4.1 Temporal variations of V contents in marsh soils
This paper found that the mean V levels in each layer of marsh soils over all sampling seasons ranged from 99.71 to 108.41 mg·kg− 1 which exceeded its background value in soils of Fujian province (78.3 mg·kg− 1) (Chen et al., 1992) but was slightly lower than its background value in terrestrial surface of China (112 mg·kg− 1) (Nie, 2011), implying that the V in marsh soils of the Min River estuary existed enrichment process. It was reported that the V enrichment in soils was mainly dependent on parent material and pedogenesis (Li et al., 2020a), and higher V contents generally occurred in soils originated from parent rock with higher V levels (Wang and Liu, 1994). Besides, atmospheric deposition and anthropogenic import also influenced the V enrichment in soils (Chen and Sun, 2020a). In this paper, since the study region located in the National Nature Reserve of the Min River estuary which was strictly protected in recent 20 years, the V levels in marsh soil, to a great extent, rested with its geochemical enrichment process.
This paper indicated that the V contents in soils differed among marshes or seasons (Fig. 2), which might rest with the differences in physical and chemical properties of soils in PAM, EM and CMM. The stepwise linear regression analyses showed that the variation of V levels in soils of PAM could be better explained by soil temperature (x1) (y=-0.93x1 + 122.764, R2 = 0.299, p < 0.001), while those of CMM could be explained by soil temperature (x1), sand (x2) and SOM (x3) (y=-1.093x1-0.362x2 + 1.798x3 + 131.3, R2 = 0.635, p < 0.001). By comparison, the variation of V contents in soils of EM could be better explained by pH (x4) and BD (x5) (y = 6.115x4-101.983x5 + 160.575, R2 = 0.456, p < 0.001). These implied that soil temperature, SOM and pH might be important factors influencing the temporal variations of V levels in soils of different marshes. Previous studies have reported that thermal conditions and SOM significantly affected the adsorption-desorption of metallic ions in marsh soils (Boyer et al., 2018; Li et al., 2020c) and higher SOM contents generally favored for enhancing V adsorption due to its strong complexing capacity (Du Laing et al., 2009; Zhu et al., 2016). In this paper, the variations of soil temperature over all sampling seasons could partly explain the temporal variations of V levels in soils of different marshes. Moreover, the relatively higher SOM contents in EM might also explain its higher V levels in soils (Table 1, Fig. 2). It was also reported that, under acidic condition, metals generally existed in free or ionized state which presented strong mobility (Huang, 2003; Lu and Yan, 2010). The marsh soils in the Min River estuary were acidity and were greatly affected by acid deposition (Pan, 2001; Li et al., 2020c). Although the pH in soils of PAM, EM and CMM showed narrow ranges over all sampling seasons (Table 1), the mobility of V in soils might be increased due to the lower pH values. This conclusion could partly explain the lower V levels in soils of PAM since the lower pH were observed (Fig. 2).
This paper implied that the spatial expansion between PA and CM generally increased the V contents in soils of EM over all sampling seasons (Fig. 2), and, compared with PAM and CMM, the mean values increased by 7.2% and 2.2%, respectively. As shown in Table 1, the physical and chemical properties of soils in EM were greatly altered during the spatial expansion, which might influence the variation of V levels in soils. Similar results were reported by Ehrenfeld (2003) and Chacón et al. (2009) who reported that the alterations of plant species, community structure and ecological traits during alien species invasion significantly affected the physical and chemical properties of soils. Compared with PA or CM communities, both PA' and CM' in ecotone showed higher densities but the former occupied the higher spaces while the latter occupied the lower spaces (Fig. 5b). Just for this reason, the special space combination between PA' and CM' in EM might be more favorable for intercepting the suspended particulate matter in tide. Compared with PAM and CMM, the fine particles (clay and silt) in topsoil of EM increased 13.85% and 29.10%, respectively (Table 1). Simultaneously, considerable V element might be imported into ecotone, resulting in the higher V levels in soils of EM. As shown in Fig. 5, the ecological traits of plants were also greatly altered during the spatial expansion, which might affect the variation of V levels in soils. Previous studies have reported that there were great differences in V absorption and accumulation among vegetations (Nawaz et al., 2018; Li et al., 2020a). Compared with PA or CM communities, both PA' and CM' in ecotone showed lower belowground and aboveground biomasses (Fig. 5a), indicating that the absorption amounts of V by the two plants might be not very high and this, to some extent, could explain the higher V levels in EM soil.
In order to better visualize the possible environmental gradients determining the temporal variation of V contents in soils of different marshes, the principal component analysis (PCA) was conducted (Fig. 6). In PAM, two principle components explained 92.98% of the variance. Principal component 1 (PC1), which explained 87.19% of the total variance, represented the gradient variations of plant height and aboveground biomass. Principal component 2 (PC2), which explained 5.79% of the total variance, showed the gradient variations of plant density, soil temperature and SOM. Further analyses indicated that the V levels showed strong correlation with PC2. By comparison, 84.73% of the total variance of environmental variables in EM was explained by two principle components (PC1 and PC2). PC1 represented the gradient variation of plant height and aboveground biomass, whereas PC2 showed the gradient variation of plant density and belowground biomass. Generally, the V levels showed close correlation with PC2. For CMM, 59.38% and 13.95% of the total variance of environmental variables were explained by PC1 and PC2, respectively. PC1 represented the gradient variations of plant height, aboveground biomass and soil pH, while PC2 showed the gradient variations of plant density and belowground biomass. As a whole, the V levels showed strong correlation with PC1. The above analyses indicated that the temporal variation of V levels in soils of EM, to a great extent, rested with the alterations of pH, SOM and plant ecological traits during the spatial expansion between PA and CM.
