Observed differences
Between soil samples either with or without O. sinensis, differences were more pronounced. For example, the moisture of the soil in which O. sinensis lived was higher (A, B, C: 34.85 ± 0.03, 35.22 ± 0.05, 37.74 ± 0.04 %) than that of the control group (28.32 ± 0.03 %). Furthermore, almost all absolute values of soil nutrients appeared to follow the same trend: samples containing O. sinensis had more TN (A, B, C: 6.02 ± 0.06, 6.11 ± 0.22, 6.54 ± 0.80 g/kg), more TP (A, B, C: 1.22 ± 0.09, 1.23 ± 0.03, 1.25 ± 0.04 g/kg) and for TK (A, B, C: 20.0 ± 0.72, 18.4 ± 0.42, 19.5 ± 1.17 g/kg), it is slightly more but overlapping with the control group (2.53 ± 0.25, 0.87 ± 0.01, and 17.8 ± 1.00 g/kg, respectively). For potassium, this trend was visible particularly for Kppm, which was much higher for samples that contained O. sinensis (465.2 ± 29.6, 495.7 ± 29.7, 425.4 ± 30.2 g/kg) than samples that did not (173.4 ± 16.0 g/kg). Finally, with the exception of sucrase (A, B, C: 4.18 ± 0.2, 4.10 ± 0.3, 5.11 ± 0.5, CK: 4.60 ± 0.2 g glucose•(g•drysoil•d)-1), all other enzymes were the lowest in the control group, including catalase (A, B, C: 13.42 ± 0.1, 13.43 ± 0.2, 13.08 ± 0.0, CK: 11.50 ± 0.3 mg H2O2•(g•drysoil•h)-1), urease (A, B, C: 6.43 ± 0.6, 5.60 ± 0.2, 7.53 ± 0.8, CK: 4.33 ± 0.0 mg NH3¬-N•(g•d)-1), cellulase (A, B, C: 7.50 ± 2.5, 6.88 ± 0.5, 7.20 ± 0.7, CK: 3.87 ± 0.5 ×10–2mg glucose•(g•drysoil•d)-1), and ACP ( A, B, C: 2.95 ± 0.0, 2.99 ± 0.1, 3.73 ± 0.0, CK: 2.66 ± 0.1 ×10–2µg p-nitrophenol•(g•h)-1). The nematode populations varied a lot among samples but appeared to be slightly higher in samples which did not contain O. sinensis (Table 1).
Table 1
Summary of soil physicochemical and nematodes effects on O. sinensis quality (Mean ± SD)
| A | B | C | CK | ANOVA |
F | P |
TN (g/kg) | 6.02 ± 0.06 | 6.11 ± 0.22 | 6.54 ± 0.80 | 2.53 ± 0.25 | 55.06 | ** |
TP (g/kg) | 1.22 ± 0.09 | 1.23 ± 0.03 | 1.25 ± 0.04 | 0.87 ± 0.01 | 25.03 | ** |
TK (g/kg) | 20.0 ± 0.72 | 18.4 ± 0.42 | 19.5 ± 1.17 | 17.8 ± 1.00 | 3.95 | 0.053 |
OM (g/kg) | 119.6 ± 7.9 | 139.1 ± 4.3 | 140.0 ± 8.1 | 51.0 ± 2.4 | 139.13 | ** |
AP (mg/kg) | 4.81 ± 0.44 | 3.90 ± 0.40 | 4.43 ± 0.28 | 1.90 ± 0.17 | 43.66 | ** |
Kppm (mg/kg) | 465.2 ± 29.6 | 495.7 ± 29.7 | 425.4 ± 30.2 | 173.4 ± 16.0 | 88.85 | ** |
catalase | 13.42 ± 0.1 | 13.43 ± 0.2 | 13.08 ± 0.0 | 11.50 ± 0.3 | 82.17 | ** |
