To study the effects of the close-to-nature model of Cunninghamia lanceolata and Phoebe bournei on the physical and chemical properties of forest soil, and to provide a scientific basis for the transformation and sustainable management of pure Chinese fir forests.

Three close-to-nature transformation modes of Cunninghamia lanceolata and Phoebe bournei (T1, Cunninghamia lanceolata∶Phoebe bournei=3∶7; T2, Cunninghamia lanceolata∶Phoebe bournei=2∶3; T3, Cunninghamia lanceolata∶Phoebe bournei=1∶1 and one pure Cunninghamia lanceolata forest control (CK) were set up, with 9 sample plots in each mode. Soil samples were taken from 0-20, 20-40 and 40-60 cm at three different slopes: upper, middle and lower. The ring knife method was used to determine the physical properties of the forest land under different modes, including soil bulk density, moisture content, maximum soil water holding capacity, capillary water holding capacity, minimum soil water holding capacity, total porosity, capillary porosity, non-capillary porosity, etc. The chemical properties of the soil, including total nitrogen, total phosphorus, total potassium, hydrolyzable nitrogen, available potassium, effective phosphorus, organic matter and pH value, were determined according to the methods specified in relevant industry standards.

The soil bulk density and mass water content differed significantly between different modes. The bulk density of the T1 was the smallest in the three soil layers, and the bulk density of the CK was the largest. The mass water content of the three soil layers was: T1 > T3 > T2 > CK. In different modes and soil layers, except for the non-capillary pore CK of the 40-60 cm soil layer in the forest land interplanted with Phoebe bournei, which was greater than that of T2 and T3, the rest were mixed forests greater than the pure control forests. Interplanting Phoebe bournei can increase the pH value of forest soil. Under different close-to-nature transformations of Phoebe bournei, except for the total phosphorus content of the 20-40 cm soil layer, there were significant differences in the effects on the total nitrogen and total potassium content of the forest soil and the total phosphorus content of 0-20 and 40-60 cm. The total nitrogen and hydrolyzable nitrogen content of the soil were CK > T1 > T2 > T3 in the three soil layers. The total phosphorus and available potassium content of the soil were T1 > T2 > T3 > CK in the three soil layers. The total potassium content of the soil was T1 > CK > T2 > T3 in the three soil layers. The organic matter content of CK was the highest in all the different modes, and there was no significant difference in the organic matter content among the three soil layers of T1, T2 and T3. The physical property factors of forest soil were generally greatly affected by the content of organic matter, total nitrogen, hydrolyzable nitrogen, total potassium, available potassium and total phosphorus.

The close-to-nature transformation model of Cunninghamia lanceolata and Phoebe bournei can improve the physical properties and nutrient conditions of forest soil, especially T1 (3 Cunninghamia lanceolata and 7 Phoebe bournei), which has good promotion value.

To study the effects of antimony stress on photosynthetic physiology of different antimony tolerance clones of Catalpa bungei.

Two kinds of C. bungei clones with different antimony tolerance were studied, namely sensitive clones (1-1) and tolerant clones (5-8). Three antimony stress concentrations (calculated by antimony element) were set up in a pollution-free red soil as the substrate for pot stress tests. The concentrations of antimony were 0 mg/kg, 1 200 mg/kg and 3 000 mg/kg. The growth, relative content of chlorophyll in leaves, chlorophyll fluorescence parameters and photosynthetic parameters of the two clones were determined at 0 d, 20 d and 40 d, respectively.

1) Under the same conditions, the growth of sensitive clones (1-1) was more affected than that of tolerant clones (5-8). 2) Under antimony stress, the relative content of chlorophyll in the leaves of both clones decreased, and the relative content of chlorophyll in the leaves gradually decreased with the extension of stress concentration and time, but the decrease range of tolerant clones was much less than that of sensitive clones. 3) The chlorophyll fluorescence parameters of the two clones were different under antimony stress. At the same time, the variable fluorescence, maximum photochemical efficiency of PSII and potential activity of PSII all decreased with the increase of antimony stress concentration, and the tolerant clones were less affected.4) Under antimony stress, the net photosynthetic rate of both clones decreased and the intercellular CO₂ concentration increased, indicating that antimony stress was caused by non-stomatal factors.

Under high concentration of antimony stress, the tolerant clones (5-8) had higher chlorophyll content than the sensitive clones (1-1), and the photosynthetic system was less affected by the damage, so they could better cope with antimony stress.