Effects of Pinus sylvestris var. mongolica afforestation on soil physicochemical properties at the southern edge of the Mu Us Sandy Land, China

With the extensive planting of Pinus sylvestris var. mongolica, its maladaptation in both premature senility and natural regeneration barrier is increasingly prominent at the southern edge of the Mu Us Sandy Land. Whether the maladaptation will lead to secondary soil degradation is unclear. Taking naturally occurring grasslands as a control, we studied the characteristics of the vegetation and soil physicochemical properties in the 0–20 cm soil layer of P. sylvestris var. mongolica plantations at 1–5, 6–10, 11–15, 20–25, 30–35 and > 40 years after afforestation. The results showed that the specific manifestation of premature senility was that maturity and recession were advanced to 20–25 years and > 40 years in P. sylvestris var. mongolica afforestation, respectively. No natural regeneration seedlings of P. sylvestris var. mongolica were found. Compared with the grasslands, the soil bulk density and particle composition were effectively improved at 20–25, 30–35 and > 40 years after afforestation. With increasing afforestation years, the contents of organic matter, total nitrogen, total phosphorus, alkali-hydrolysable nitrogen, available phosphorus and available potassium fluctuated, and the minimum values occurred at 11–15 years. The contents of a few soil nutrient indices dramatically decreased at > 40 years. Significant correlations between the stands and the soil physicochemical properties were found, and soil nutrients showed signs of secondary degradation caused by the maladaptation of the plantations at > 40 years. Thus, we suggest that fertilization practices need to be conducted to promote more sustainable growth of trees after 11–15 years, and stand rebuilding measures need to be implemented to prevent soil nutrient degradation at > 40 years.