Background Due to the special geographical location, the ecological environment of Tibet Plateau-Loess Plateau transition zone is fragile and its soil fertility is quite poor. Plantation is the main measure of ecological restoration in this area. Studying the soil ecological stoichiometric characteristics of plantation is conducive to understanding the effect of vegetation improvement in this area and revealing the limiting elements of vegetation growth, providing theoretical basis and technical guidance for vegetation restoration and reconstruction.

Methods The study selected 5 standard plots (20 m×20 m) for each plantation(Picaea crassifolia, Sabina przewalskii, Pinus tabuliformis Carr., Larix principis-rupperchii, Populus cathayana Rehd, and Betula platyphylla Suk) in this area, and selected 3 standard plots (20 m×20 m) of grassland as a control. We collected 0-60 cm soil samples to determine their C, N and P contents, using one way ANOVA and Pearson correlation analysis to explore the soil ecological stoichiometric characteristics of typical plantations and their influencing factors.

Results 1) The soil C, N and P contents of 6 plots were 11.34-20.86, 1.16-1.74 and 0.51-0.72 g/kg. The soil nutrient contents of each plantation were higher than that of grassland, while the soil nutrient of broad-leaved forest was higher than that of coniferous forest. 2) The soil C/N, C/P and N/P of 6 plantations were 9.30-11.60, 16.56-41.98 and 1.71-3.64. The soil C/P and N/P were lower than the average level of soil in China. 3) Soil C, N and P decreased with the increase of soil depth, and there were significant differences between different soil layers (P < 0.05). Soil C/N decreased with the deepening of the soil depth, and its coefficient of variation was small. Soil C/P and N/P of different plantations showed different trend of soil depth, while they had no significant difference between different soil layers. 4) The correlation analysis showed that there was a very significant positive correlation between soil C and N, indicating that the changes of their contents were consistent. C/P and N/P were more affected by P. Soil nutrients and their stoichiometric ratio were significantly correlated with soil saturated hydraulic conductivity and alkali-decomposed nitrogen (P < 0.05).

Conclusions This study indicates that plantation has a certain improvement effect on soil nutrients, while broad-leaved forests are better than coniferous forests. N is found to be the main restricted nutrient in this area. In the process of plantation management, attention should be paid to the supplement of N to promote the healthy development of plantation ecosystem.