Soil Properties In The Litchi Orchards
The soil samples of litchi orchards that were maintained under conventional (CA) and sustainable (SA) agriculture were collected trimester-wise successively from October 2016 and April 2017 to observe the impact of agricultural management and temporal change. The temperatures and relative humidity at the sampling times, October 4, 2016 (CA1 and SA1), January 4, 2017 (CA2 and SA2), and April 11, 2017 (CA3 and SA3), were 29.8°C, 23.5°C, 29.9°C, and 72%, 70%, and 53%, respectively. The soil pH was ranged from 4.70 to 5.37 in the litchi orchards (Fig. 1). The pH of SA soil was slightly lower than that of CA soil. No obvious change between the EC values of CA and SA soils was observed in October 2016 and January 2017. However, a high soil EC value was detected in the CA soil of April 2017 due to the application of fertilizer in March 2017. The soil organic matter on average was 4.0% and 4.7% in the CA and SA soils, respectively, but there was no significant difference. The result of total nitrogen indicated that the content in the SA soil was slightly higher than in the CA soil. In addition, the total nitrogen was the highest in January 2017, reaching 1.8 mg g− 1 soil in both soils. No significant difference was perceived between the CA and SA soils in the extractable elements including Ca, Mg, Mn, and Cu (Table 1). The contents of P and Zn in the CA soil were higher than that in the SA soil. By contrast, the Fe trend was SA > CA. Moreover, the extractable element of K was the highest in the CA soil of April 2017. According to the Pearson correlation analysis of soil properties, soil pH had a high relationship with the extractable elements (Table 2). A positive correlation between organic matter and total nitrogen, and EC and potassium content was also documented.
Table 1
Temporal variations in the soil extractable elements of litchi orchards that were maintained under conventional (CA) and sustainable (SA) agricultural practices. 1, 2 and 3 indicate successive trimester samplings carried out on October 2016, January 2017 and April 2017, respectively. The extractable elements including P, K, Ca, Mg, Fe, Mn, Cu, and Zn were determined by inductively coupled plasma emission spectroscopy. Results are shown as mean ± standard deviation.
Elements Soils | P | K | Ca | Mg | Fe | Mn | Cu | Zn |
mg kg− 1 soil |
CA1 | 2.3 ± 1.0bc | 211.0 ± 54.7b | 583.3 ± 143.1a | 160.3 ± 18.5a | 453.3 ± 79.0a | 104.3 ± 47.5a | 4.0 ± 2.1a | 3.5 ± 0.5b |
SA1 | 1.9 ± 0.7c | 145.1 ± 52.3b | 703.5 ± 371.1a | 195.9 ± 70.5a | 428.1 ± 15.3ab | 114.5 ± 38.1a | 2.9 ± 1.5a | 4.0 ± 1.3b |
CA2 | 4.3 ± 1.2ab | 251.3 ± 88.2b | 882.6 ± 292.8a | 174.1 ± 29.9a | 419.2 ± 59.7ab | 77.0 ± 42.5a | 5.7 ± 1.8a | 7.0 ± 1.0a |
SA2 | 3.4 ± 1.2abc | 139.0 ± 47.1b | 614.9 ± 371.0a | 160.7 ± 73.3a | 471.1 ± 21.1a | 73.0 ± 22.8a | 4.3 ± 1.9a | 4.9 ± 2.2ab |
CA3 | 4.7 ± 1.5a | 543.0 ± 504.3a | 911.5 ± 411.2a | 149.5 ± 16.7a | 378.2 ± 64.6b | 84.2 ± 22.5a | 5.1 ± 1.9a | 7.0 ± 0.7a |
SA3 | 4.0 ± 1.7ab | 138.3 ± 39.6b | 633.3 ± 387.9a | 144.6 ± 67.5a | 420.5 ± 10.7ab | 99.5 ± 47.8a | 4.4 ± 2.2a | 5.5 ± 2.4ab |
Table 2
Pearson correlation analysis of soil properties. Significance is indicated by **p-value < 0.01, and *p-value < 0.05. O.M. and T.N. are organic matter and total nitrogen, respectively.
