Abstract: The expansion of toxic weeds represents a key symptom of grassland degradation and exerts profound effects on ecosystem structure and function. These species often facilitate their establishment by forming a fertile island effect, yet how this process varies across large geographic scales and its underlying mechanisms remain poorly understood. Here we conducted large-scale field sampling at 20 grassland sites spanning over 3000 km to investigate the soil fertile island effect of a dominant toxic weed (Stellera chamaejasme L.) in China. We found that the presence of S. chamaejasme coincided with increased contents of soil organic carbon, dissolved organic carbon, total nitrogen, nitrate, and ammonium, with the most pronounced fertile island effects observed for soil organic carbon, dissolved organic carbon, and ammonium. Furthermore, these fertile island effects declined with increasing aridity, either directly or indirectly through microbial processes. These findings suggest that S. chamaejasme is more effective at forming the fertile island effect and promoting its expansion in wetter regions, highlighting the importance of regionally adapted strategies for toxic weed control.

Key words: Poisonous weed, Plant expansion, Soil nutrient, Plant-soil feedback, Climate gradient