Abstract: Previous studies reported that Sonneratia apetala inhibited growth of invasive weeds Spartina alterniflora through allelopathy. In order to elucidate mechanism of soil in the allelopathy, allelopathetic effects of the soil extracts from three kinds of plant communities, i.e. S. apetala community (A), S. apetala ＋ S. alterniflora community (B) and S. alterniflora community (C), on ryegrass (Lolium perenne) were studied through allelopathetic bioassay. Results show that the soil extracts from the three kinds of communities all displayed allelopathic effects, and the effects were concentration-dependent. The effects of the soil extracts from community A and Community B were positive on growth of the roots and shoots of ryegrass when the concentration of the extracts was low, and negative when the concentration was high. The soil extracts from the three kinds of communities did not affect much germination and growth of the ryegrass when their concentration was at 0.25 g•mL^-1, but did significantly to the growth of the roots and shoots of ryegrass when the concentration was raised up to 1.0 g•mL^-1. The soil extact from Community A was higher than that from Community C in allelopathy inhibiting growth of ryegrass shoot (P＜0.01). Through root excretion and litter decomposition, S. apetala may release more allelochemicals than any other into the soil, changing soil properties, and hence inhibiting apread of S. alterniflora.