pH | –0.220 | –0.085 | –0.099 | 0.549** | 0.165 | 0.753** | 0.660** | –0.368* | 0.388* | 0.705** | 0.505* |
| EC | –0.005 | –0.189 | 0.140 | 0.714** | 0.204 | –0.181 | –0.185 | –0.204 | –0.019 | 0.312 |
| | O.M. | 0.510* | 0.173 | 0.093 | 0.245 | –0.020 | –0.092 | –0.081 | 0.165 | 0.049 |
| | | T.N. | 0.228 | –0.009 | 0.138 | –0.076 | 0.233 | –0.449* | 0.181 | 0.199 |
| | | | P | 0.230 | 0.548** | 0.241 | –0.570** | 0.106 | 0.923** | 0.731** |
| | | | | K | 0.477** | –0.063 | –0.286 | –0.156 | 0.153 | 0.225 |
| | | | | | Ca | 0.728** | –0.336 | 0.298 | 0.637** | 0.639** |
| | | | | | | Mg | –0.110 | 0.606** | 0.465* | 0.463* |
| | | | | | | | Fe | –0.440* | –0.557** | –0.287 |
| | | | | | | | | Mn | 0.291 | 0.126 |
| | | | | | | | | | Cu | 0.680** |
| | | | | | | | | | | Zn |
Soil Enzymatic Activities In The Litchi Orchards
Soil enzymes are associated with soil quality and responsible for biochemical processes (Ai et al. 2012; Schimel et al. 2017). Thus, the soil enzymatic activities of acid phosphatase, arylsulfatase, β-glucosidase, urease, and N2-fixing activities involved in phosphorus, sulfur, carbon, and nitrogen cycles were measured (Fig. 2). The result revealed that the enzymatic activities were affected not only by agricultural management but also temporal changes. Acid phosphatase and arylsulfatase in the SA soil had higher activity than in the CA soils. The enzymatic activity of arylsulfatase dropped in April 2017. By contrast, β-glucosidase and urease activities in the CA soil were higher than in the SA soils. Urease activity tended to be high in January 2017. The N2-fixing activity in soils was performed by the acetylene reduction assay. The result indicated that the N2-fixing activity in the CA soil of October 2016 was higher than that of the other soils. No significant difference was observed in the CA and SA soils of January 2017 and April 2017.
RM-ANOVA was used to assess the effect of agricultural management on different enzymatic activity across temporal change (Table 3). The result indicated significant variations in enzymatic activity with sampling time. However, a significant interaction between temporal change and agricultural management only appeared in the arylsulfatase, β-glucosidase, and urease activities.
Table 3
Significance of p-value for repeated measures ANOVA on acid phosphatase, arylsulfatase, β-glucosidase, urease, and N2-fixing activity from the litchi orchard soils.
Model term | Enzymatic activity |
Acid phosphatase | Arylsulfatase | β-Glucosidase | Urease | N2-fixing |
F | P | F | P | F | P | F | P | F | P |
Test of within-subjects effects | | | | | | | | | | |
Time | 7.785 | 0.001** | 111.941 | 0.000** | 7.130 | 0.004** | 34.217 | 0.000** | 7.596 | 0.009** |
Time × Management | 2.835 | 0.069 | 3.918 | 0.027* | 18.992 | 0.000** | 3.586 | 0.047* | 3.774 | 0.060 |
Test of between-subjects effects | | | | | | | | | | |
Intercept | 1725.628 | 0.000** | 310.086 | 0.000** | 324.388 | 0.000** | 738.748 | 0.000** | 20.710 | 0.000** |
Management | 3.433 | 0.077 | 10.950 | 0.003** | 14.856 | 0.001** | 12.037 | 0.002** | 3.692 | 0.068 |
Significance is indicated by **p-value < 0.01, and *p-value < 0.05 |
Soil bacterial community through 16S rRNA analysis using Illumina MiSeq
To analyze the diversity of the bacterial community with the soil samples, the high-throughput sequencing was used for the hypervariable V6 region of the 16S rRNA by Illumina MiSEq. Raw reads with adapter contamination, ambiguous base, and low complexity were trimmed and removed. The average clean reads and data were 119,495 reads and 31.176 Mb in each sample, respectively (Table S2). Venn diagram represented the common and unique OTUs of CA and SA soil samples (Fig. S2). The number of unique OTUs in CA and SA soils was obviously low in January 2017. In addition, the unique OTUs in the SA soil were considerably higher than those of the CA soil in October 2016 and January 2017.
