Objective  This paper aims to study the changes and differences of soil nutrient and stoichiometry of Pinus tabuliformis and Robinia pseudoacacia plantations under different densities to strengthen the stand management and ecological restoration of artificial forests in the Loess Plateau of northern China.

  Method  P. tabuliformis and R. pseudoacacia plantations were taken as research objects, and they were divided into three groups of stand density: high (2 000−2 700 plant/ha), medium (1 100−1 600 plant/ha) and low (800−1 100 plant/ha). Four kinds of stands with different densities were selected for each group, and soil samples were taken in layers respectively to measure the physical and chemical properties of soil.

  Result  (1) Two-factor analysis of variance showed that the stand type had a significant effect on total P, C:P, N:P, the stand density only had a significant effect on total P, and the stand type and stand density had a jointly significant effect on organic carbon, total N, total P, C:P, N:P. (2) The organic carbon and total N of P. tabuliformis and R. pseudoacacia forests with different stand densities were as follows: high density P. tabuliformis forests (Pinus H) > medium density P. tabuliformis forests (Pinus M) > low density R. pseudoacacia forests (Robinia L) > high density R. pseudoacacia forests (Robinia H) > low density P. tabuliformis forests (Pinus L) > medium density R. pseudoacacia forests (Robinia M ), and the total P is as follows: Robinia M > Pinus H > Robinia L > Robinia H > Pinus M > Pinus L. With the increase of stand density, the contents of organic carbon and total N in all soil layers of P. tabuliformis forest showed gradual increase, while the changes of total P content were relatively stable and had no significant difference. The contents of organic carbon and total N in all soil layers of R. pseudoacacia forest first decreased and then slightly increased, while the contents of total P first increased and then decreased. Under the same density and different stand types, the soil nutrient content of P. tabuliformis forest was better than R. pseudoacacia forest at high density and medium density, but it was opposite at low density. (3) The C∶N ratios of P. tabuliformis and R. pseudoacacia forests with different stand densities were as follows: Pinus H > Robinia H > Robinia L > Pinus M > Robinia M > Pinus L and the C∶P and N∶P ratios were as follows: Pinus H > Pinus M > Robinia L > Robinia H > Pinus L > Robinia M. With the increase of stand density, the soil C∶P and N∶P in P. tabuliformis forest gradually increased, the availability of phosphorus gradually decreased, while the soil C∶P and N∶P in R. pseudoacacia forest decreased first and then increased, the availability of phosphorus first increased and then decreased. The availability of phosphorus in P. tabuliformis forest soil was lower than that in R. pseudoacacia forest with the same density under high and medium density, but the opposite was true under low density. Soil organic carbon and total N determined the levels of C:P and N:P respectively to a great extent. Under different stand densities, soil C:N was relatively stable, soil nitrogen content was relatively deficient, and growth process was limited by nitrogen. (4) The soil nutrient content of P. tabuliformis and R. pseudoacacia under different forest densities showed an “ surface aggregation phenomenon ” and soil organic carbon, total N, total P, C∶P, N∶P gradually decreased with the increase of soil depth. C∶N had no obvious law. With the increase of stand density, the variation intensity of soil properties of P. tabuliformis forest firstly decreased and then increased, while that of R. pseudoacacia forest increased slowly. Compared with P. tabuliformis forest, stand density had less influence on the vertical variation of soil nutrients and stoichiometry of R. pseudoacacia forest, and the vertical variation tended to be more stable. (5) The change of stand density will change the influence of soil physical properties on soil nutrient and its stoichiometric ratio to different extent. The effects of soil bulk density on soil nutrient content and stoichiometric ratio were the greatest under different stand densities, followed by non capillary porosity.

  Conclusion  Generally, under different densities of the same stand type, P. tabuliformis forest is at a better level in soil nutrient content and its vertical variation, phosphorus availability and nitrogen limitation at medium density, while R. pseudoacacia forest was at low density in comparison. Under the same density and different stand types, the comprehensive performance of P. tabuliformis forest in high density and medium density was better than that of R. pseudoacacia forest with the same density, but it is opposite in low density.