The effects of continuous planting on the growth of Casuarina equisetifolia (C. equisetifolia) have severely restricted the sustainable development of the industry. In this study, we investigated the diversity and functional changes of bacteria and fungi in the rhizosphere soil of continuously planted C. equisetifolia and their effects on soil nutrient transformation and C. equisetifolia growth. The results showed that after continuous planting, C. equisetifolia growth was significantly inhibited, the activities of nutrient transformation-related enzymes in rhizosphere soil were reduced, available nutrient content of the soil decreased, and soil bacterial diversity decreased, while fungi diversity increased. After continuous planting, 9 genera of significantly altered characteristic bacteria in the rhizosphere soil of C. equisetifolia were functionally enriched in animal parasites or symbionts, aromatic compound degradation, and nitrate reduction, with contributions mainly from the 3 characterisic bacteria such as Planctopirus, Bacillus, and Acinetobacter. After continuous planting, 7 genera of significantly altered characteristic fungi in the rhizosphere soil of C. equisetifolia were functionally enriched in soil saprotroph, lichen parasite, undefined saprotroph, endophyte, animal pathogen, wood saprotroph, litter saprotroph and plant pathogen, with contributions mainly from the 6 characteristic fungi such as Aspergillu, Fusarium, Saitozyma, Tolypocladium, Mortierella, and Funneliformis. Functional analysis and PLS-SEM equation analysis showed that the growth inhibition of C. equisetifolia due to continuous planting was the result of the joint action of the characteristic bacteria and fungi, but there was a difference between the functions of the two. The function of characteristic bacteria was mainly to provide conditions for the propagation of pathogenic organisms, which reduced soil nutrient content and hindered nutrient uptake by C. equisetifolia. The function of characteristic fungi was primarily to damage soil texture, nourish pathogenic bacteria to infest C. equisetifolia, and damage the root system to inhibit nutrient uptake. Characteristic bacteria and fungi together accelerated the effect of continuous planting on the growth of C. equisetifolia. This study provides an important reference for the cultivation regulation of continuously planted C. equisetifolia.

Chromated copper arsenate (CCA) is a wood preservative containing Cr, Cu, and As and leaching of these heavy metals into agricultural fields raises concern about food safety. The heavy metals enter the plants due to hydrophilic nature and prevalent mobility by damaging the photosynthetic process and hindering metabolism of plants. Phytoremediation is one of the constructive methods employed in mitigation of toxic metals from the soil. Nevertheless, only limited studies have been conducted on the effect of heavy metals and chelating agents on biochemical parameters in tree species. Overall, the present study advocates the possible effects of CCA components, EDTA and DTPA, on total carbohydrates, protein, and chlorophyll content of Acacia auriculiformis and Casuarina equisetifolia seedlings. Experiments on the effect of CCA and chelating agents on biochemical parameters of seedlings were conducted in potting medium. The plastic pots transplanted with 1-month-old seedlings of Acacia auriculiformis and Casuarina equisetifolia were supplied with sufficient air-dried soil and compost followed by treatment of six different concentrations (250-2500 mg kg(-1) soil) of CCA. Simultaneously, in a separate set of experiments, pots with planting media were supplied with CCA (1000 mg kg(-1) soil) followed by treatment of four different concentrations of EDTA and DTPA (0.1-1.5 mu M kg(-1)). The leaves of 6-month-old seedlings of both tree species exposed to different concentrations of CCA and chelating agents were harvested and the amount of total carbohydrates, protein, and chlorophyll content was estimated by spectrophotometric methods. The results of different treatments were compared with the control. Results of the study showed significant decrease (p <= 0.05) in the amount of total carbohydrates, proteins, and chlorophyll content with an increase in concentrations of CCA (750-2500 mg kg(-1)) in the seedlings of A. auriculiformis and C. equisetifolia. Similarly, the seedlings of both tree species treated with 1.5 mu M kg(-1) each of EDTA and DTPA showed significantly (p <= 0.05) increased total carbohydrate, proteins, and chlorophyll content. The studies conclude that the CCA components affect the total carbohydrate, protein, and chlorophyll content of the A. auriculiformis and C. equisetifolia seedlings. However, moderate to higher concentrations of EDTA and DTPA were effective in ameliorating CCA toxicity.