Background Soil and water loss in Pisha sandstone area is serious and the ecological environment in it is very severe. Understanding the spatial distribution characteristics of soil particles and eco-stoichiometric characteristics after Grain for Green project in Pisha sandstone area is of great significance for exploring the mechanism of ecological restoration in this area.

Methods In order to explore the response of soil particles and soil eco-stoichiometric characteristics to reforestation measures in Pisha sandstone area, we selected Pinus tabuliformis Carr. forest (YS), Caragana korshinskii Kom. forest (NT), Populus simonii Carr. forest (XY) and Stipa bungeana Trin. grassland(ZM) as the research objects, and buckwheat slope farmland(QM) as the control. The soil particle distribution (PSD), soil carbon(C), nitrogen (N), phosphorus (P) and stoichiometric ratio were studied by using multi-fractal theory, and the correlation between them was analyzed.

Results 1) Soil under different measures well reflected multi-fractal character. Grain for Green Project significantly increased the distribution, heterogeneity and dispersion of PSD. It also increased the amount of adsorbed soil nutrients by increasing the content of fine grains, and thus promoted the increase of soil C, N and P content. 2) The average contents of C, N and P in soil were 4.940, 0.570 and 0.056 g/kg. Among them, the content of C and N was low, and P was seriously deficient and was the limiting element in this area. Soil C/N, C/P and N/P ranged from 8.02 to 9.91, 77.65 to 94.34 and 8.50 to 11.03, with little fluctuation among different measures and profiles. 3) Clay, silt and fine sand were the main soil particle sizes that determined the content of C, N and P elements in the region. They can better explain the distribution characteristics of C, N and P elements in the soil. Singular index (Δα) and capacity dimension (D₀) were positively correlated with soil C and N content (P < 0.01). Δα and singular function (Δf) were positively correlated with the measurement ratio of C, N and P (P < 0.01).

Conclusions This study shows that C. korshinskii Kom. forest had the best improvement effect on soil PSD and soil nutrients (C, N, P content), and was an excellent tree species for ecological restoration in Pisha sandstone area. Fractal parameters demonstrate promising application prospects in characterizing soil eco-stoichiometric characteristics.