#These two authors contributed equally to this work
Soil potentially hazardous metal (PHM) is continually attracting public attention worldwide, due to its highly toxic properties and potentially huge damage to human being through food chain. Phytoremediation is an effective and eco-friendly way in remediation technology. A pot experiment was carried out to investigate the effect of different organic materials (biogas residue (BR), mushroom residue (MR), and bamboo-shoot shell (BS)) application on phytoremediation of two PHM-contaminated soils (Fuyang soil as ‘heavily-polluted soil’ and Wenzhou soil as ‘moderately-polluted soil’, respectively) by
Potentially hazardous metal (PHM) pollution of arable soils is attracting more and more attention worldwide due to the high toxicity, persistence and easy-bioaccumulation characteristics of PHMs [
Applying organic fertilizer or organic material into soils is a practicable method, which could improve soil fertility and activate soil PHMs by dissolved organic matter (DOM) such as small molecule organic acids, leading to the availability change of PHMs for plants uptake [
In China, a large number of organic wastes are produced in the process of agricultural production, including biogas residue (BR), mushroom residue (MS), bamboo shell (BS) and so on. The application of organic wastes as manure not only reduces their pollution to the environment, but also reduces the use of chemical fertilizer [
The Cd-contaminated farmland soils were collected in two places (Wenzhou and Fuyang cities). Based on GB 15618-2018 in China [
Experimental materials | Moderately-polluted soil | Heavily-polluted soil | Biogas residue | Mushroom residue | Bamboo shell |
---|---|---|---|---|---|
pH | 5.32 | 5.64 | 6.50 | 7.80 | 6.80 |
OM (%) | 4.05 | 2.86 | 52.72 | 75.11 | 56.22 |
CEC (cmol kg−1) | 20.60 | 9.19 | – | – | – |
TN (%) | – | – | 1.72 | 1.14 | 2.24 |
TP (%) | – | – | 0.20 | 0.10 | 0.44 |
TK (%) | – | – | 0.91 | 0.60 | 1.35 |
AN (mg kg−1) | 246.96 | 197.72 | – | – | – |
AP (mg kg−1) | 74.86 | 9.19 | – | – | – |
AK (mg kg−1) | 365.50 | 88.50 | – | – | – |
TCu (mg kg−1) | 84.64 | 1183.48 | 25.50 | 10.00 | 8.43 |
TZn (mg kg−1) | 228.15 | 2403.15 | 256.25 | 85.40 | 7.59 |
TPb (mg kg−1) | 77.35 | 817.75 | 1.75 | 3.30 | 0.23 |
TCd (mg kg−1) | 2.65 | 13.15 | 0.25 | nd1) | nd |
ACu (mg kg−1) | 8.14 | 768.73 | – | – | – |
AZn (mg kg−1) | 54.20 | 1140.36 | – | – | – |
APb (mg kg−1) | 11.71 | 515.01 | – | – | – |
ACd (mg kg−1) | 0.49 | 2.61 | – | – | – |
Note: “nd” means the content was undetected; OM: organic matter; CEC: cation exchange capacity; TN: total nitrogen; TP: total phosphorus; TK: total potassium; AN: available nitrogen; AP: available phosphorus; AK: available potassium; TCu: total copper; TZn: total zinc; TPb: total lead; TCd: total cadmium; ACu: available copper; AZn: available zinc; APb: available lead; ACd: available cadmium.
