Bamboo and soil properties
There were negligible differences in the heights, DBH and above-ground biomass among the moso bamboo plants of four different ages (Fig. 1). The average height, DBH and above-ground biomass of moso bamboos ranged from 13.9 to 14.9 m, 10.4 to 10.6 cm and from 16.25 to 23.33 kg, respectively.
The organic matter and total N contents in rhizospheric soil of the 0.5-yr bamboo plant were significantly (P < 0.05) higher than those of the other three counterparts, with the lowest organic matter and total N contents being in the rhizospheric soils of the 6.5-yr bamboo plant (Figs. 2a, b). The NH4+-N contents did not significantly differ among the four treatments (Fig. 2c), and the highest and lowest NO3--N contents were present in the rhizospheric soils of 0.5-yr and 4.5-yr bamboo plants, respectively (Fig. 2d).
Biological properties related to soil N transformations
The N transformation rates, functional gene abundances and urease activities of the rhizospheric soils were significantly but differently affected by moso bamboo ages (Fig. 3). The lowest N mineralization rate was detected in the rhizospheric soil of the 6.5-yr bamboo plant, which was significantly lower than those of the 2.5-yr and 4.5-yr bamboo plants (Fig. 3a). The nprA gene abundances in the four different rhizospheric soils ranged from 8.16×107 to 1.57×108 copies g-1 dry soil, and nprA gene abundances in the rhizospheric soils decreased with bamboo ages (Fig. 3b). The trend of chiA gene abundances among the four different rhizospheric soils was similar to that of N mineralization rates, and chiA gene abundance in the rhizospheric soils of the 6.5-yr bamboo plant was significantly (P < 0.05) lower than those of the other three treatments (Fig. 3c). The highest nitrification rate was in the rhizospheric soil of 4.5-yr bamboo, while the rhizospheric soil of the 2.5-yr bamboo had the lowest nitrification rate (Fig. 3d). Soil AOA amoA gene abundances decreased in order of 4.5-yr > 0.5-yr > 6.5-yr > 2.5-yr (Fig. 3e). However, the AOB amoA gene abundance in the rhizospheric soil of 0.5-yr bamboo plant was the highest among the four different treatments (Fig. 3f). Relative to that of 0.5-yr bamboo plant, average urease activities in rhizospheric soils of 2.5-yr, 4.5-yr and 6.5-yr bamboos decreased by 81.4%, 81.1% and 88.2%, respectively (P < 0.05; Fig. 3g).
Endophytic and rhizospheric microbial community diversities
After filtering, average sequence length ranged from 374 to 377 bp and from 229 to 270 bp for bacterial and fungal high-quality reads in different samples, respectively (Table S2), and the measured rarefactions of bacterial and fungal sequences all reached saturation plateaus (Fig. S1). The OTU similarities and differences of the above-ground tissues, roots and rhizospheric soils of different ages were present in four-set Venn diagrams (Fig. S2). The 0.5-yr and 6.5-yr above-ground tissues had the highest bacterial and fungal OUT numbers (Figs. S2a, d). For the roots, the highest bacterial and fungal OUT numbers were both in the 6.5-yr root (1294 and 2233, Figs. S2b, e). The unique bacterial OTUs were 69, 76, 96 and 90 for the rhizospheric soils of 0.5-yr, 2.5-yr, 4.5-yr and 6.5-yr bamboo plants, respectively (Fig. S2c), and these four different rhizospheric soils shared 500 fungal OTUs (Fig. S2f).
Both bamboo plant age and niche differentiation significantly (P < 0.05) influenced the diversity indices and richness estimators of the bacterial community (Fig. 4). Among the above-ground tissues of different ages, the 0.5-yr above-ground tissue always had the highest diversity indices and richness estimators of the bacterial community, which were significantly higher than those in the 4.5-yr tissues (Figs. 4a-d). However, for the roots, the highest diversity indices and richness estimators were in the 6.5-yr plants, and significantly (P < 0.05) different diversity indices were observed only between the 0.5-yr and 6.5-yr bamboo plants. Rhizospheric soils had significantly (P < 0.05) higher diversity indices and richness estimators than their above-ground tissue counterparts with the same age, although the four different rhizospheric soils shared similar values of community diversity. In the roots, bacterial richness estimators increased with the bamboo root ages (6.5-yr > 4.5-yr > 2.5-yr > 0.5-yr). Bamboo plant ages only significantly (P < 0.05) affected fungal community diversity in the above-ground tissues, with negligible impacts being observed in the roots and rhizospheric soils (Figs. 4e, f). Meanwhile, there were no significant differences in fungal insimpson diversity among the above-ground tissues, roots and rhizospheric soils in the same age group. In contrast, apart from the above-ground tissue, fungal richness estimators in the roots were also significantly influenced by the bamboo ages, with the lowest richness estimators being in the 0.5-yr roots (Figs. 4g, h).
Endophytic and rhizospheric microbial structures
All the bacterial OTUs belonged to the predominant phyla (> 0.1% relative abundance): Actinobacteria, Acidobacteria, Armatimonadetes, Bacteroidetes, Chlamydiae, Chloroflexi, Dependentiae, Elusimicrobia, Entotheonellaeota, Firmicutes, Gemmatimonadetes, Latescibacteria, Nitrospirae, Planctomycetes, Proteobacteria and Verrucomicrobia (Fig. S3a). At the genus level, the Enterobacter has a higher relative abundance (Fig. S3b), and in terms of the biological function, the relative abundance of bacteria related with carbohydrate transport and metabolism decreased in the order of above-ground tissue > root > soil (Fig. S3c).