urease | 6.43 ± 0.6 | 5.60 ± 0.2 | 7.53 ± 0.8 | 4.33 ± 0.0 | 23.17 | ** |
sucrase | 4.18 ± 0.2 | 4.10 ± 0.3 | 5.11 ± 0.5 | 4.60 ± 0.2 | 7.68 | * |
NR | 3.50 ± 0.4 | 1.72 ± 0.4 | 2.54 ± 0.2 | 1.56 ± 0.3 | 22.92 | ** |
cellulase | 7.50 ± 2.5 | 6.88 ± 0.5 | 7.20 ± 0.7 | 3.87 ± 0.5 | 3.95 | 0.061 |
ACP | 2.95 ± 0.0 | 2.99 ± 0.1 | 3.73 ± 0.0 | 2.66 ± 0.1 | 142.32 | ** |
nematodes | 199 ± 32 | 142 ± 90 | 206 ± 70 | 337 ± 119 | 2.90 | 0.102 |
pH | 5.79 ± 0.01 | 5.89 ± 0.01 | 5.77 ± 0.06 | 5.83 ± 0.02 | 8.64 | * |
Moisture (%) | 35.22 ± 0.05 | 37.74 ± 0.04 | 34.85 ± 0.03 | 28.32 ± 0.03 | 96.07 | ** |
Catalase (mg H2O2•(g•drysoil•h)−1); urease (mg NH3¬-N•(g•d)−1); Sucrase (g glucose•(g•drysoil•d)−1); nitratase reductase (NR) (µg NO2-N•(g•drysoil•d)−1); Cellulase (×10− 2mg glucose•(g•drysoil•d)−1); acid phosphatase (ACP) (×10− 2µg p-nitrophenol•(g•h)−1). TN: total nitrogen, TP: total phosphorus, TK: total potassium, OM: organic material, AP: organic phosphorus, Kppm: rapidly available potassium. A: high-quality O. sinensis groups; B: general-quality O. sinensis groups; C: poor-quality O. sinensis groups; CK: control group (without O. sinensis). Significance of main effect: *P < 0.05, **P < 0.01. |
Among the different quality categories (A, B, and C) of O. sinensis, we only observed a few differences. For example, a content of ca. 120 g/kg of OM appeared to be associated with the best quality (A: 119.6 ± 7.9 g/kg), whereas lower qualities had more OM (B: 139.1 ± 4.3, C: 140.0 ± 8.1 g/kg), and the control group (D) was characterized by less OM (51.0 ± 2.4 g/kg). A similar situation was found for AP, for which the best quality had the highest content (A: 4.81 ± 0.44 mg/kg), lower qualities containing slightly less (B: 3.90 ± 0.40, C: 4.43 ± 0.28) and the control containing only little (CK: 1.90 ± 0.17). Finally, samples showing a higher activity of the NR were also the group with the best quality of O. sinensis (A: 3.50 ± 0.4 [µg NO2-N•(g•drysoil•d)−1), less for those groups with lower quality O. sinensis (B: 1.72 ± 0.4, C: 2.54 ± 0.2), the control group displaying the least activity (CK: 1.56 ± 0.3).
Finally, some soil characteristics did not appear to vary among samples. This was the case of the pH, which was typically between 5.77 and 5.90 (A: 5.79 ± 0.01, B: 5.89 ± 0.01, C: 5.77 ± 0.06, CK: 5.83 ± 0.02), and the sucrase activity (A: 4.18 ± 0.2, B: 4.10 ± 0.3, C: 5.11 ± 0.5, CK: 4.60 ± 0.2 g glucose•(g•drysoil•d).
