Accumulation and distribution of N in flue-cured tobacco growing in different soils
Accumulation dynamics of N in different soils
Nitrogen gradually increased in loam soil, clay loam, and sandy loam soils with plant growth (Fig. 1), attaining a maximum at the mature-plant stage(2.10 g/plant, 1.43 g/plant, and 2.90 g/plant, respectively). Nitrogen accumulation was lower in plants grown in clay loam than in plants grown in loam soil and sandy loam during the entire growth period, indicating that the N supply capacity of clay loam was relatively weak, and tobacco plants grown in this soil had the lowest levels of N uptake and utilisation. The N uptake and accumulation in flue-cured tobacco grown in loam soil and sandy loam were basically the same before the ceiling stage, but at the mature stage, N accumulation was significantly higher in plants grown in sandy loam than in plants grown in loam soil and clay loam (P < 0.05).
Accumulation dynamics of N in different soils.
Nitrogen use efficiency of flue-cured tobacco in different texture soils
Flue-cured tobacco N use efficiency was similar among plants grown in clay loam and loam soils (Fig. 2); in both soils, N use efficiency improved steadily from the rosette stage to the ceiling stage, at which point it reached a maximum(40.7% and 34.5%, respectively). The N use efficiency of flue-cured tobacco grown in clay loam decreased significantly after reaching the ceiling stage. At the mature stage, N use efficiency of plants grown in clay loam and loam soils fell to 21.7% and 29.2%, respectively.
N use efficiency of flue-cured tobacco in different texture soils.
In sandy loam, N use efficiency of flue-cured tobacco increased during the growth period, from 5.3% at the rosette stage to 43.7% at the mature stage. We observed significant differences between N use efficiency between tobacco grown in sandy loam and in clay loam (P < 0.05), an indication that different soil textures had a significant effect on tobacco N uptake and utilisation during the growing phase. In sandy loam soil, tobacco N use efficiency was lower in the early growth stages and gradually increased over the course of the growth period, whereas that of plants grown in clay soil and loam was lower in the later growth stages than in the earlier growth stages.
N accumulation in different organs of flue-cured tobacco grown in different soils
Prior to the ceiling stage, no significant differences were observed in N accumulation in the roots of flue-cured tobacco grown in soils of different textures (P > 0.05), as shown in Table 2. Following the ceiling stage, however, N accumulation in the roots was significantly higher in plants grown in loam soil and sandy loam than in plants grown in clay loam (P < 0.05). At the mature stage, N accumulation in the roots of plants grown in loam soil, clay loam, and sandy loam was 0.82 g/plant, 0.40 g/plant, and 0.70 g/plant, respectively. N accumulation in the stems of tobacco grown in all three soils increased with growth phase. At the rosette stage, N accumulation in the stems of plants grown in clay loam was significantly higher than in plants grown in loam soil and sandy loam (P < 0.05); from the ceiling stage to the mature stage, N uptake by tobacco grown in sandy loam increased, reaching a maximum (1.62 g) at the mature stage, whereas N absorption ceased in stems of tobacco grown in loam soil. Changes in N accumulation in leaves of tobacco grown in loam soil and clay loam were consistent from the rosette to the mature stage, and maximum N accumulation in leaves of tobacco grown in these two soils occurred in the ceiling stage (2.82 g/plant and 2.72 g/plant, respectively); values fell rapidly after this stage, however. Prior to the budding stage, N accumulation in leaves of plants grown in sandy loam was lower than in plants grown in loam soil and clay loam, and gradually increased after this stage, reaching the highest level (3.32 g/plant) at the mature stage.
Effects of soil texture and N fertilisation on soil bacterial community diversity and structure
Richness and alpha diversity indices of soil bacterial communities
The effects of soil texture and N fertilisation on soil bacteria richness and diversity are shown in Table 3. In the control treatment (CK), the bacterial community richness (Chao1 and ACE) of sandy loam was higher than that of loam soil and clay loam, whereas in the N fertilisation treatments (T1 and T2), Chao1 and ACE indices were higher for sandy loam and loam soil than clay loam. Chao1 and ACE were also higher in loam soil and clay loam soils under the T1 treatment than the T2 and CK treatments, whereas for sandy loam soils, these indices were highest under the T2 treatment, followed in order by the CK and T1 treatments. These results suggest that N fertilisation can increase bacterial community richness in loam and clay loam soils but reduce bacterial community richness in sandy loam soil. Shannon and Simpson indices were used to investigate bacterial community diversity. In the CK treatment, the Shannon index was highest in loam soil and lowest in clay loam soil, whereas in the T1 treatment, it was highest in sandy loam and lowest in clay loam soils, with a similar trend o observed in the T2 treatment. This pattern indicated that bacterial diversity was lowest in clay loam. In contrast, the Simpson index exhibited an opposite trend to that of the Shannon index, indicating that bacterial diversity was greatest in clay loam soil regardless of N fertilisation. Results of ANOVA testing indicated that both N fertilisation and soil texture significantly affected bacterial community diversity and structure.
Beta diversity analysis of bacterial community structure
The results of a PCA based on the OTU levels are shown in Fig. 3. Using PCA, it was possible to extract two axes that reflected the differences between the samples. The PCA suggested an obvious separation of the bacterial communities among the different treatments, indicating that the bacterial communities in soils with different textures differed significantly under the same fertilisation treatments. In sandy loam and loam soils, bacterial communities were similar under T1 and T2 conditions, but both differed considerably from CK, whereas in clay loam soil, the bacterial communities of CK, T1, and T2 differed greatly. The results indicated that soil texture and N fertilisation were important factors affecting changes in soil bacterial communities.
