Soil nitrogen contents and deep profile distributions in apple orchards of the Loess Plateau

Contents of soil total nitrogen, ammonium nitrogen and nitrate nitrogen in 0-300 cm soil layers of 6-28 year-old apple orchards were measured at 6 apple production bases of Fengxiang, Baishui, Changwu, Xifeng, Yanan and Jingning in the Loess Plateau, and profile distributions of soil total nitrogen, ammonium nitrogen and nitrate nitrogen contents between different ages of apple orchards at different sites were analyzed and compared. The results showed that the range of soil total nitrogen contents is from 0.19 g/kg (Yanan) to 1.28 g/kg (Baishui), the range of soil ammonium nitrogen contents is from 5.19 mg/kg (Jingning) to 39.46 mg/kg (Changwu), and the range of soil nitrate nitrogen contents is from 3.97 mg/kg (Yanan) to 352.86 mg/kg (Baishui) at different sites and of different ages of apple orchards. There are not significant differences of soil total nitrogen contents in apple orchards at different sites except Yanan where the content is obviously lower than those of other sites. The soil nitrate nitrogen contents are significantly different at different sites, while the soil ammonium nitrogen contents are less different. The soil total nitrogen contents in 0-60 cm soil layers of the apple orchards are obviously higher than those in the deeper soil layers. The soil nitrate nitrogen contents of old apple orchards are higher than those of young apple orchards, and at the same time the nitrate nitrogen accumulation in soil profiles below 100 cm soil layers occurs in varying degrees. The contents of soil total nitrogen, ammonium nitrogen and nitrate nitrogen of apple orchards at Yanan are obviously lower than those at other experiment sites, which means more nitrogen fertilizer should be used appropriately to increase apple yields. However, nitrate nitrogen contents in deep soil layers of apple orchards at Fengxiang, Baishui, Changwu, Jingning and Xifeng are relatively higher, hence nitrogen fertilizer utility should be kept at present level or reduced.