Characterization of organic fertilizers
The elemental analysis results were shown in Table 2. The contents of C, H, O, N, and S were 10.05%~37.03% (MO>HO>SO), 1.27%~4.76% (MO>HO>SO) and 20.52%~50.73% (MO>SO>HO), 0.89%~2.29% (HO>MO>SO), and 0.19%~0.5% (HO>SO>MO), respectively. This indicated that the main components of MO were C, H, and O, which were present at significantly higher abundances than in the HO and SO. Therefore, it might be assumed that MO contained more functional groups than HO and SO.
Table 2
Elemental content of HO, MO and SO.
Sample
|
Elemental content (%)
|
C
|
H
|
O
|
N
|
S
|
HO
|
16.77
|
1.96
|
20.52
|
2.29
|
0.5
|
MO
|
37.03
|
4.76
|
50.73
|
2.11
|
0.19
|
SO
|
10.05
|
1.27
|
20.58
|
0.89
|
0.4
|
It was seen from the SEM analysis that the surface of three organic fertilizer particles was rough and uneven (Fig. 1), and the structure with more obvious pores was observed on the MO surface. The BET surface area of MO (14.60 m2·g−1) was also larger than those of HO (1.10 m2·g−1) and SO (0.87 m2·g−1). The elemental mapping analysis was performed to study the distribution of C, O, N, P, and S elements in HO, MO, and SO (Fig. 2d, e, f). The O and N distribution were matched with those of organic fertilizer. It was well-noted that MO had higher C content than HO and SO, although EDS was not sensitive to the detection of C element (red), which was matched with the result of elemental analysis (Table 2).
Organic fertilizers exhibited similar FTIR spectra characteristics (Fig. 2) with characteristic peaks for nutrients such as phosphate (483 cm−1, 555 cm−1) (Mazeika et al. 2016) and amorphous SiO2 (805 cm−1). The peak at 896 cm−1 was attributed to the C-H deformation vibration of cellulose(Hussain et al. 2017), where the higher cellulose content in SO results in higher peak strength. Strong intensity at 1048 cm−1 was attributed to C-O stretching vibration. The high protein substances in soybean meal will decompose during fermentation (Yasar et al. 2020), the higher peak of COO symmetric stretching of amino acids or COH in-plane bending vibration of carboxylic acids (1430 cm−1) can be observed in SO. The broadband at 1638 cm−1 was attributed to C=O in amides (Jegatheesan et al. 2012), with higher peak intensity in MO, which indicated the presence of more humic-like substances (A et al. 2000). The peaks observed between 2960 and 2850 cm−1 in HO and MO were attributed to C–H stretching (Carballo et al. 2008), while the broadband between 3360 and 3392 cm−1 in MO was attributed to -OH groups. Overall, MO contains more−-OH, C-H, and C=O groups, and humic-like substances which could exchange ions and complexes with heavy metals. In addition, SO contains many amino acids and carboxylic acids decomposed by amino acids, which can provide lots of active adsorption sites for As.
Effects of organic fertilizers on general chemical properties of soil
The application of all organic fertilizers to soil can improve the general chemical properties of the soil, such as pH, OM, and CEC values (Fig. 3). The increase of these values was proportional to the application rates of organic fertilizer added. In this study, the addition of MO increased soil pH by 0.33~0.59, which was higher than HO and SO (< 0.3 unit). With the increase of pH, Cd in the soil is more likely to precipitate and reduce Cd availability(Wen et al. 2020), while As is more likely to be reduced to arsenate and gradually released, increasing As availability(Liu et al. 2006). The soil pH after MO treatment was close to the optimum pH of 6.8 by evaluating the trade-off value, which can control the availability of Cd and As in paddy soil simultaneously (Yao et al. 2021). Under the MO treatments, the OM content in soil increased by 26.58%~69.11%. Compared with SO, there was no significant difference but showed a more noticeable increase than HO. SO treatments increased the CEC content in the soil to a greater extent (30.69%~51.05%), followed by MO treatments (31.52%~39.91%). The pH, OM, and CEC, as critical factors of soil properties, affected the chemical speciation of heavy metals in the soil primarily via their adsorption/desorption, precipitation/dissolution, and complex formation reactions (Cao et al. 2019), and finally decided their mobility and availability (Brokbartold et al. 2012).
