Magnesium Influence on 13C and 15N in Apple

Thirty apple trees with the same crop loads (5.5–6.2 fruit per cm² cross-sectional area of fruiting branch), developmental attributes, and number of autumn branches were selected to reduce the individual differences in the sources of photoassimilates in the fruits. In the experiment, there were five treatments, namely, CK, Mg₅₀, Mg₁₀₀, Mg₁₅₀, and Mg₂₀₀, which represented 0, 50, 100, 150, and 200 kg/ha of pure Mg application (equal to 0, 250, 500, 750, and 1,000 kg/ha of MgSO₄), respectively. The date of flowering was 25th April, and the date of fruit maturity stage was 20th October.
To exclude the natural abundance of ¹³C and ¹⁵N in the apple trees affected by different Mg treatments, each treatment was divided into two groups of three replicates, the first group for isotope labeling and the second for the determination of natural abundance and other indicators. Fertilization treatment was conducted on 1st August (95 days after blooming) in 2018 and 2019. Each tree of group 1 was supplied with 100 g of common urea and 20 g of ¹⁵N-urea (abundance of 10.28%), each tree of group 2 was supplied with 120 g of common urea (200 kg/ha of pure N), and a circular ditch with a depth and width of 20 cm was dug 40 cm away from the center trunk when fertilizing. MgSO₄, as the only Mg source, mixed with urea/(¹⁵N-urea) was evenly watered in the circular ditch after being dissolved in water. All apple trees were subjected to destructive sampling at the 20th October (180 days after blooming, the period of fruit maturity stage).