Soil and water conservation is an important function of forest ecosystems; however, it remains unclear which forest type is best suited for water and soil conservation under the same site conditions. In order to clarify the soil and water conservation function of different plantations in the northern and southern mountains of Lanzhou city, we investigated several soil and water conservation function indicators (thickness and accumulation of litter, maximum water holding capacity and rate of litter, water holding capacity and water absorption rate of litter, soil infiltration rates, soil water content, soil bulk density, soil porosity, and soil water storage) of five plantation types (Platycladus orientalis plantations (Po), Robinia pseudoacacia plantations (Rp), Populus alba var. pyramidalis plantations (Pa), P. alba var. pyramidalis + R. pseudoacacia mixed plantations (Pa + Rp), and P. orientalis + R. pseudoacacia mixed plantations (Po + Rp)) and evaluated them using the gray correlation method. The results indicated the accumulation of litter varied from 13.50 to 47.01 t·hm⁻² and increased in the order of Pa < Rp < Po < Po + Rp < Pa + Rp. The maximum water holding capacity of litter varied from 35.29 to 123.59 t·hm⁻² and increased in the order of Pa < Rp < Po < Po + Rp < Pa + Rp. The soil physical properties (soil infiltration, porosity, and bulk density) of mixed plantations were better than those of pure plantations. The soil maximum water storage was significantly different among plantation types (p < 0.05), with an average varying from 3930.87 to 4307.45 t·hm⁻², and was greater in mixed plantations than in pure plantations. Gray correlation analysis revealed that mixed plantations had the best conservation function of the five plantation types, followed by broad-leaved plantations and coniferous plantations. This suggests that the planting of mixed plantations dominated by Pa + Rp is therefore recommended in the future construction of plantations in the northern and southern mountains of Lanzhou to realize sustainable forest development.

Soil salinization is an important environmental problem worldwide and has a significant impact on the growth of plants. In recent years, the mechanisms of plant salt tolerance have received extensive attention from researchers. In this paper, an experiment was implemented to assess the potential effect of different NaCl and NaHCO₃ (sodium bicarbonate—an alkaline salt) concentrations (25 mmol·L⁻¹, 50 mmol·L⁻¹, 100 mmol·L⁻¹, 150 mmol·L⁻¹ and 200 mmol·L⁻¹) on the growth, antioxidant enzymes, osmoprotectants, photosynthetic pigments and MDA of Viola tricolor L. to reveal the physiological response and explore the maximum concentrations of NaCl and NaHCO₃ stress that V. tricolor can tolerate. The results showed that NaCl and NaHCO₃ treatments had significant effects on osmoprotectants, antioxidant enzymes, photosynthetic pigments, MDA content and the plant height growth of V. tricolor. On day 14 of the NaCl and NaHCO₃ stress, the height growth of V. tricolor was significantly greater than CK when the concentration of NaCl and NaHCO₃ was less than 100 mmol·L⁻¹. Soluble protein (SP) was significantly greater than CK when the NaCl concentration was less than 150 mmol·L⁻¹ and the NaHCO₃ concentration was less than 200 mmol·L⁻¹; soluble sugar (SS) was significantly greater than CK under all NaCl and NaHCO₃ treatments; proline (Pro) was significantly greater than CK when the NaCl concentration was 150 mmol·L⁻¹ and the NaHCO₃ concentration were 150 and 200 mmol·L⁻¹, respectively. Peroxidase (POD) was significantly greater than CK when the NaCl concentration was less than 200 mmol·L⁻¹ and the NaHCO₃ concentration was less than 150 mmol·L⁻¹; superoxide dismutase (SOD) was significantly greater than CK when the NaCl concentration was 50 mmol·L⁻¹ and the NaHCO₃ concentrations were 50, 100 and 150 mmol·L⁻¹, respectively; catalase (CAT) was significantly greater than CK when the NaCl and NaHCO₃ concentrations were 25, 50 and 100 mmol·L⁻¹, respectively. Chlorophyll (Chl) was significantly lower than CK when the NaCl and NaHCO₃ concentrations were greater than 100 mmol·L⁻¹. Malondialdehyde (MDA) gradually increased with the increase in the NaCl and NaHCO₃ concentrations. Membership function analysis showed that the concentrations of NaCl and NaHCO₃ that V. tricolor was able to tolerate were 150 mmol·L⁻¹ and 200 mmol·L⁻¹, respectively. Beyond these thresholds, osmoprotectants and antioxidant enzymes were seriously affected, Chl degradation intensified, the photosynthetic system was seriously damaged, and the growth of V. tricolor was severely affected. According to a comprehensive ranking of results, the degree of NaCl stress on V. tricolor was lower than that from NaHCO₃ when the treatment concentration was lower than 50 mmol·L⁻¹, but higher than that from NaHCO₃ when it exceeded 50 mmol·L⁻¹.

Climate change is causing the soil environment to change (soil temperature and soil moisture), which affects the soil nitrogen cycling of vegetation. To examine this, the soil of three typical communities (grassland, shrub forests, and Picea wilsonii Mast. forests) was selected and cultured under three different moisture levels (20, 40, and 60% of the field water-holding capacity, referred to as W20, W40, and W60, respectively) and three different temperatures (15, 25, and 35 °C, referred to as T15, T25, and T35, respectively) to evaluate the effects of different vegetation types on soil nitrogen mineralization in the Xinglong Mountains, Gansu Province, China. The results showed that under the three moisture levels, the cumulative mineralization of soil nitrogen in different vegetation types was the largest under the T35 treatment. Under three temperature levels, the cumulative mineralization of soil nitrogen in different vegetation types was the largest under the W60 treatment. Under the three vegetation types, the cumulative mineralization of soil nitrogen in the P. wilsonii Mast. forest was the largest, which was 2.23% and 24.79% higher than those of grasslands and shrub forests, respectively. We concluded that soil nitrogen mineralization in the P. wilsonii Mast. forest was the best option under T35 and W60 in this study.