Latitudinal variation in plant–soil feedbacks : contrasting patterns between the global plant invader Spartina alterniflora and a native competitor in China

Plant–soil feedbacks (PSFs) can strongly influence plant coexistence and invasions of non-native plant species. However, it is unclear how PSFs vary across large biogeographic gradients and influence interactions between native and invasive plant species. Understanding such variation and associated mechanisms will have important implications for the management of invasive plants on large geographical scales. We examined the strength of PSFs in field soils conditioned by a global plant invader Spartina alterniflora and its native competitor Phragmites australis, across the major latitudinal range of S. alterniflora (22.4–37.9°N) in Chinese coastal wetlands. We disentangled the influence of abiotic and biotic drivers underlying PSFs by using soil sterilization (to eliminate microbes) and sterilization + activated carbon addition (to eliminate microbes and secondary chemicals) in a greenhouse experiment. We found that PSFs of native P. australis shifted from neutral/negative to positive with increasing latitude, whereas PSFs of invasive S. alterniflora were positive across latitudes, with little latitudinal variations. Soil sterilization weakened positive PSFs of the invader, but enhanced positive PSFs of native P. australis at lower latitudes, thereby diminishing the latitudinal gradient of PSFs in P. australis. These results suggest that soil biota-mediated PSFs may promote S. alterniflora invasion across latitudes, with a more pronounced role at lower latitudes. Our findings illustrate contrasting latitudinal gradients in PSFs between invasive and native plant competitors, highlighting the importance of considering geographical context when evaluating the role of PSFs in plant invasions and ecological restoration.