Background Horqin sandy land is a national key ecological environment construction area and a typical ecological fragile area. In recent years, wind erosion and soil erosion are serious in this area. Elm forest is the most stable vegetation type in sandy land. It is not only the climax community of vegetation succession in sandy land, but also the ecological tree species most suitable for local climate conditions. However, due to the limitation of soil moisture and nutrients, the ecological function degradation and low efficiency of some elm forests has become increasingly prominent with the growth of elm forests. At present, the focus of vegetation restoration and ecological reconstruction in sandy land should be on the management of the existing plantation.

Methods Taking young 18 a, middle-aged 26 a, mature 32 a elm trees of three different forest ages in Wengniute banner as the research object, the nutrient change characteristics of 0-10, 10-20 and 20-40 cm soil layers were analyzed. With the PCA-TOPSIS evaluation system, the data sets were firstly subjected to dimension reduction analysis using principal component analysis method, and the dynamic scores of soil nutrients at different forest ages were further discussed and evaluated combined with TOPSIS method, and then the nutrient indexes of the appropriate age for the sandy land were screened out. The nutrient index and plant growth index of MDS (minimum data set) were calculated by weighted TOPSIS, and the result score was calculated.

Results The cumulative PCA contribution rates of soil available nitrogen, available potassium and polyphenol oxidase were all greater than 0.9, indicating that they had a positive effect on soil quality, and the more the index, the better. The highest available nitrogen score in Z treatment (26 a) was 1, followed by Y treatment (18 a), which was 0.467. The highest value of soil available potassium was 0.84 in C treatment (32 a). The highest value of soil polyphenol oxidase score was Z treatment (1), followed by C treatment (0.645), and the lowest value was Y treatment (0.159). Result The score linear regression passed the F test (F=398.629, P < 0.05), R²=0.996, and the TOPSIS score of available nitrogen represented 99.6% of the growth index, indicating that available nitrogen was the nutrient factor next to available potassium. Polyphenol oxidase R²=0.990, which had a high regression fitting degree with growth factors, and the score value was reliable after verification, and can also be used as a positive effect index of elm forest soil.

Conclusions There are varied different differences in soil nutrient and enzyme activity indexes at different forest ages, and the same index presents has significant differences at different forest ages. According to the entropy weight score of soil index, the proportion and cycle of chemical fertilizer application should be properly controlled or adjusted at different forest ages to better improve the growth of local elm trees and enhance the soil fertility.