Soil characterization
It has been reported that plants could alter soil properties via nutrient cycling and litter decomposition (Cao et al., 2014). After 2 years of plantation, the physicochemical properties of soil in different conditions are presented in Table 1. The pH of MUD was 8.54, the pH of TDB was higher than TLB. Soil salinity was lower in soils under plant-covered sites than in MUD. These results indicated that T. chinensis could decrease soil pH and desalinization was occurred, the living status of T. chinensis exhibited higher capacity of decreasing saline-alkali soil than the death condition of plants. The SOM of plant-covered soils was obviously higher than MUD, with the highest SOM value in TLB. This result might be related to the litter decomposition and below-ground biomass (Cao et al., 2014; Chaudhary et al., 2015; Shao et al., 2015). A previous study showed that salinity and alkalinity could reduce the SOM accumulation in coastal wetlands (Xie et al., 2017). Wang et al. (2010) also illustrated that the increase in SOM could decrease the accumulation of salinity by improving soil porosity and water permeability. Levels of TN, TP, STK, and SAK decreased in the following order: MUD༞TDB༞TLB. These results indicated that the higher uptake of N, P, and K by T. chinensis to regulate N and P mineralization, osmotic adjustment (Rathore et al., 2017). Overall, the physicochemical properties of bulk soil were quite different between TLB and TDB.
Table 1
Soil physicochemical parameters of T. chinensis coverage
| TLB | TDB | MUD |
pH | 7.14 | 8.51 | 8.54 |
Moisture (%) | 19.8 | 24.2 | 63.3 |
SOM (g/kg) | 97.3 | 90.1 | 16.6 |
Salinity (g/kg) | 5.05 | 14.1 | 28.7 |
TN (mg/kg) | 316 | 350 | 504 |
AN (mg/kg) | 34.0 | 39.2 | 40.6 |
TP (mg/kg) | 174 | 484 | 520 |
AP (mg/kg) | 11.3 | 18.4 | 14.2 |
STK (%) | 1.01 | 1.49 | 1.62 |
SAK (mg/kg) | 153 | 416 | 693 |
Bacterial community diversity
Soil microbial diversity is one of the direct indicators of soil health, fertility, and function (Xu et al., 2019). The responses of soil microbial communities were investigated by high throughput sequencing. As shown in Fig. 2, it can be seen that the rarefaction curve reached plateau, suggesting the bacterial diversity covered sufficiently at the sequencing depth. Chao1 richness was analysed to compare the levels of alpha diversity indices in different treatment (Fig. 3). The indices of Chao1 in TDR was markedly higher than TLR, indicating that the bacterial communities in the rhizosphere soil of death status were more diverse. In addition, there were no obvious differences in the bulk soils of the plant species in different conditions. Liu et al. (2016) exhibited the Chao1 index of dense T. chinensis-covered bulk soils was 1,475 in November and 1,607 in September, which is in line with the present study.
The sequence data was subjected to PCoA based on weighted UniFrac distance analysis. As shown in Fig. 4 (a), the first two PCs explained a total of 41.94% variance of bacterial communities in rhizosphere soils, the bacterial communities were clearly clustered into two groups, the condition of living plant species was separated from those of death treatment. Similarly to the rhizosphere soil communities, the first two PCs explained a total of 41.33% variance of bacterial communities, there was distinct two groups clustered between the living and death T. chinensis-covered bulk soils (Fig. 4 (b)). These results indicated that the different status of plant species in both rhizosphere and bulk soils could dramatically change the bacterial community diversity.
Predominant bacterial community composition at the phylum level
The composition of the bacterial communities at the phylum level in the bulk soil and rhizosphere soil from all the samples are presented in Fig. 5. In rhizosphere soils, the major phyla were Proteobacteria, Bacteroidota, Chloroflexi, Acidobacteriota, Desulfobacterota, Planctomycetota, Gemmatimonadota, Actinobacteriota, Myxococcota, Latescibacterota, which accounted for more than 90% of the bacterial sequences obtained from the rhizosphere soils. Proteobacteria, Bacteroidota, Chloroflexi were dominant in rhizosphere samples. As for bulk soils, the main phyla were Proteobacteria, Bacteroidota, Chloroflexi, Acidobacteriota, Actinobacteriota, Gemmatimonadota, Planctomycetota, and Myxococcota. Similarly to the rhizosphere soil communities, Proteobacteria, Bacteroidota, Chloroflexi were dominant in bulk soils. It has been reported that Proteobacteria, Chloroflexi, Acidobacteria, and Bacteroidetes were the dominant bacterial phyla in welands across China (An et al., 2019). Furthermore, Jing et al. (2019) demonstrated that Proteobacteria, Bacteroidota, Chloroflexi were the dominant bacterial phyla in the rhizosphere and bulk soils of halophytes in the saline coastal wetlands of the Yellow River Delta. However, a study in coastal saline-alkali soil under T. chinensis woodland at Bohai Bay revealed that Proteobacteria, Acidobacteria, and Actinobacteria were the dominant phyla (Liu et al., 2016). It is likely that the soil type, vegetation type, seasonality, the distance to seashore line might play important role in soil characteristics and shape the composition of the bacterial communities (Liu et al., 2016; Shao et al., 2015).
Proteobacteria was the major phylum in all the samples, which is consistent with the findings of previous studies on other halophyte covered soils such as Atriplex triangularis, Suaeda glauca, Messerschmidia sibirica, Salicornia europaea, and Phragmites australis (Jing et al., 2019; Tian and Zhang, 2017; Zhang et al., 2019; Zhao et al., 2016). The phylum Proteobacteria accounted for the largest proportion across samples might be attributed to its capacity to metabolize various substrates (Eilers et al., 2010; Zhao et al., 2020). Previous report showed that Bacteroidota might participate in the decomposition of organic matter (An et al., 2022). Ward et al. (2009) reported that Acidobacteria played a vital role in the cycling of organic matter derived from plants. Therefore, the relative abundance of theses phyla in the present study might be due to the higher root exudate in coastal soils of Zhoushan Island. For comparison, higher abundance of Bacteroidota was observed in TLB as compared with TLR, indicating the bacteria in bulk soils had higher capacity of degrading organic matter than that of rhizosphere soils in the healthy ecosystem of T. chinensis. Higher abundance of Acidobacteria was found in TDR and TDB samples compared with TLR and TLB, suggesting the bacteria in death plant soils exhibited higher decomposition of organic matter derived from plants. Besides, Proteobacteria, Chloroflexi, Acidobacteriota are fast growing Gram-negative bacteria, and their carbon sources are simple and easy to use (Yin et al., 2018). Thus, it is great potential for using halophytes such as T. chinensis to ecological restore the environment. Overall, bacterial community might be used as a bioindicator for coastal ecological restoration project and wetland degradation of T. chinensis.