (CK) and sand mulching (SM), and the sub-factors with two different nitrogen application levels of 5g 15N-urea (N1) and 5g 15N-urea + 75.5g urea (N2). Using 15N isotope tracing technique, the 15N abundance and total nitrogen of different plant organs were tested during June (fruit enlargement), August (new shoot stop growing), and October (before defoliation). The results indicated that groundcover management and nitrogen application levels improved the aboveground and underground biomass of apple trees to different extents, and nitrogen application was beneficial to the accumulation of aboveground and underground biomass under sand mulching condition in the late growth stage of apple tree. Groundcover management could enhance biomass accumulation of apple tree. Higher nitrogen application level (CKN2) could effectively increase the dry weight of root under conventional tillage, but the root growth rate of SMN1 was very rapidly before defoliation, and there was no significant difference between SMN1 and CKN2 (P>0.05). The Ndff (percentage of nitrogen content of every organisms come from fertilizer nitrogen) of fruit, shoot, perennial branch, central trunk and coarse root were significantly affected by groundcover management and nitrogen application levels (P<0.05), and the effect of their interactions significantly affected the Ndff of leaf, fruit, perennial branch, and fine root (P<0.05). The accumulation effect of two factors interaction on Ndff of fruit was greater than perennial branch. In June and August, the Ndff of perennial branch and fine root which was the highest under sand mulching (SMN1), and there were 2.26%, 3.21% and 3.67%, 5.89%, respectively. The effect of two factors and their interactions extremely significantly affected the 15N allocation rate of fruit (P<0.01), and partially behaved significant (P<0.05) or extremely significant effects (P<0.01) on other organs. The storage organ was the largest 15N utilization organ of apple tree, followed by vegetative organ and reproductive organ. During the whole growth stage, the utilization rate of 15N was 3.38%-38.00%, which showed that sand mulching was greater than conventional tillage in groundcover management, and except October, high nitrogen application level was greater than low nitrogen level. Groundcover management significantly increased 15N the utilization rate of apple trees (P<0.05). However, the effect of increasing nitrogen application level on 5N the utilization rate was not significant in most cases (P>0.05). After comprehensive analysis, the low nitrogen application level (N1) and effective groundcover management (SM) under the experimental conditions which could not only promote the accumulation of shoot and total biomass of young trees, but also improve the efficiency of nitrogen utilization, thus optimizing the input of nitrogen fertilizer in agricultural ecosystems.