Pearson correlation between environmental variables and eigenvalue of O. sinensis
In order to build up our hypothesis, we needed to identify potential correlations. Hence, we tentatively performed statistical analyses despite pseudo-replication. These results need to be cautiously evaluated and the p-values provided are only indicative. Pearson correlation was applied to analyse interrelation of each environmental variable and eigenvalue of O. sinensis (Table 2). We found that the polypide, stroma and the fresh weight (FW) of O. sinensis were likely to be strongly correlated with each other (indicative p < 0.01), and that some environmental variables may be correlated to all three of them. These environmental variables may include catalase, TN, TP, OM, AP, Kppm, and moisture. Additional, yet probably weaker correlations (indicative p < 0.05), may exist between the eigenvalue of O. sinensis and other edaphic factors, such as cellulase, NR, TK, and nematode populations. As expected, based upon the mean values themselves (see above), the eigenvalue of O. sinensis had no distinct relationship with pH, but a positive correlation may be visible with some soil enzyme activity, moisture, and all measured soil nutrients. Among these factors, some may be correlated. For example, the respective correlations between catalase and urease with other enzymes appeared significant (indicative p < 0.01). Also, each type of soil nutrient appeared to have a significant correlation with each other, and correlations between enzymes and nutrients are also mostly significant, soil moisture did not show any significant correlations with most of the enzyme activity, but did with most of the nutrients (p < 0.01). In addition, the pH appeared to be positively correlated only to sucrase and TK, two factors that did not vary much among groups. Nematodes population showed opposite correlations as those observed for moisture (indicative p < 0.05), and nematode populations were negatively correlated (p < 0.01) with moisture. In conclusion, the quality of O. sinensis showed relationships with most of the soil enzyme activity, all soil nutrients, as well as moisture and nematode populations.
Table 2
Correlations between environmental variables and eigenvalue of O. sinensis
| Catalase | Urease | Sucrase | NR | Cellulase | ACP | TN | TP | TK | OM | AP | Kppm | pH | Moisture | Nematode | Polypide | Stroma |
Catalase | | | | | | | | | | | | | | | | | |
Urease | 0.649* | | | | | | | | | | | | | | | | |
Sucrase | –0.255 | 0.238 | | | | | | | | | | | | | | | |
NR | 0.549 | 0.608* | –0.136 | | | | | | | | | | | | | | |
Cellulase | 0.795** | 0.708* | 0.009 | 0.439 | | | | | | | | | | | | | |
ACP | 0.469 | 0.864** | 0.570 | 0.304 | 0.495 | | | | | | | | | | | | |
TN | 0.929** | 0.748** | 0.064 | 0.487 | 0.768** | 0.682* | | | | | | | | | | | |
TP | 0.903** | 0.713** | –0.027 | 0.507 | 0.652* | 0.615* | 0.968** | | | | | | | | | | |
TK | 0.600* | 0.506 | 0.301 | 0.623* | 0.688* | 0.430 | 0.681* | 0.595* | | | | | | | | | |
OM | 0.909** | 0.734** | –0.012 | 0.377 | 0.697* | 0.681* | 0.971** | 0.966** | 0.488 | | | | | | | | |
AP | 0.895** | 0.770** | –0.050 | 0.724** | 0.711* | 0.567 | 0.928** | 0.949** | 0.724** | 0.876** | | | | | | | |
Kppm | 0.969** | 0.611* | –0.241 | 0.466 | 0.711* | 0.452 | 0.944** | 0.959** | 0.545 | 0.948** | 0.911** | | | | | | |
pH | –0.063 | –0.404 | –0.630* | –0.573 | –0.287 | –0.438 | –0.267 | –0.199 | –0.741** | –0.078 | –0.366 | –0.020 | | | | | |
Moisture | 0.952** | 0.571 | –0.295 | 0.325 | 0.730* | 0.440 | 0.900** | 0.890** | 0.395 | 0.939** | 0.802** | 0.969** | 0.152 | | | | |
Nematode | –0.652* | –0.391 | 0.292 | –0.146 | –0.527 | –0.244 | –0.608* | –0.616* | –0.117 | –0.684* | –0.561 | –0.683* | –0.264 | –0.722** | | | |
Polypide | 0.981** | 0.705* | –0.234 | 0.591* | 0.784** | 0.496 | 0.941** | 0.928** | 0.586* | 0.927** | 0.933** | 0.974** | –0.108 | 0.947** | –0.677* | | |
