A long-term fertilizer experiment has been conducted since 1973 in the apple ‘Jonathan’ orchard in Fukushima prefecture, Japan. The soil type in the orchard is a brown forest soil. This fertilizer experiment is equipped with a 0N plot (0 kg N 10 a−1) and a 4N plot (20 kg N 10 a−1 as annual application of ammonium nitrate). In this field of study, we evaluated the effects of long-term nitrogen (N) fertilization on the changes in the yield, fruit quality, and mineral contents of apple trees, and soil chemical properties using previous (1974–1994) and our recent data (2014–2016). We also collected soil samples in the year 2014, 41 years after the experiment began, and evaluated the effects of long-term nitrogen application on the changes of the contents of different forms of soil N (NO3−, exchangeable NH4+, total fixed NH4+, and organic N). The results were summarized as follows.
1) The long-term nitrogen fertilization decreased the fruit acidity and increased the N contents of the fruit and leaves. On the other hand, the yield, fruit number, fruit weight, fruit firmness, and brix sugar were not affected by nitrogen application and annually fluctuated.
2) Nitrogen fertilization decreased the pH of the surface soil (0–20 cm). The soil acidification was accompanied by a significant decrease in the contents of soil exchangeable Ca, Mg, and K and a significant increase in the content of soil exchangeable Al.
3) In the 0N and 4N plots, the content of Ca in the leaves in the year 2016 was lower than the average content in the years 1975, 1976, and 1984, which corresponded to the remarkable decrease of the content of soil exchangeable Ca during these three decades.
4) The contents of total N and total C in the surface soil (0–5 cm) of both plots gradually increased during the course of the experiment. The increase in total N and total C in the 4N plot was greater than that in the 0N plot.
5) On comparing the forms of soil N in the 0N and 4N plots in the year 2014, it was observed that the contents of NO3− and total fixed NH4+ were not significantly different at all soil depths. The content of exchangeable NH4+ was not significantly different except in the 70–90 cm soil layer. The content of organic N in the 0–5 cm and 70–90 cm layers was significantly higher in the 4N plot. More than 80％ of the soil N existed in an organic form, regardless of nitrogen application and soil depth.
6) It was suggested that most of the applied N was lost by leaching of NO3− and was hardly accumulated in the soil. However, it was also suggested that a part of the applied nitrogen was absorbed by the undergrowth plants, and was accumulated in the surface soil (0–5 cm) after their decomposition. The leaching of NO3− was accompanied with the leaching of Ca2+.