The PHM hyperaccumulator
The biogas residue (BR) was collected from a pig manure digester in an organic farm of Yuhang County, Hangzhou City, Zhejiang Province. The mushroom residue was collected in a farm of Jiaxing City, Zhejiang Province, where the black fungus cultivation was carried out. The main raw material of fungus-stick was mulberry sawdust (about 80%). The auxiliary materials were wheat bran, cotton seed hull and lime. The bamboo-shoot shell was collected from an agricultural product processing factory in Lin’an County, Hangzhou City, Zhejiang Province. After mushroom residue (MR) and bamboo-shoot shell (BS) were fermented, three organic materials were air-dried and were ground for sieving through 2 mm nylon mesh. The basic properties are shown in
Each organic material (BR, MR, BS) was mixed with the soil according to the organic material-soil ratio of 1% and 5%, respectively. Therefore, a total of 7 treatments were set up including a control (no organic material application) (
Treatments | Moderately-polluted soil (mg kg−1) | Heavily-polluted soil | ||||||
---|---|---|---|---|---|---|---|---|
ACu | AZn | APb | ACd | ACu |
AZn |
APb |
ACd |
|
CK | 7.6 ± 0.8 b | 46.96 ± 4.1b | 11.05 ± 1.5a | 0.47 ± 0.03a | 0.8 ± 0.06bc | 0.99 ± 0.12c | 0.49 ± 0.04d | 1.8 ± 0.13b |
1% BR | 7.0 ± 0.7b | 44.22 ± 3.4b | 9.35 ± 1.4bcd | 0.35 ± 0.05d | 0.84 ± 0.07ab | 1.00 ± 0.1b | 0.53 ± 0.05ab | 1.8 ± 0.20b |
5% BR | 8.4 ± 1.1a | 68.51 ± 7.1a | 10.93 ± 1.3a | 0.37 ± 0.04cd | 0.70 ± 0.05d | 0.87 ± 0.05d | 0.41 ± 0.03e | 1.5 ± 0.08c |
1% MR | 7.1 ± 0.5b | 45.26 ± 5.0b | 9.70 ± 0.7bc | 0.41 ± 0.03b | 0.81 ± 0.1b | 1.03 ± 0.08bc | 0.52 ± 0.05bc | 1.9 ± 0.12 b |
5% MR | 5.3 ± 0.6c | 44.23 ± 3.6b | 8.05 ± 0.9d | 0.39 ± 0.06bc | 0.77 ± 0.07c | 1.00 ± 0.08bc | 0.47 ± 0.05d | 1.7 ± 0.46b |
1% BS | 6.9 ± 0.7b | 40.38 ± 5.1b | 8.85 ± 1.0cd | 0.31 ± 0.05e | 0.86 ± 0.09a | 1.11 ± 0.09a | 0.56 ± 0.04a | 2.3 ± 0.12a |
5% BS | 7.2 ± 1.2b | 43.63 ± 3.5b | 10.62 ± 1.3ab | 0.40 ± 0.04bc | 0.76 ± 0.06c | 1.03 ± 0.07bc | 0.49 ± 0.04cd | 1.9 ± 0.11b |
Note: ACu: available copper; AZn: available zinc; APb: available lead; ACd: available cadmium. Different lower cases on the same column indicated significant differences between treatments (
Plant samples were washed with deionized water and were measured their fresh weight. Then
Soil samples were air-dried and sieved to pass a 5 mm nylon mesh. Soil pH was directly determined using a pH meter at a soil to water ratio of 1:2.5. (
The available PHMs of soil samples were extracted by HCl (0.1 mol L−1) and determined by ICP-OES (Optima 7000 DV, PerkinElmer) [
The one-way analysis of variance (ANOVA) was used to determine the significant differences among different treatments at the significant level of 0.05. The ANOVA was carried out using SPSS 20.0. All the plots were produced by origin software.
