Continuous planting barrier is an important limiting factor for the growth of Casuarina equisetifolia (C. equisetifolia), an ecological and economic tree varieties, but its underlying microbial mechanism has not been fully elucidated. This study revealed microbial-driven barriers by examining the impact of continuous planting on root endophyte communities in C. equisetifolia. The results showed that continuous planting significantly reduced the antioxidant enzyme activities, root activity, and nutrient accumulation in the root system of C. equisetifolia, and inhibited root length and plant height growth. Microbial community analysis revealed that continuous planting led to a significant decrease in the abundance of Nitrobacter, a key endophytic bacterium in the root system, which weakened the nitrogen metabolism functions mediated by nitrate reduction, nitrogen respiration, and nitrate respiration, and undermined the nitrogen conversion efficiency of the root system. At the same time, continuous planting promoted the enrichment of pathogenic endophytic fungi (Phomopsis, Pseudocercospora and Diaporthe) in the root system and enhanced their plant pathogen functions, which further reduced the antioxidant and nutrient uptake capacities of the C. equisetifolia root system. It was shown that continuous planting inhibited C. equisetifolia growth through a dual microbial mechanism: on the one hand, it reduced functional flora, weakening nitrogen metabolism and stress tolerance; on the other hand, it increased pathogenic fungi and intensified disease impact. This study provides a new perspective for analyzing the endogenous microecological mechanism of the continuous planting disorder, and provides a theoretical basis for alleviating the disorder by regulating the root microbial community.
IMPORTANCEAs an important ecological and economic tree species, the sustainable management of Casuarina equisetifolia plantations is severely challenged by the continuous planting barrier. While prior studies focused on soil changes, the role of root endophytic microbes remains unclear. This study reveals coordinated changes in root endophytic microbial structure and function under continuous planting and their effects on root physiology. Continuous planting disrupts microbial stability and inhibits growth by weakening nitrogen metabolism and promoting disease. These findings deepen understanding of plant-microbe interactions and offer theoretical and practical guidance for alleviating plantation degradation via microbial regulation, aiding the development of healthy, stable, and efficient forest ecosystems.