The bacterial distribution in the soils was analyzed against the RDP database (Cole et al. 2014). The result revealed that Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, and Proteobacteria were the predominant phyla (Fig. 3). No significant difference was observed in the Actinobacteria and Chloroflexi between the CA and SA soils. The Acidobacteria in the SA soil was higher than that of CA soil. However, Bacteroidetes and Proteobacteria in the CA soil had a higher amount than that of SA soil. Moreover, the relative abundance of Acidobacteria, Bacteroidetes, and Proteobacteria was considerably influenced by sampling time. Relative abundance of Acidobacteria in the CA and SA soils was high in April 2017 than in October 2016 and January 2017, whereas the lower proportion of Proteobacteria was detected in April 2017. Additionally, the relative abundance of Bacteroidetes in October 2016 was 5.1- and 2.4-fold higher than that in January 2017 and April 2017, respectively. Further taxonomic division at the class-level indicated that Gammaproteobacteria was the most dominant with a relative abundance of 17.1% and 14.3% in the CA and SA soils on average, respectively.
The observed species (SOBS) and alpha-diversity including Chao, ACE, Shannon, and Simpson were evaluated under the impact of agricultural management and temporal change (Fig. 4). The result revealed that the CA soil had a higher amount of bacterial species and greater richness than that of SA soil. Nevertheless, the bacterial diversity was variable at different sampling time. The trend of bacterial diversity in both soils was following the order April 2017 > October 2016 > January 2017 according to the analysis of Shannon and Simpson indices. Beta-diversity with weighted UniFrac analysis was used to estimate the differences of soil samples in species complexity (Fig. 5). The result indicated that the five clusters can be divided, suggesting that the temporal changes distinguished from the soil samples in addition to the treatment of agricultural management.
Repeated measures analysis of variances (ANOVAs) was used to evaluate the effect of agricultural management on the bacterial predominant phyla and alpha diversity across temporal change (Table 4). The result indicated that a significant difference with temporal change was observed in Acidobacteria and Proteobacteria. A marginally significant temporal change was apparent in Bacteroidetes. However, no significant agricultural management effect that differed over temporal change was perceived in these predominant phyla. On the other hand, no significant difference with time and agricultural management was found in the SOBS, Chao, and ACE index corresponding to bacterial richness (Table 5). Nevertheless, a significant difference with time appeared in the Shannon and Simpson index corresponding to bacterial diversity.
Table 4
Significance of p-value for repeated measures ANOVA on bacterial distribution including Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, and Proteobacteria.
Model term | Bacterial distribution |
Acidobacteria | Actinobacteria | Bacteroidetes | Chloroflexi | Proteobacteria |
F | P | F | P | F | P | F | P | F | P |
Test of within-subjects effects | | | | | | | | | | |
Time | 6.331 | 0.013* | 2.743 | 0.104 | 5.084 | 0.061 | 2.498 | 0.124 | 6.382 | 0.013* |
Time × Management | 1.025 | 0.388 | 1.072 | 0.373 | 0.113 | 0.762 | 0.891 | 0.436 | 0.171 | 0.845 |
Test of between-subjects effects | | | | | | | | | | |
Intercept | 27.494 | 0.002** | 82.485 | 0.000** | 35.065 | 0.001** | 55.087 | 0.000** | 161.974 | 0.000** |
Management | 1.365 | 0.287 | 0.712 | 0.431 | 1.724 | 0.237 | 0.036 | 0.855 | 0.776 | 0.412 |
Significance is indicated by **p-value < 0.01, and *p-value < 0.05. F and P indicates the probability and significance test. |
Table 5
Significance of p-value for repeated measures ANOVA on alpha diversity of bacteria including SOBS, Chao, ACE, Shannon, and Simpson indices.
