Objective  This study clarified the relationship between stand spatial structure characteristics and soil nutrient content in Fraxinus mandshurica plantation, revealed the factors of stand spatial structure affecting soil nutrient, and obtained the optimal measures of stand spatial structure based on improving soil nutrients, providing scientific basis for precise management and effective soil management of Fraxinus mandshurica plantation.

  Method   Fraxinus mandshurica plantation with 4 kinds of stand density and 2 kinds of mixed proportion was taken as the object in Maoershan region of Heilongjiang Province of northeastern China. Correlation analysis and path analysis were used to find out the relationship between soil nutrient content and stand structure factors, such as mix ratio, stand density, angular scale, size ratio, opening degree and competition index.

  Result   The surface soil nutrient content of mixed forest was significantly higher than that of the four density pure forests, and the contents of organic carbon, total nitrogen, total phosphorus, total potassium, available phosphorus and available potassium were increased by 37.5%, 46.7%, 22.2%, 4.6%, 17.9% and 6.8%, respectively. There were significant differences in soil nutrient contents among different densities of pure plantations (P < 0.05), and soil nutrient contents decreased with stand density increasing. The contents of surface soil organic carbon, total nitrogen, total phosphorus, total potassium, available phosphorus and available potassium of the minimum density pure plantation were increased by 74.6%, 54.8%, 24.7%, 9.9%, 70.5% and 39.7% compared with those of the maximum density pure plantationt, respectively. The soil nutrient content increased with the increasing of angle scale, size ratio and openness, but decreased with the increasing of competition index. Stand density and competition index had direct negative effects on most soil nutrient contents, and angular scale, size ratio and openness were positively correlated with most soil nutrient contents. The interaction of structural characteristic factors directly or indirectly affected soil nutrient content.

  Conclusion  The soil nutrient content of mixed Fraxinus mandshurica plantations is higher than that of pure Fraxinus mandshurica plantations. The soil nutrient contents of low density of pure plantations are higher than that of high density of pure Fraxinus mandshurica plantations. In the process of management of Fraxinus mandshurica plantation, more mixed forests should be built to keep reasonable stand density as the general keynote and reduce the competition index among trees. On the basis of adjusting the angle scale to make the trees tend to random distribution, the inferior trees should be cut down according to the size ratio to increase the opening degree, gradually change the state of unreasonable growth space of forest trees and then improve the nutrient condition of forest land.