Latitudinal Variation in the Responses of Invasive Alternanthera philoxeroides and Native Digitaria sanguinalis to Soil Legacy Effects

Plant–soil interactions are increasingly recognized as a key driver of plant invasion, yet whether soil legacy effects associated with invaded habitats differentially influence invasive and native plants across broad geographic ranges remains poorly understood. We hypothesized that soil legacy effects would differ among soil sources from different latitudes and would affect the two species asymmetrically. Here, we address this gap by examining how latitudinal variations in soil legacy effects alter the growth and photosynthetic performance of both invasive Alternanthera philoxeroides and the co-occurring native Digitaria sanguinalis. We collected soil samples from 40 A. philoxeroides-invaded plots spanning 21° N to 37° N in China and pooled soils from five geographically adjacent sites at similar latitudes, generating eight composite soil sources (clusters 1–8, from low to high latitudes). We then conducted pot experiments with the soils originating from different latitudinal clusters to examine the effects of soil legacies on the morphology, biomass, nutrient content, and photosynthetic fluorescence of A. philoxeroides and D. sanguinalis. We found that soil legacy effects increased the maximum stem length and leaf area of D. sanguinalis, while they decreased the overall root–shoot ratio of plants at low and middle latitudes. Soil legacy effects altered the nitrogen–phosphorus ratio (N:P) of A. philoxeroides in most latitudinal clusters, with a pronounced shift toward greater phosphorus investment. In latitudinal cluster 8, soil legacy effects resulted in higher F0 and Fm values in D. sanguinalis than in A. philoxeroides under monoculture, indicating stronger PSII reaction center activity in the native species. In mixed culture, soil legacy effects eliminated the photosynthetic superiority of A. philoxeroides over D. sanguinalis. With increasing latitude, soil legacy effects shifted the photosynthetic fluorescence characteristics of the two studied species from being associated with plant growth to being associated with ecological stoichiometry. Our study indicates that soil legacy effects in A. philoxeroides-invaded habitats may not consistently promote A. philoxeroides invasion. Instead, their regulation of A. philoxeroides and D. sanguinalis performance is latitude-dependent. These findings provide a new perspective for understanding the geographic variation in mechanisms underlying plant invasion.