Background  The project of Returning Farmland to Forests is the largest ecological project in China and even in the world. The study on the soil nutrient characteristics of different land after Returning Farmland to Forests will provide the basis for soil remediation and improvement of different land where Returning Farmland to Forests is conducted.

Methods  In order to explore the characteristics of soil nutrient in different lands where Returning Farmland to Forests was conducted, taking the different lands after Returning Farmland to Forests in the Chakou Watershed in the loess hilly and gully region as study object, soil samples of ecological forest land (Robinia pseudoacacia, Platycladus orientalis) and economic forest land (Juglans regia) were collected. Taking soil samples of existing slope cropland as the reference substance, multiple comparisons of nutrients in soil layers of 0-20 cm and 20-40 cm in the different plots were conducted by one-way ANOVA, correlation analysis of soil nutrients by Pearson correlation coefficient, and the modified Nemerow formula was used to evaluate the soil nutrients in different plots.

Results  1) The soil nutrient contents of lands after Returning Farmland to Forests in Chakou Watershed was mostly moderate variation, variation coefficient of each nutrient content was concentrated in 20%-30%, nutrient content in the topsoil (0-20 cm) was higher than that in the 20-40 cm soil layer, reflecting the surface aggregation effect of forest soil, among them, the surface aggregation effect of organic matter and available nutrients were more obvious, the soil nutrient content of 0-20 cm soil layer was generally 30% more than that of 20-40 cm soil layer. 2) Soil nutrient content in different plots ranked in the order of ecological forest > economic forest > slope cropland on the whole, however, we found that there was no significant difference in total phosphorus content among different plots, it may be related to the parent material in the soil. 3) The correlation between the contents of total nitrogen, available nitrogen and potassium with organic matter was positively significant. 4) The soil comprehensive fertility index of R. pseudoacacia, P. orientalis and J. regia were 0.9 (intermediate level), 0.85 (intermediate level), and 0.74 (low level), this showed that the soil nutrient status of ecological forest was better than that of economic forest.

Conclusions On the whole, the benefit of soil nutrient conservation for land where Returning Farmland to Forests was conducted were better than those of existing slope cropland in Chakou Watershed, however, the characteristics of soil nutrients in different plots varied. The comprehensive study on soil nutrients of ecological forest and economic forest will provide basis for soil ecological restoration of different models of returning farmland to forest.