Background The ecological environment of the Loess Plateau is fragile and its soil fertility is quite low, the "Grain for Green Project" is a key measure for soil and water conservation and restoration of degraded land in this region. The study of the plant-litter-soil stoichiometric characteristics of typical plantation forests and their interrelationships may reveal the limiting elements of vegetation growth. Comparing differences in ecological stoichiometric characteristics between pure and mixed forests can help to find out which of the two silvicultural modes is the better choice in the process of vegetation restoration in the region and provide scientific support for the reconstruction mode of vegetation on the Loess Plateau.

Methods We selected four types of plantation forests (pure Pinus tabuliformis forest, P. tabuliformis + Caragana korshinskii mixed forest, pure Larix principis-rupprechtii forest, and L. principis-rupprechtii + C. korshinskii mixed forest) in Jingle county, Shanxi province, and set up 3 sampling sites in each forest. We collected tree layer organs (leaves, branches, stems and roots), litter, and soil (0-30 cm) samples and determined the C, N and P content using potassium dichromate oxidative heating and AA3 continuous flow analyzer. We used ANOVA, correlation analysis and structure equation modelling to analyze the ecological stoichiometric characteristics and their interactions among tree layer organs (leaves, branches, stems and roots), litter, and soil (0-30 cm).

Results 1) The changes of C, N and P contents of trees in 4 plantation forests were in the form of leaf>branch>stem>root. Only L. principis-rupprechtii leaf N content significantly increased by 3.18 g/kg after mixing, leaf C ∶N and C ∶P decreased, and regional tree growth showed P limitation. 2) Only L. principis-rupprechtii litter N content increased significantly by 1.35 g/kg after mixing, while litter C ∶N was reduced, and there were no significant changes in litter of P. tabuliformis forests. 3) The C and P contents of plantation forest soil both decreased with increasing soil depth, in which the C and P contents of 0-10 cm soil layer were significantly higher than those of other soil layers, which were 8.92 and 0.31 g/kg, respectively. Mixing increased the C contents of each soil layer of L. principis-rupprechtii, in which the 0-10 cm soil layer increased the most significantly, with a significant increase of 49.15%. 4) Pearson and SEM analyses showed that the N contents of the litter layer directly influenced the soil layer C ∶N, which in turn influenced the nutrient pattern of the arbor layer, which in turn positively influenced the litter layer.

Conclusions Mixing can effectively improve the nutrient status of plantation forests. However, the introduction of the same hybrid shrubs varied significantly among tree species, while the L. principis-rupprechtii + C. korshinskii mixed forest is optimal. In the future, mixed afforestation should be preferred for vegetation restoration in loess hilly and gully region.