Model term | Alpha diversity |
SOBS | Chao | ACE | Shannon | Simpson |
F | P | F | P | F | P | F | P | F | P |
Test of within-subjects effects | | | | | | | | | | |
Time | 2.024 | 0.175 | 0.353 | 0.709 | 0.529 | 0.602 | 9.585 | 0.003** | 10.641 | 0.002** |
Time × Management | 1.885 | 0.194 | 0.349 | 0.712 | 0.703 | 0.514 | 0.692 | 0.520 | 0.435 | 0.657 |
Test of between-subjects effects | | | | | | | | | | |
Intercept | 389.136 | 0.000** | 386.770 | 0.000** | 380.278 | 0.000** | 1828.267 | 0.000** | 24.436 | 0.003** |
Management | 4.988 | 0.067 | 4.647 | 0.075 | 5.036 | 0.066 | 0.460 | 0.523 | 0.038 | 0.851 |
Significance is indicated by **p-value < 0.01, and *p-value < 0.05. F and P indicates the probability and significance test. |
Relationship between the bacterial community, soil enzymatic activity, and soil properties
The relationship between bacterial community, enzymatic activity, and soil properties was further explored by redundancy analysis (Fig. 6). The RDA components interpreted 68.68%, 93.33%, and 99.26% of the variation in enzymatic activity, bacterial community, and diversity, respectively. Concerning the enzymatic activity, the first component explained 41.36% of the total variation (Fig. 6A). Soil samples were clustered along the sampling time. Acid phosphatase, β-glucosidase, and urease activities were related to total nitrogen in the soils of January 2017, whereas arylsulfatase activity occurred predominantly in the SA soil of October 2016 and January 2017 and negatively correlated with EC. Moreover, β-glucosidase was associated with the available phosphorus. Regarding the dominant phyla of bacteria, clustering between soil samples was not obvious (Fig. 6B). A related correlation was found in EC and available potassium with the abundance of Proteobacteria. However, the relationship of the other dominant bacteria, such as Acidobacteria, Actinobacteria, and Chloroflexi, and soil EC, and available potassium was the opposite. A similar result of the negative association was also observed between Bacteroidetes, and soil organic matter and available phosphorus. SOBS, Chao, and ACE indices were predominated in the CA soils and positively associated with soil pH (Fig. 6C). In addition, a positive correlation between bacteria diversity such as Shannon index, and soil EC and organic matter was dominantly perceived in the soils of October 2016 and April 2017.
Multivariate linear regression ANOVA with stepwise method was used to model the relationship between the enzymatic activity, bacterial community and soil properties. Pearson correlation was developed as shown in Table 6 and Table 7. No significant association with soil property was observed in the N2-fixing activity and the dominant bacteria including Acidobacteria, Actinobacteria, Bacteroidetes, and Chloroflexi. In addition, under p-value < 0.05, acid phosphatase, arylsulfatase, β-glucosidase, SOBS, Chao, ACE, Shannon, and Simpson were used as the dependent variable in the multivariate linear regression with stepwise method. Equations were shown as follows:
β-Glucosidase = 0.015 + 0.05 × P (Model r2 = 0.390, F = 14.041, p-value = 0.001) | (1) |
Acid phosphatase = 0.212 + 0.106 × Total nitrogen (Model r2 = 0.295, F = 9.186, p-value = 0.006) | (2) |
Arylsulfatase = 0.127 + 0.463 × Total nitrogen – 0.002 × EC (Model r2 = 0.344, F = 5.495, p-value = 0.012) | (3) |
SOBS = − 924.424 + 543.719 × pH + 1.962 × EC (Model r2 = 0.769, F = 34.859, p-value = 0.000) | (4) |
Shannon = 2.963 + 0.004 × EC + 0.479 × pH (Model r2 = 0.338, F = 5.357, p-value = 0.013) | (5) |
Chao = − 1432.642 + 793.366 × pH + 2.232 × EC – 832.455 × P (Model r2 = 0.771, F = 22.501, p-value = 0.000) | (6) |
ACE = − 1601.525 + 830.783 × pH + 2.439 × EC – 892.694 × P (Model r2 = 0.790, F = 25.031, p-value = 0.000) | (7) |
Simpson = 0.063–3.203 × organic matter + 0.075 × total nitrogen (Model r2 = 0.604, F = 16.021, p-value = 0.000) | (8) |
Table 6
Pearson correlation between enzymatic activity, bacterial community, and soil properties.
