In this study, we investigated the soil physicochemical parameters and responses of rhizospheric fungal communities of Hippophae rhamnoides to Mn stress under different sexual competition patterns. The results showed that competition significantly affects soil physicochemical properties, enzyme activity, and rhizosphere-associated fungal community structures. Under Mn stress, soils with intersexual competition had higher levels of N supply than those with the intrasexual competition. Moreover, fungal communities under intersexual interaction were more positive to Mn stress than intrasexual interaction. Under intrasexual competition, female plants had higher total phosphorus content, neutral phosphatase activity, and relative abundance of symbiotic fungi in soils to obtain phosphorus nutrients to alleviate Mn stress. In contrast, male plants had relatively stable fungal communities in soils. In the intersexual competition, rhizosphere fungal diversity and relative abundance of saprophytic fungi in male plants were significantly higher than in female plants under Mn stress. In addition, female plants showed greater plasticity in the response of rhizosphere microorganisms to their neighbors of different sexes. The microbial composition in soils of female plants varied more than male plants between intrasexual and intersexual competition. These results indicated that sex-specific competition and neighbor effects regulate the microbial community structure and function of dioecious plants under heavy metal stress, which might affect nutrient cycling and phytoremediation potential in heavy metal-contaminated soils.

(1) Background: In recent years, Hippophae rhamnoides has been used extensively to prevent desertification in China due to its nitrogen (N) fixation and sand stabilization abilities. However, as a dioecious species, few studies have focused on the sexual dimorphism of H. rhamnoides in response to sand burial, which frequently presents in desertification areas. (2) Methods: In this paper, we explored the ecophysiological responses of female and male saplings of H. rhamnoides under unburied treatment (control) and different sand burial depths (denoted as T33, T67, T90 and T133, corresponding to sand burial depths of 33, 67, 90 and 133 percent of the mean initial height of the saplings, respectively). (3) Results: Compared with unburied controls, the T33 treatment significantly promoted biomass accumulation and photosynthetic capacity, whereas T67 and T90 treatments inhibited biomass and physiological parameters of the two sexes. Deeper sand burial treatments, i.e., T90 and T133, significantly decreased the survival rates of the two sexes. Furthermore, the sex-specific responses of the two sexes of H. rhamnoides were affected by different depths of sand burial. Males had higher levels of stem starch and root sucrose and exhibited a larger increase in root nodule biomass under the T33 treatment, indicating better carbohydrate utilization and N fixation, whereas females showed lower total biomass and fewer root nodules, as well as more inhibition of photosynthetic and chlorophyll fluorescence parameters, water potential and root carbohydrates, indicating more negative effects on females than males under the T67 treatment. (4) Conclusions: We conclude that sex-related response and adaptation to sand burial depths may potentially affect the colonization, sex ratio and ecological function of the two sexes of H. rhamnoides in desertification areas.