Analysis of growth inhibition of continuously planted Casuarina equisetifolia in relation to characteristic soil microbial functions and nutrient cycling

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.