PCA analysis of different treatments.
Relative abundance of dominant bacterial communities at different taxonomic levels
The RDP classifier Bayesian algorithm was used to classify and analyse 97% of OTU representative sequences at similar taxonomic levels. The community composition and relative abundance of bacteria in each sample were calculated at various taxonomic levels (phylum, class, order, family, genus, and species) (Table 4).
Under the different soil textures in CK treatments, the relative abundance of dominant bacterial communities at these six taxonomic levels was highest in clay loam followed by loam soil and sandy loam, the latter two of which were similar to each other. In the T1 treatment, the relative abundance of dominant communities did not significantly differ between soils of different textures at the six taxonomic levels. In the T2 treatment, the dominant abundance of bacteria at five taxonomic levels (phylum, class, order, family, and genus) was highest in clay loam followed by loam soil, and was lowest in sandy loam. In contrast, at the species level, the relative abundance of dominant communities was lowest in clay loam and highest in sandy loam. When comparing different fertilisation treatments in the same soil texture, N fertilisation was found to have a bigger impact on the relative abundance of dominant communities in clay loam than in the other two soils at the class, order, family, and genus levels. Moreover, N fertilisation reduced the relative abundance of dominant bacterial communities in clay loam and loam soils, but did not have the same effect in sandy loam soil.
Analysis of bacterial community composition
Sample sequences were classified at the phylum level, with a total of 39 taxa identified; phyla with relative abundance above 1% are shown in Fig. 4. Both soil texture and N fertilisation significantly influenced the relative abundance of the most prominent bacterial phyla in the soil samples. Most bacterial phyla and classes detected in the samples were affected by soil texture, regardless of N fertilisation.
Relative abundance of the bacterial predominant phyla for different samples. LS loam soil, SL sandy loam, CL clay loam, CK no fertiliser, T1 conventional fertilisation, T2 no nitrogen fertiliser.
The dominant bacterial phyla differed among soil types. In loam soil, Proteobacteria (32.10–34.75%), Firmicutes (17.91–25.58%), Acidobacteria (14.20–23.79%), and Bacteroidetes (5.02–9.98%) were the dominant phyla, whereas in clay loam soil, abundances of Firmicutes (20.41–50.94%), Proteobacteria (20.09–35.17%), Acidobacteria (13.04–14.32%), and Bacteroides (5.01–5.79%) exceeded 5%. In sandy loam, abundances of five species of bacteria exceeded 5%, consisting of Proteobacteria (24.38–38.96%), Firmicutes (17.65–28.31%), Acidobacteria (12.79–22.31%), Chloroflexi (4.95–7.59%), and Actinobacteria (2.69–9.45%).
N fertilisation had a limited and variable effect on the abundances of the primary bacterial phyla. Compared with the loam soil CK, the T1 and T2 fertilisation treatments had no significant effect on the abundance of Proteobacteria, whereas Acidobacteria declined by 31.09%and Firmicutes and Bacteroidetes increased by 44.39% and 26.31%, respectively.
To further examine the effects of soil texture and N fertilisation on soil bacterial community structure, the classification and relative abundance of OTUs were analysed at the genus level. A total of 43 genera were detected in all N fertilisation-soil texture combinations (Fig. 5). The number of species and abundances of bacteria differed among treatments, indicating that the bacterial distribution in the various samples was extremely diverse. Of these, Lactococcus (11.52–46.59%), Pseudomonas (1.21–7.47%), Subgroup_6_norank (1.49–9.74%), Unclassified (2.48–6.16%), Uncultured (9.98–20.60%), and Uncultured_norank (1.00–2.91%) were the most common groups of bacteria in the soils of all treatments, while Uncultured and Lactococcus were the dominant types in all treatments. In addition, the top three unique types in loam, sandy loam, and clay loam soils in the CK treatment were Subgroup_6_norank (9.74%), Xanthomonas (8.45%), and Acinetobacter (4.46%); in the T1 treatment, Unclassified(5.33%), Bacillus(8.49%), and Xanthomonas (8.19%); and in the T2 treatment, Massilia (5.58%), Unclassified (6.16%), and Pseudomonas(7.47%).
Relative abundance of the bacterial predominant genus for different samples.
Relative abundance of N-transforming bacteria in soils of different textures
The effects of soil texture and N fertilisation on the estimated OTUs of soil N-transforming bacteria are shown in Table 5. Bacillus, Nitrobacter, Nitrosospira, Nitrospira, and Rhizobium were the most common N-transforming bacterial genera identified in all soil samples. Bacillus and Nitrospira OTUs were more abundant than those of the other N-transforming bacteria; the OTUs of Nitrosospira and Rhizobium in clay loam soil were higher than those in loam and sandy loam soils, while the OTUs of Bacillusin sandy loam soil were the highest among all soil types. In addition, Bacillus and Nitrobacter were both higher in sandy loam soil than in the other soils. In terms of the fertilisation treatments, the OTUs of Bacillus, Nitrosospira, and Nitrobacter were consistently higher in T1 than in T2 and CK across all soil types, while the OTUs of Nitrospira were slightly lower in T1 than in T2 and the OTUs of Rhizobium were highest in CK.
Source: Ecology - nature.com
