Intercropping with oat enhances growth, active ingredient accumulation, soil fertility, and rhizosphere microbial homeostasis in Astragalus mongholicus

Long-term continuous monocropping causes rhizosphere microecological dysbiosis and constrains the yield, medicinal quality and soil sustainability of Astragalus mongholicus , a geo-authentic medicinal herb. Intercropping is a well-established ecological measure against continuous cropping obstacles, yet how companion crops with contrasting functional traits regulate the rhizosphere remains poorly understood. A two-year (2024–2025) field experiment was conducted on soil monocropped with A. mongholicus for three consecutive years, comparing monoculture (AT-AT) with sunflower- (HA-AT), maize- (ZM-AT) and oat- (AE-AT) intercropping at a 2:4 row ratio. Plant and rhizosphere soil samples were collected at the vegetative, flowering and harvest stages in both years, and bacterial (16S rRNA) and fungal (ITS) communities were sequenced for the 36 samples from 2024. Oat intercropping performed best, increasing root dry mass to 20.05 and 23.10 g per plant and raising calycosin-7-O-β- D -glucoside (0.94 and 0.88 mg/g) and astragaloside IV (0.92 and 0.85 mg/g) above all other treatments. Maize intercropping was intermediate, whereas sunflower intercropping performed poorly and even lowered soil organic matter and total nitrogen below monoculture levels. The superiority of oat was attributed to root niche complementarity between its shallow, finely branched fibrous roots and the deep taproot of A. mongholicus , which improved rhizosphere nutrient availability (at harvest in 2025, soil organic matter 22.87 vs. 16.06 g/kg and total nitrogen 1.28 vs. 0.95 g/kg under AE-AT vs. AT-AT) and selectively enriched beneficial taxa such as Bacillus and Mortierella while suppressing the pathogen Fusarium , thereby increasing the complexity and stability of the microbial co-occurrence networks. These coordinated changes in soil fertility, microbiome and root growth promoted bioactive compound accumulation. Oat- A. mongholicus intercropping therefore alleviates continuous cropping obstacles through root niche complementarity and microbial homeostasis, providing a practical strategy for the sustainable production of medicinal plants.