Bamboo plant ages significantly (P < 0.05) affected the relative abundances of Actinobacteria and Firmicutes in the above-ground tissues, Bacteroidetes, Chloroflexi, Elusimicrobia, Nitrospirae and Planctomycetes in the roots, and Gemmatimonadetes, Latescibacteria and Verrucomicrobia in the rhizospheric soils (Fig. S4). The relative abundances of Acidobacteria, Chloroflexi, Dependentiae, and Nitrospirae in rhizospheric soils were the highest among the three niches and significantly (P < 0.05) higher than those in the above-ground tissues. Notably, there were no Latescibacteria detected in any above-ground tissues (Fig. S4j), and phyla Elusimicrobia, Nitrospirae, Planctomycetes and Verrucomicrobia were not detected in the 2.5-yr or 4.5-yr above-ground tissues, either.
The predominant phyla of fungal community were: Ascomycota, Basidiomycota, Chytridiomycota, Glomeromycota, Mortierellomycota, Mucoromycota and Rozellomycota (Fig. S5a). The genus Mortierella were detected in all samples, and 70% of the total Mortierella were from the soil samples (Fig. S5b). The relative abundances of endophyte fungi in the 48 samples ranged from 2.90% to 28.8% (Fig. S5c). In the above-ground tissues, roots and rhizospheric soil, the largest phylum was Ascomycota among all the phyla, (Fig. S6a). The relative abundances of Basidiomycota, Chytridiomycota, Glomeromycota and Mortierellomycota in the above-ground tissues and the relative abundances of Ascomycota and Basidiomycota in the roots were significantly (P < 0.05) affected by bamboo ages (Figs. S6b-e).
Comparisons of endophytic and rhizospheric microbial structures
The PCoA1 and PCoA2 of two-dimensional plots explained 27.0%-60.0% of the total variances in endophytic and rhizospheric microbial structures, and as elicited in Fig. 5, divergences in endophytic and rhizospheric microbial community structures among the same niches with different ages occurred, although to various extents. There were significant (P = 0.01 and P = 0.001) differences in bacterial and fungal communities among the above-ground tissues of different ages (Figs. 5a, d). Significant differences in the bacterial communities occurred between the 6.5-yr and 0.5-yr or 2.5-yr above-ground tissues (Fig. 5a). For the fungal communities in the above-ground tissues, significant differences were found between the 6.5-yr and the other three age groups (Fig. 5d). In the roots, bacteria rather than fungi had significantly (P = 0.024) different community structures among different age groups (Figs. 5b, e). Significant differences in bacterial communities occurred between the 6.5-yr and 0.5-yr or 2.5-yr roots, which was consistent with the results of the bacteria in above-ground tissues. Compared with the microbial community in the above-ground tissues and roots, microbes (both bacteria and fungi) in the rhizospheric soils markedly overlapped (P > 0.05) among different age groups (Figs. 5c, f).
Niche selection of key microbes
On the whole (48 samples), there were no significant differences in the bacterial communities detected in the above-ground tissues, roots or rhizospheric soils of different age bamboos (Fig. 6). However, bacterial communities were significantly different between the above-ground tissues and their corresponding rhizospheric soils, but identical in the above-ground tissues and roots (Fig. 6a). The fungal community had the same trend with its bacterial counterpart, with the microbes in the above-ground tissues and rhizospheric soils being significantly different in the PCoA1 value (Fig. 6b). The same results were also evident in the unweighted UniFrac clustering analyses (Figs. 6c, d). Regardless of bamboo ages, fungal communities in the above-ground tissues were firstly separated, and the fungi in the roots and rhizospheric soils tended to be grouped together (Fig. 6d). The microbes (both bacteria and fungi) from the above-ground tissues had longer distance from their counterparts in the rhizospheric soils, relative to the microbes in the roots. Based on the above, sample niches outweighed bamboo ages in shaping key microbial structures in the moso bamboo plant-soil system. Random forest analysis revealed that the phylum Nitrospirae displayed the most variable importance in the bacterial composition in the above-ground tissues, roots and rhizospheric soils (Fig. 6e). For the fungal community, the phylum Glomeromycota had the largest decrease accuracy (Fig. 6f).
Comprehensive linkages among moso bamboo growth, soil nutrient and microbes
The R2 values in stepwise regression models of N mineralization and nitrification rates to the regulating factors were 0.264 and 0.483, respectively (Table 1). The N mineralization rate was best described and positively correlated with chiA gene abundances. The potential nitrification rate was positively correlated with both AOB amoA gene abundance and bamboo age. The moso bamboo biomass had positive relationships with the total N and NH4+-N contents of rhizospheric soils (Table S3). Among all the predominant bacteria and fungi, only the relative abundances of phylum Chytridiomycota in the above-ground tissues, roots and soils were all positively correlated with the moso bamboo biomass (Table S3).
Table 1
Quantitative relationships among N transformation rates, functional gene abundances and soil and bamboo properties
Stepwise regression models
|
R2
|
P value
|
Net mineralization rate = 1.10 ×10-9 × chiA gene abundance – 0.25
|
0.264
|
0.042
|
Potential nitrification rate = 1.81 ×10-8 ×AOB amoA gene abundance + 0.874 × bamboo age – 4.91
|
0.483
|
0.033
|