Compared with CK, pH values of most treatments decreased, except 5% MR treatment (
Adding 5% organic material had a marked increase in soil organic matter (SOM). However, adding 1% organic materials had no significant effect on SOM (
For Moderately-polluted soil, compared to the control, most treatments (except the Cu and Zn in 5% BR treatment) decreased the available Cu, Zn, Pb and Cd concentration in the soil (
Treatments | Moderately-polluted soil | Heavily-polluted soil | |||||||
---|---|---|---|---|---|---|---|---|---|
ACu | AZn | APb | ACd | ACu | AZn | APb | ACd | ||
Original | 9.57% | 23.40% | 15.13% | 18.47% | 64.93% | 47.38% | 62.97% | 19.84% | |
CK | 8.93% | 20.29% | 14.21% | 17.72% | 67.57% | 41.14% | 59.92% | 13.69% | |
1% BR | 8.23% | 18.99% | 12.14% | 13.20% | 70.95% | 41.55% | 64.81% | 13.69% | |
5% BR | 9.87% | 29.79% | 14.21% | 13.95% | 58.28% | 36.15% | 50.13% | 11.40% | |
1% MR | 8.35% | 19.43% | 12.53% | 15.46% | 68.42% | 41.55% | 63.59% | 14.45% | |
5% MR | 6.23% | 18.99% | 10.46% | 14.70% | 65.04% | 41.55% | 57.47% | 12.93% | |
1% BS | 8.11% | 17.27% | 11.50% | 11.69% | 72.64% | 45.71% | 68.48% | 17.49% | |
5% BS | 8.46% | 18.99% | 14.21% | 15.08% | 64.19% | 41.55% | 59.92% | 14.45% |
Note: The availability of PHMs means (available concentration of a metal)/(total concentration of the metal) * 100%.
The available concentration of PHMs varied depending on the types and dosages of organic materials in heavily-polluted soil (
The results from our pot experiment showed that organic material types and their application rates had different effects on the concentrations of available four PHMs in soils and their availability. Obviously, this was related to the complex reaction process in the soil system. The effects of organic materials on the bioavailability of PHMs depend on the properties of organic materials, the internal redox, the soil types and the characteristics of PHM ions [
For Cu and Zn in moderately-polluted soil, 5% BR increased their contents, while 1% BR had no significant effect on their contents. However, the inverse phenomena were found in heavily-polluted soil. Both the nutrient contents and cation exchange capacity in moderately-polluted soil were higher than heavily-polluted soil, while the contents of PHMs were lower than heavily-polluted soil (
For Cd, almost all treatments in this study restrained the available Cd to some extent. However, only the BS treatments had an activation effect on Cd in Heavily-polluted soil. Previous studies revealed that the root exudates and rhizosphere microorganisms (such as arbuscular mycorrhiza and endophytic mycorrhiza) of
Overall, the addition of organic materials enhanced the shoot dry weight of
Compared with control, the PHM contents in shoots of
Treatments | Moderately-polluted soil | Heavily-polluted soil | ||||||
---|---|---|---|---|---|---|---|---|
Cu |
Zn |
Pb |
Cd |
Cu |
Zn |
Pb |
Cd |
|
CK | 4.6 ± 0.35bc | 4.03 ± 0.30c | 1.0 ± 0.06a | 36 ± 3.0a | 35 ± 3.0a | 16.82 ± 1.00a | 0.19 ± 0.01a | 0.47 ± 0.01 a |
1% BR | 4.3 ± 0.20c | 4.45 ± 0.35a | 0.42 ± 0.03d | 31 ± 2.0b | 30 ± 2.0ab | 15.86 ± 0.8b | 0.14 ± 0.01b | 0.41 ± 0.01ab |
