Dynamic characteristics of soil properties in a Robinia pseudoacacia vegetation and coastal eco-restoration

Precipitation is one of the major determinants of soil moisture in plantations growing in the saline-alkali soil in the Yellow River Delta. The present study was conducted on a 28-yr old Robinia pseudoacacia plantation, a major vegetation type of the Yellow River Delta, and the results indicated strong seasonal patterns in soil moisture, salt, nutrient and enzyme activities at varying depths with respect to annual precipitation. Soil moisture during the growing season of March to October fluctuated drastically in response to precipitation events and was generally higher than that from November to February. Furthermore, soil water content in the 0–60cm soil layer was positively correlated with precipitation during the same period but not for the value in the 60–80cm soil layer. Salt content in different soil layers increased gradually from November to February, decreased from February to September and then increased in October. Precipitation was strongly and negatively correlated with soil salt content in different soil layers (except 20–40cm). Soil enzymes became less active and soil nutrient contents decreased with soil depth, though different enzymes and nutrients showed seasonal variations. The activity of polyphenol oxidase increased in spring, reached the maximum values in June, and decreased in later months. The activity of alkaline phosphatase, proteinase and urease fluctuated throughout the growing season, with the maximum values in October. Available phosphorous increased in the early months and decreased after August, whereas available potassium, hydrolysable nitrogen and organic substances content in soil gradually increased throughout. Therefore, it is suggested that precipitation is the crucial limiting factor to tree growth through impacts on soil moisture, salt, nutrient and enzyme activities in saline-alkali soils in the Yellow River Delta.