Stroma | 0.970** | 0.659* | –0.305 | 0.652* | 0.772** | 0.402 | 0.901** | 0.894** | 0.607* | 0.871** | 0.930** | 0.955** | –0.126 | 0.915** | –0.653* | 0.991** | |
FW | 0.934** | 0.548 | –0.425 | 0.673* | 0.728* | 0.239 | 0.818** | 0.821** | 0.587* | 0.777** | 0.884** | 0.914** | –0.090 | 0.865** | –0.618* | 0.952** | 0.984** |
Significant correlations (p < 0.05) are given in italics. * represents significant correlation (p < 0.05), ** represents highly significant correlation (p < 0.01). Abbreviations: NR: nitrate reductase, ACP: acid phosphatase, TN: total nitrogen, TP: total phosphorus, TK: total potassium, OM: organic material, AP: organic phosphorus, Kppm: rapidly available potassium, FW: fresh weight of the O. sinensis |
Network of multiple environmental variables on O. sinensis in SEM
The Kaiser-Meyer-Olkin (KMO) value and Bartlett’s test of sphericity both revealed the rationality of analysis (enzyme activity: KMO = 0.736, Bartlett Sig.<0.01; nutrient: KMO = 0.787, Bartlett Sig.<0.01; O. sinensis: KMO = 0.504, Bartlett Sig.<0.01). Likewise, the accumulated variance of the factor analysis satisfied the standard requirements (enzyme activity: 72.912; nutrient: 95.361; O. sinensis: 98.358).
Based on previous studies [33–35] and our analysis, we developed an initial model by assuming that all the variables were directly correlated with the quality of O. sinensis, and that there were connections between nutrients and enzyme activity, as well as causal relationships between soil moisture, pH and soil nutrients, enzyme activity and nematodes. After fitting the model, we made progressive modifications to the original design with the remainders of the parameters until an optimized model fit was achieved (χ2 = 4.249, p = 0.643). The final model fitting is presented in Fig. 1 and Supplementary Table S4 [see Additional file 1], and constitute the testable hypothesis we provide for future research.
In this path analysis, soil moisture, nutrients and enzyme activity were associated with the higher quality of O. sinensis. First, we assumed that soil nutrients would have a strong direct effect, yet it appeared that soil nutrients possibly affected the quality of O. sinensis by altering enzyme activity. The path coefficient from nutrients to O. sinensis was only 0.10 (indicative p = 0.719), and the path coefficient from nutrients to enzyme activity was 0.92 (indicative p < 0.001). Soil moisture was also positively associated with soil nutrients (path coefficient = 0.93, p < 0.001), in addition to having a direct effect on the quality of O. sinensis. Moreover, lower pH values were associated with higher nutrient content, which in turn was associated with larger nematode populations. The values of pH did not appear to have any noticeable relationships with nematode populations.
Finally, in order to investigate the relationship between soil nutrients and O. sinensis in more details, we disassembled the one-dimensional nutrient data and estimated the contribution of the different nutrient components (which included TN, TP, OM, AP and Kppm) to O. sinensis. A similar procedure was conducted with data on soil enzyme activity (which included catalase, urease, NR and cellulase). The results indicated that TN made a large contribution to the quality of O. sinensis (92.59%) and that the remaining nutrient variables only accounted for small fractions (Kppm = 6.20%, AP = 0.90%, OM = 0.31%, TP < 0.01%). Enzyme activity, catalase and NR were found to potentially have a major contribution to the quality of O. sinensis (i.e., catalase = 74.57%, NR = 22.78%), with the remaining enzymes contributed less than 3.0% altogether (i.e., urease = 1.73%, cellulase = 0.92%). These seemingly strong results, as all of our results, are however only indicative.