4.2 Accumulation and transference of V in marsh plants
This paper implied that the V contents in PA, PA', CM' and CM differed among tissues, and, over all sampling seasons, the values in roots were significantly higher than those in other tissues (Fig. 3). Previous studies have indicated that, as V existed in growing medium, bioaccumulation was a key adaptive strategy for most plants (Saco et al., 2013; Hou et al., 2014). The bioaccumulation for V generally occurred in roots, which was 2 ~ 1000 folds of aboveground parts (Aihemaiti et al., 2020). Moreover, the V in roots could form stable compound with calcium through the chelating and complexating the polar compound in cytoderm, which retarded the transference of soluble vanadium ion and reduced the V bioaccumulation in aboveground parts (Kaplan et al., 1990). In this paper, the R/S and R/L ratios in PA, PA', CM' or CM were larger than 1 (Table 2), implying that the V contents in roots of the four plants were significantly higher than those in aboveground parts. This conclusion could be verified by the higher [BCF] in roots of different plants over all sampling seasons (Table 3). Besides, the S/L ratios in marsh plants were less than 1 (Table 2), indicating that the limited V nutrient transferred from roots to aboveground parts might be preferentially allocated to leaves and this was favorable for the biosynthesis of chlorophyll and the metabolism of carbohydrates in photosynthesis process (Nawaz et al., 2018). It was noting that, over all sampling seasons, the V contents in standing litters were much higher that those in stems and leaves (Fig. 3), which might be dependent on the nutrients (including V) absorbed by plants and the nutrients retained in standing litters (Chen and Sun, 2020b). As mentioned above, the V in living bodies could form stable compound with calcium in cytoderm, which indicated that, as plant withered, the V in these compounds could be stranded in standing litters in large numbers due to their poor mobility.
This paper showed that the V contents in tissues differed among species. Over all sampling seasons, the V levels in tissues of PA' were generally higher than those of PA and, similarly, the values in organs of CM' were much higher than those of CM (Fig. 3). The probable reason was related to the alteration of plant ecological traits and the competitive absorption for nutrients (including V) by different plants during the spatial expansion. Compared to the pure community (PA or CM), the aboveground and belowground biomasses of PA' (or CM') significantly decreased (Fig. 5a), indicating that both the living spaces of PA' and CM' in ecotone were squeezed severely and their competitiveness for nutrient might be intense. Previous studies have found that, in habitat with limited nutrient, the competition advantage of plants generally rested with their conserved utilization for limited resources (Sardans and Peñuelas, 2014; Wang et al., 2018). It was reported that the spatial expansion of PA and CM in the Min River estuary was bi-directional (He et al., 2018) and the competitions between them rested with their ecological adaptation strategies (Li et al., 2020a). In this paper, the V contents and the [BCF] in tissues of CM' were generally higher than those of PA' over all sampling seasons (Fig. 3, Table 3), which indicated that the PA' and CM' in ecotone were very likely adopt different strategies for V absorption and utilization to maintain their competitiveness. Compared to PA, the density of PA' increased but its height, R/S and R/L ratios decreased greatly (Fig. 5, Table 2), implying that the P. australis might compete primarily by increasing the number of tillering and transferring the V accumulated in roots to the photosynthetic organ (leaf) preferentially. However, compared with CM, the density of CM' decreased but its height, R/S and R/L ratios generally increased (Fig. 5, Table 2), indicating that the C. malaccensis might resist the spatial expansion of P. australis by increasing the bioaccumulation of V in roots, decreasing the number of tillering and expanding the space of aboveground parts.
This paper indicated that the V levels in tissues of PA, PA', CM' and CM differed among seasons, which could be better interpreted by the differences in growth rhythm and ecological traits among plants. In this study, the V contents in roots and leaves of PA (or PA') during spring were the highest, while those in summer and autumn were much lower (Fig. 3), implying that, compared to the vigorous growth stage, the roots of PA (or PA') at initial growth stage showed higher V bioaccumulation and higher transference from roots to leaves. By comparison, the V levels in roots and stems of CM (or CM') achieved the higher values in winter (Fig. 3). Previous studies have reported that the aboveground parts of PA (or PA') almost withered in winter, while those of CM (or CM') were not dead and, particularly, stem was the main body of aboveground parts (>94%) (Wu et al., 2020; Li et al., 2020a). Thus, in order to keep alive and enhance the stress resistance for the lower temperatures in winter, the V accumulated in roots of CM (or CM') might be greatly transferred to stems. This paper also indicated that, over all sampling seasons, both the V stocks in roots of PA' and CM' were much higher, but the values in stems and leaves of the former were generally higher than those of the latter (Table 4). As mentioned above, the two plants might adopt different strategies for V absorption and utilization during the spatial expansion. Compared to CM', the PA' in ecotone might preferentially transfer the V in roots to the aboveground parts to maintain its competitiveness, which resulted in the higher V stock in its aboveground parts.