The AHL-driven cascade of rhizosphere microbes, enzymes, and nutrients contributes to the growth decline of Casuarina equisetifolia in continuous planting systems

Acyl homoserine lactones (AHL), which are extensively studied quorum sensing (QS) signaling molecules in Gram-negative bacteria, play a significant role in regulating plant growth and shaping the rhizosphere ecosystem. In this study, we investigated the effects of continuous planting across multiple generations of the Australian pine Casuarina equisetifolia (Casuarinaceae) on AHL accumulation in its rhizosphere soil. The study aims to identify key AHL and further employs an exogenous supplementation approach to assess their influence on the growth of C. equisetifolia and the associated rhizosphere soil ecosystem. Our findings reveal a progressive increase in total AHL content, rising from 1.76 to 3.65 ng/g with continuous planting generations. The key AHL that significantly alter rhizosphere soil properties under continuous planting conditions are identified as C4-HSL, 3-oxo-C10-HSL, 3-oxo-C12-HSL, and 3-oxo-C14-HSL. Following exogenous treatment with C4-HSL, an increase in its concentration was correlated with a significant enhancement in both the root length and plant height of C. equisetifolia. Conversely, treatments with 3-oxo-C10-HSL, 3-oxo-C12-HSL, and 3-oxo-C14-HSL resulted in a significant reduction in these growth parameters. While all four key AHL contributed positively to the proliferation of soil fungi and actinobacteria, their effects on bacterial populations exhibited variability. Following the exogenous application of the four key AHL, a significant reduction in the activities of urease and protease in the soil was observed. In contrast, the activities of acid phosphatase and cellulase were enhanced, leading to a decrease in the soil's available nitrogen and potassium content, while the available phosphorus content increased. Interaction effect analysis reveals that these key AHL collectively exert a strong positive regulatory effect on soil microbial abundance (0.979∗∗). Furthermore, soil microorganisms show a significant positive correlation with soil enzyme activity (0.997∗∗), whereas soil enzyme activity exhibits a strong negative correlation with the soil's available nutrient content (−0.995∗∗). Additionally, the soil's available nutrient content positively regulates the growth of C. equisetifolia (0.970∗∗). The inhibitory effect of continuous planting on C. equisetifolia growth primarily stems from reduced 3-oxo-C10-HSL levels coupled with elevated 3-oxo-C12-HSL and 3-oxo-C14-HSL concentrations in the rhizosphere. This alteration leads to a decrease in the bacterial population within the soil, which significantly reduces the activities of soil urease and protease, as well as the availability of nitrogen and potassium in the rhizosphere of C. equisetifolia. Consequently, these changes result in markedly diminished root length and dry weight of C. equisetifolia. This study provides a critical theoretical framework for the exogenous application of AHL to modulate C. equisetifolia growth in forest ecological system.