To study the fractal dimensions of the soil particle size distributions (PSDs) within different plantations (of Pinus densiflora, Quercus acutissima, Robinia pseudoacacia, and Larix kaempferi) and evaluate PSDs as an indicator of the likelihood of Robinia pseudoacacia invasion, the soil porosity of 0–20 cm soil layers was measured at different plantations in the Yaoxiang Forest Farm, Shandong Province, China. The results showed that the fractal dimension (Dm) values varied from 2.59 to 2.70 among the different plantations and were significantly negatively correlated to sand content and positively correlated to silt content and clay content. Significant negative correlations were observed between Dm and both soil organic matter (SOM) (P < 0.05) and available phosphorus (P < 0.01). The multifractal entropy dimension (D1) and entropy dimension/capacity dimension (D1/D0) parameters were not significantly correlated with SOM, although significant correlations were found between SOM and each of D0, Δα, and Δf(α). Compared with the other plantations, the Robinia pseudoacacia plantation had higher nutrient contents, higher D0 and D1 values and lower Dm values. Based on principal component analysis (PCA) ordination, we concluded that Robinia pseudoacacia and Pinus densiflora shared a similar habitat and that Robinia pseudoacacia is more likely to invade Pinus densiflora plantations for soil.
Three species of aquatic plants (Scirpus validus, Phragmites australis and Acorus calamus) were used as experimental materials to study their capacity to purify contaminated water and their effects on water pH and dissolved oxygen (DO). The water was contaminated with different concentrations of nitrate (5 mg/L, 15 mg/L and 25 mg/L). The results indicated that the concentration of nitrate, species of aquatic plant and their interaction significantly impacted denitrification (P = 0.00). Under the same concentrations, the three species of aquatic plants provided varying degrees of purification. Acorus calamus provided effective purification under all three concentrations of nitrate wastewater, with removal percentages of 87.73%, 83.80% and 86.72% for nitrate concentrations of 5 mg/L, 15 mg/L and 25 mg/L, respectively. In terms of the purification ability by unit fresh weight, Acorus calamus exhibited the worst purification capacity, whereas the capacities of Scirpus validus and Phragmites australis were higher. The purification capacity of Scirpus validus for the three concentrations was as follows: 0.08 mg/(L•g FW), 0.29 mg/(L•g FW), and 0.51 mg/(L•g FW). The capacity of Phragmites australis was 0.07 mg/(L•g FW), 0.25 mg/(L•g FW), and 0.53 mg/(L•g FW). The capacity of Acorus calamus was 0.04 mg/(L•g FW), 0.12 mg/(L•g FW), and 0.21 mg/(L•g FW). Under increased concentrations of nitrate, the three species of aquatic plants exhibited various degrees of increased purification capacity. Under the different concentrations of nitrate, the three species exhibited the same trends with respect to water pH and DO, increasing first and then falling. The pH remained at approximately 7.5, and the DO fell to 4.0 mg/L. A comprehensive analysis reveals that Acorus calamus provides excellent nitrate purification, although by unit fresh weight, both Scirpus validus and Phragmites australis provide superior purification capacity.
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