Fumigation-Andrographis paniculata residue integration reshapes root exudate metabolism and enhances growth in continuous cropping pepper systems

Continuous cropping obstacles in pepper ( Capsicum annuum L.) arise from soil degradation and pathogen accumulation. The combination of soil fumigant and Traditional Chinese Medicine Residue (TCMR) strategies may overcome these limitations, but their synergistic regulation of root exudate metabolism remains unclear. This study conducted pot experiments comparing six treatments: untreated continuous cropping rhizosphere soil (CK), dazomet fumigation (M1), metam sodium fumigation (W1), Andrographis paniculata residue amendment alone (C1), and combined fumigation-residue treatments (MC and WC). Growth parameters, soil properties, and root exudates were analyzed via photosynthesis measurements, enzymatic assays, and untargeted metabolomics. The combined treatments (MC and WC)markedly enhanced plant growth, with substantial increases in fresh weight (73.4–152.0%, p < 0.05), dry weight (75.0–139.1%, p < 0.05), chlorophyll content (119–148%, p < 0.01), and net photosynthetic rate (71–81%, p < 0.05), while photorespiration was strongly suppressed (58–60% reduction, p < 0.05). Soil nutrient conditions were also improved, as indicated by increases in soil organic matter (SOM, 18–24%, p < 0.05), total nitrogen (TN, 22–28%, p < 0.05), and available phosphorus (AP, 35–40%, p < 0.05). Metabolomics identified 1,167 metabolites, with MC/WC enriching nitrogen metabolism and amino acid biosynthesis. Weighted Gene Co-Expression Network Analysis (WGCNA) revealed two functional modules: the Turquoise module (amino acid metabolism) showed positive correlations with nitrogen mineralization-related parameters, while the Blue module (flavonoid/citrate metabolism) showed positive correlations with ammonium nitrogen but negative correlations with nitrate nitrogen. The combination of soil fumigant and TCMR integration is associated with alleviated continuous cropping obstacles, as suggested by improved soil fertility and altered root exudate metabolic profiles. However, as correlative analyses cannot establish causality, the proposed mechanisms require direct experimental validation.