Objective To investigate the spatial distribution characteristics of soil soluble nitrogen component in Hippophae rhamnoides L. subsp. sinensis Rousi plantation with different stand ages in loess hilly region.

MethodsBased on the Hippophae rhamnoides plantation with three different stand ages of 5-year-old, 15-year-old and 20-year-old were selected in Jinding Town of Zhidan County, soil samples were collected in 0-10 cm, 10-20 cm and 20-30 cm soil layers, and seasonal dynamic variation of the proportion of soil soluble nitrogen component were analyzed. The grassland was treated as control.

ResultsWith the extension of the recovery years, the content of soil soluble nitrogen components in the Hippophae rhamnoides plantations increased. For soil nitrate nitrogen and soluble organic nitrogen (SON), there were significant differences between Hippophae rhamnoides plantations in 20 years and 5 years, while there was no significant difference between grassland and Hippophae rhamnoides forests in 5 years. There was no significant difference in soil ammonium nitrogen among different restoration years. The soil nitrate nitrogen and SON showed the same trend, with 20-year-old>15-year-old> grassland>5-year-old. For the ammonium nitrogen, soluble organic nitrogen in forests with 15-year-old was the largest, followed by 20-year-old,5-year-old, and grassland, and the average content of ammonium nitrogen in the surface layer was the lowest. There was no significant difference in soil soluble nitrogen components among different soil layers. Soil nitrate nitrogen was the highest in summer and the lowest in winter or autumn. Ammonium nitrogen was the highest in summer or autumn, and the lowest in winter. SON was higher in spring and winter than in summer and autumn. The proportion of SON, nitrate nitrogen and ammonium nitrogen in total soluble nitrogen was 80.3%, 10.2% and 9.5% respectively. The proportion of SON in total soluble nitrogen of Hippophae rhamnoides forests with different years was higher in spring and winter than in summer and autumn, and most of nitrate nitrogen and ammonium nitrogen were higher in summer and autumn than in spring and winter.

Conclusion In the loess hilly area of northern Shaanxi, Hippophae rhamnoides plantations can effectively improve soil soluble nitrogen components. With the increase of forest age, the soil nitrogen accumulation increases.