5% BR | 5.0 ± 0.45ab | 4.80 ± 0.32a | 0.41 ± 0.05d | 29 ± 2.0b | 24 ± 1.0c | 14.07 ± 0.50c | 0.10 ± 0.01c | 0.31 ± 0.01cd |
1% MR | 3.5 ± 0.32d | 3.33 ± 0.30d | 0.50 ± 0.05d | 23 ± 1.0c | 24 ± 2.0c | 12.70 ± 0.50d | 0.11 ± 0.01c | 0.32 ± 0.01cd |
5% MR | 3.0 ± 0.25d | 2.72 ± 0.25e | 0.43 ± 0.03 d | 21 ± 2.0c | 11 ± 0.80d | 11.92 ± 0.40d | 0.05 ± 0.01d | 0.20 ± 0.01e |
1% BS | 4.8 ± 0.40abc | 4.40 ± 0.50b | 0.75 ± 0.05b | 36 ± 3.0a | 25 ± 2.0bc | 14.74 ± 0.50c | 0.13 ± 0.01b | 0.36 ± 0.01c |
5% BS | 5.3 ± 0.68a | 3.66 ± 0.25d | 0.52 ± 0.04c | 24 ± 2.0c | 20 ± 2.0c | 11.83 ± 0.50d | 0.10 ± 0.01c | 0.27 ± 0.01d |
Note: Different lower cases on the same column in the table indicated significant differences between treatments (
Organic material reduced the content of Cd in plant shoots. The 5% MR treatment decreased by 57.6%, compared to the control. In heavily-polluted soil, Cu, Zn, Pb and Cd contents in shoots of
To evaluate the efficiency of
Treatments | Moderately-polluted soil | Heavily-polluted soil | ||||||
---|---|---|---|---|---|---|---|---|
Cu |
Zn |
Pb |
Cd |
Cu |
Zn |
Pb |
Cd |
|
CK | 6.5 ± 0.5 b | 5.51 ± 0.40cd | 1.40 ± 0.10a | 48.96 ± 4bc | 39.47 ± 3ab | 19.63 ± 1.00cd | 0.22 ± 0.02a | 0.53 ± 0.05bc |
1% BR | 6.6 ± 0.60b | 5.99 ± 0.50c | 0.51 ± 0.05e | 41.03 ± 4c | 44.33 ± 4a | 22.86 ± 1.00bc | 0.21 ± 0.02ab | 0.60 ± 0.05a |
5% BR | 8.7 ± 0.70a | 7.69 ± 0.60a | 0.75 ± 0.06d | 43.48 ± 3c | 45.44 ± 3a | 24.68 ± 1.20ab | 0.16 ± 0.01c | 0.56 ± 0.06ab |
1% MR | 4.5 ± 0.37c | 4.28 ± 0.40e | 0.82 ± 0.07cd | 32.32 ± 3d | 35.96 ± 2b | 20.03 ± 1.00cd | 0.18 ± 0.01bc | 0.46 ± 0.05de |
5% MR | 6.0 ± 0.60b | 5.13 ± 0.35d | 1.10 ± 0.10b | 37.80 ± 3d | 21.59 ± 1d | 24.02 ± 1.10ab | 0.10 ± 0.01d | 0.40 ± 0.04e |
1% BS | 8.4 ± 0.70a | 7.31 ± 0.60ab | 1.20 ± 0.10ab | 60.02 ± 5a | 35.39 ± 3b | 19.96 ± 2cd | 0.18 ± 0.01c | 0.49 ± 0.05cd |
5% BS | 9.3 ± 1.0a | 6.82 ± 0.50b | 1.00 ± 0.10c | 47.30 ± 4c | 42.16 ± 2a | 27.17 ± 1.50a | 0.22 ± 0.01a | 0.60 ± 0.05a |
Note: Different lower cases on the same column in the table indicated significant differences between treatments (
The properties of organic materials, as well as their application rates can influence the soil PHMs’ availability. Therefore, more attention should be paid to the availability of soil PHMs and the growth of
The BR, MR and BS application could decrease the pH in two soils (moderately-polluted soil and heavily-polluted soil) except the 5% MR treatment. The contents of soil organic matter with 5% organic materials application were significantly higher than that with 1% organic materials. For the available PHM concentrations in soil, 5% BR treatment had the best activation effect on Cu and Zn in moderately-polluted soil, while 1% BS has the best effect on Cu, Zn, Pb and Cd in heavily-polluted soil.
Based on the biomass production and PHM contents in shoots of
The proper addition of BR and BS could promote the phytoremediation efficiency of soil PHMs by
We thank Chunying Dou for his help in the laboratory analysis and Ying He for her help in the figure production.