| Hippophae rhamnoides, with strong adaptability and resistance,is the main specie that be used for maintaining soil and water, greening barren hills and slopes, and improving sandy and saline land on Loess Plateau of China. So it plays a vital role on water and soil maintenance in the forestation area. But in recent years, large area of Hippophae rhamnoides have been declining, and scholars have undergone many researches and analysis for that. However the reason is yet to be determined.In this study, we chosen different types of Hippophae rhamnoides (Hippophae rhamnoides pure stand, Hippophae rhamnoides×Robinia pseudoacacia, Hippophae rhamnoides×Pinus tabulaeformis, Hippophae rhamnoides×Platycladus orientalis Franco) and different growth stages (8a,13a and 18a) as participants of study in Malian Beach Yongshou county of Shaanxi. We studied the stand structure and its effects on soil nitrogen mineralization rates, total nitrogen, available nitrogen, phosphatase activity of Hippophae rhamnoides, And combining with stand structure, root activity and natural regeneration capacity, we expect that the study can provide theory evidence for decline of Hippophae rhamnoides and its health assessment and management. As following conclusions:(1)Interspecific competition is the chief reason for decline of Hippophae rhamnoides in the mixed stand. Density of dead trees has negative correlation with the high and crown of its mixed species. Mixed species have a great impact on the decline and dead of Hippophae rhamnoides. Because Its height and crown take advantage,and it make a strong shading affect on Hippophae rhamnoides. As a result of lacking light and basic vegetation space for Hippophae rhamnoides, it gradually decline and eventually die out. The nitrogen mineralization rate, total nitrogen, available nitrogen and phosphatase activity of Hippophae rhamnoides pure stand beyond to others.(2)Intraspecific competition is the chief of decline of Hippophae rhamnoides in pure stand. The height, crown and diameter growing as the age of Hippophae rhamnoides, Owing to abundance clone being generated, its crown close gradually, and the competition became fiercer. The trees disadvantaged in height lack of light long-term, gradually died. But The average height under branch of alive increased gradually. The number of the dead were correlation with the age. Very few died and no dead branch in 8a Hippophae rhamnoides forest. But in 13a dead trees account for 19% and height under branch was 0.77m,and in 18a dead trees account for 42.1% and height under branch was 3.67m. Distribution of diameter class showed that 8a Hippophae rhamnoides stand is in growing phase,13a more stable and 18a in decline phase.(3)The activity of the roots was normal varies with age. Root activity showed that, Ocm-10cm layer:absorbing roots (0.5mm～2mm) activity among 8a,13a,18a Hippophae rhamnoides were no obvious difference(P>0.05); 10cm～30cm layer:The difference of absorbing root activity of the three Hippophae rhamnoides plantation was significant(P <0.05). Exhibiting a law:13a> 8a> 18a. The thick roots (10mm～20mm) were mainly distributed in 10cm～30cm soil layer, the roots vitality of 8a and 13a were significantly different comparing with 18a(P<0.01).(4)The natural regeneration capacity of Hippophae rhamnoides was weaker as the age. The density of natural regeneration of 8a(1567 trees/hm2) was significantly higher comparing with 13a (833 trees/hm2) and 18a(242 trees/hm2) (P<0.01).13a was significantly higher comparing with 18a(P<0.01). The results of natural regeneration showed that the health status of Hippophae rhamnoides was worse as the age increasing.(5)The soil physicochemical properties of Hippophae rhamnoides plantations of different age showed that, the 13a was the best,18a followed, the last was 8a. The amount of nitrogen mineralization of different layer soil was significantly different (P<0.05), and negatively correlated with soil depth.(6)The nitrogen mineralization rate of 13a was significantly higher (P<0.05) than 8a. There were no obvious difference between 8a and 18a,13a and 18a.30cm～50cm:13a and 18a were significantly higher(P<0.05) than 8a. the nitrogen mineralization rate determined by ion exchange culture highly correlated with the fertility index,which is including soil total nitrogen, available nitrogen and soil phosphatase activity (P<0.01).It can reflect the dynamic characteristics of soil nitrogen mineralization well, and consistent with health status of Hippophae rhamnoides stand. The method was basicly reliable to determine soil N mineralization.(7)The soil nitrogen mineralization rate of 0cm～10cm soil layer of different age Hippophae rhamnoides plantations had greater seasonal variations, and it had correlation with air temperature.It manifested as July> April> November. But the nitrogen mineralization rate of 10cm～30cm and 30cm～50cm layer had no rules to follow. The effects of temperature on soil nitrogen mineralization was stronger than water, which played a leading role in the process of soil nitrogen mineralization.
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