Soil replacement mitigates pear replant disorder by reshaping rhizosphere microbial communities and reducing putatively phytotoxic alkaloids

Replant disease frequently results in stunted tree growth, thereby constituting a pressing problem to be resolved during the renovation of old orchard. In this study, we investigated the growth of pear trees in newly established orchards and collected rhizosphere soil samples from the unrenovated old orchard, renovated orchards without soil replacement, and renovated orchards with soil replacement. The soil nutrient elements were determined using inductively coupled plasma-optical emission spectrometry and an Auto Analyzer 3. High-throughput sequencing was used to analyze rhizosphere microbial communities, and ultra-high-performance liquid chromatography-tandem mass spectrometry was used for metabolite identification. A seed germination assay of Pyrus betulifolia was further performed using the compounds obtained from metabolomic analysis. This study aimed to elucidate the effects of soil replacement on pear tree growth, soil nutrient status, rhizosphere microbial communities, and soil metabolites. Soil replacement alleviated the inhibitory effect of continuous cropping on pear tree growth. The trunk cross-sectional area, east–west crown diameter, north–south crown diameter, branch number per plant, average branch number per 667 m 2 , and yield per plant of trees planted with replaced soil were all significantly higher than those of the renovated orchard without soil replacement, with increases of 69.20%, 86.29%, 63.64%, 69.05%, 24.11%, and 131.37%, respectively. Soil replacement increased bacterial diversity in pear orchard rhizosphere soil. Acaulium , especially Acaulium retardatum , was significantly enriched in the renovated orchard without soil replacement, with relative abundances of 14.79% and 2.88%, respectively. Rhizosphere metabolites differed between soil-replacement and non-replacement treatments. In the non-replacement orchard, the content of finaconitine was significantly higher than that in the replacement orchard and the old orchard, and its structural analog aconitine inhibited seed germination and seedling growth of Pyrus betulifolia . In the soil-replaced orchard, amino acids including threonine and proline were abundant among rhizosphere metabolites, and the mixture of these two amino acids significantly promoted seed germination, epicotyl growth, and radicle growth of Pyrus betulifolia . Correlation analysis showed that the contents of finaconitine, amino acids and their derivatives were significantly positively correlated with the abundance of Gemmatimonadota. Soil replacement reduced soil Zn content, the relative abundance of microorganisms Acaulium and Acaulium retardatum , and the content of the alkaloid finaconitine. The structural analogue of finaconitine, aconitine, significantly inhibited seed germination and seedling growth of Pyrus betulifolia.