| pH | EC | Organic matter | Total nitrogen | P | K |
Acid phosphatase | –0.021 | –0.172 | 0.278 | 0.543** | 0.174 | –0.245 |
Arylsulfatase | 0.116 | –0.452* | 0.163 | 0.452* | –0.249 | –0.281 |
β-Glucosidase | 0.607** | 0.156 | 0.181 | 0.154 | 0.624** | 0.266 |
Urease | –0.111 | 0.184 | –0.132 | 0.555** | 0.148 | 0.290 |
N2-fixing | 0.221 | –0.187 | –0.066 | –0.276 | –0.305 | –0.055 |
Acidobacteria | –0.260 | –0.124 | 0.161 | 0.082 | 0.147 | –0.151 |
Actinobacteria | 0.024 | –0.109 | –0.051 | –0.125 | –0.081 | –0.108 |
Bacteroidetes | 0.116 | –0.183 | –0.322 | 0.046 | –0.309 | –0.086 |
Chloroflexi | –0.129 | –0.130 | 0.082 | –0.101 | 0.019 | –0.201 |
Proteobacteria | 0.258 | 0.227 | 0.141 | 0.053 | 0.003 | 0.392* |
SOBS | 0.699** | 0.362* | –0.137 | –0.244 | 0.384* | 0.456* |
Chao | 0.731** | 0.257 | –0.299 | –0.258 | 0.332 | 0.362* |
ACE | 0.727** | 0.281 | –0.284 | –0.255 | 0.330 | 0.387* |
Shannon | 0.268 | 0.444* | 0.305 | –0.294 | 0.343* | 0.388* |
Simpson | –0.013 | –0.305 | –0.432* | 0.335 | –0.069 | –0.210 |
Significance is indicated by **p-value < 0.01, and *p-value < 0.05 |
Table 7
Pearson correlation between enzymatic activity and bacterial community.
| Acidobacteria | Actinobacteria | Bacteroidetes | Chloroflexi | Proteobacteria |
Acid phosphatase | 0.168 | 0.049 | –0.135 | 0.200 | –0.184 |
Arylsulfatase | –0.286 | 0.103 | 0.106 | –0.059 | 0.369* |
β-Glucosidase | –0.147 | –0.181 | 0.053 | 0.001 | 0.142 |
Urease | 0.048 | –0.382* | 0.263 | –0.079 | –0.075 |
N-fixing | –0.311 | –0.135 | 0.425* | –0.135 | 0.300 |
Significance is indicated by **p-value < 0.01, and *p-value < 0.05 |
On the other hand, the soil enzymatic activities were used as the dependent variable, and the dominant bacterial phyla were used as the independent variable in the stepwise multiple regression model. Under p-value < 0.05, only N2-fixing was a significant difference with the dominant bacteria. The equation was shown as follows:
N2-fixing = 0.004 + 0.001 × Bacteroidetes (Model r2 = 0.180, F = 4.843, p-value = 0.039) | (9) |