Abstract: To reveal the process of soil bioactivity and fertility restoration of Robinia pseudoacacacia plantation in loess hilly region, using the space for time substitution method, a chronosequence of 17, 27 and 42 years old of R. pseudoacacia plantation was selected, with a sloping cropland (0 year old) as control. We examined the dynamics of carbon, nitrogen and phosphorus contents in 0-30 cm soil layer, and the relationship between the contents of carbon, nitrogen and phosphorus and the activities of four enzymes. The results showed that soil nutrient content and enzyme activity of R. pseudoacacia plantation decreased from 0-10 cm to 20-30 cm soil layer. Soil organic carbon, total nitrogen and dissolved organic carbon in the layer of 0-30 cm increased with the increases of restoration duration, being 1.4, 4.2 and 5.1 times higher than those in sloping cropland, respectively. The content of dissolved organic nitrogen in soil was slightly different among different restoration durations, with a mean value being 1.8 times higher than that in sloping cropland. Similarly, from 15 to 45 years old, the activities of four enzymes in the soil layer of 0-30 cm significantly increased. The increments of alkaline phosphatase and sucrase activities reached 3.4-9.5 times and 8.1-15.6 times, and that of catalase and urease activity was only 0.2-0.3 times and 1.8-2.2 times, compared with that in cropland, respectively. The results of redundancy analysis showed that soil nutrients could well explain the variability of enzyme activity, with a significantly positive correlation (P<0.01). The effects of soil nutrients on enzyme activity were in order of total nitrogen > dissolved organic carbon > soil organic carbon > soil water content > dissolved organic nitrogen > available phosphorus. The alkaline phosphatase activity was positively correlated with dissolved organic carbon, dissolved organic nitrogen and available phosphorus, which could be used as an index to reflect soil restoration in R. pseudoacacia plantation.

Key words: climate factor, soil nutrient, architecture trait, redundancy analysis, three-dimensional laser