TCLP extractable heavy metals concentration in soil
Currently, the Toxicity Characteristic Leaching Procedure (TCLP) method is most widely used for evaluating the availability of heavy metals in soil globally. This study also referred to this method to evaluate the immobilizing effects of different organic fertilizers on heavy metals Cd and As. The results of TCLP-Cd were shown in Fig. 4a, where the leaching amount was inversely proportional to the application rates of treatment agents. Compared to the CK treatment (513.67 µg·kg−1), the lowest TCLP-Cd concentrations were observed under MOM (425.33 µg·kg−1) and MOH (389.33 µg·kg−1) treatments, showing a reduction by 17.20%, and 24.21%, respectively. That might be due to MO significantly increasing the soil pH, as confirmed by the soil pH changes observed (Section 3.2) and the experimental results previously reported (Wang et al. 2021, Xun et al. 2016). The increased soil pH resulted in soil colloids with negative charge increased on the soil surface, which enhanced the adsorption capacity of soil for cationic heavy metals (Wei et al. 2021), thereby reducing their migration throughout the soil and reducing their capacity for absorption and accumulation by plants (Zhong et al. 2020).
HO treatments increased the concentration of TCLP-As in the soil (Fig. 4b), while MO and SO significantly reduced the leaching concentration of TCLP-As by 9.10%~19.04% and 9.82%~36.93%. However, the reduction was inversely proportional to the application rates of the treatment agent, which may be attributed to the increase of soil pH value making As more activated in the soils (Wei et al. 2021). Under a low application rate, more TCLP-As concentration reduced by SO than the others might be due to the presence of more amino acid and carboxyl functional groups (Fig. 3), the separation of carboxyl groups can produce three active adsorption sites for As (Deeprasert et al. 2021). MO treatment reduced the concentration of TCLP-Cd and TCLP-As in the soil simultaneously. This may be because the MO surface has more functional groups ( C = O, C = C, − OH ) (Fig. 3), and the treated soil is at the optimal pH, while reducing the availability of Cd and As(Yao et al. 2021), thus increasing the immobilized heavy metals in the soil (Li et al. 2016).
Effects of organic fertilizers on heavy metal concentration in rice
The concentrations of Cd and As in different parts of rice under different treatments are shown in Fig. 5. Based on the results shown in Fig. 5a, the Cd concentration in the shoots was significantly reduced under MO treatments (P <0.05). Compared with CK, the Cd concentration in the shoots decreased by 23.35%, 34.24%, and 44.36% following MOL, MOM, and MOH treatments. All treatments had no significant effect on the Cd concentration in the rice roots. The MO and SO treatments reduced the As concentrations in the shoots shown in Fig. 5b. MOL, MOM, MOH, SOL, SOM, and SOH treatments reduced the As concentrations in the shoots by 11.30%, 16.19%, 17.61%, 17.75%, 15.97%, and 18.24%. It was shown that MO reduced the accumulation of Cd and As in rice shoots simultaneously. HO treatment reduced the accumulation of Cd in the shoots but promoted the accumulation of As in the roots of rice. Section 3.3 showed that the availability of As increased under HO treatment, more As was accumulated in rice, and most of them were retained in Fe-plaque on the root surface(Yin et al. 2017).
Effects of organic fertilizers on accumulation factors
As two valuable indicators of the accumulation and mobilization of metals in plants, BCF and TF were calculated based on metal availability and uptake by plants in soil-plant systems (Mendez Monica &Maier Raina 2008, Radziemska 2018). BCF value > 1 indicated that the metal tended to transfer from soils to plants and accumulated in plant tissues (Yoon et al. 2006). TF value was used to measure the transportability of the plant for the accumulated metal from plant roots to shoots. It was reported previously that rice was characterized by a BCF > 1 and TF < 1 (Yang et al. 2020), and smaller BCF and TF values indicated a more effective treatment agent (Li et al. 2019). The BCF and TF results obtained were shown in Fig. 6, which were determined from the characteristic analysis mentioned above. The BCF indexes of Cd were higher than those of As, but its TF indexes were lower than the latter, indicating that a larger proportion of Cd was absorbed by the rice and accumulated in rice roots. MO reduced the BCF and TF index values of Cd and As simultaneously, while HO only reduced the BCF and TF index values of Cd, and SO only reduced the BCF and TF index values of As compared with CK. It meant that under the combined pollution of Cd and As, the application of MO to the soil reduced the mobility of Cd and As effectively, made them more stable in the soil, and reduced their accumulation and toxicity into the plants. It was well-noted that the BCF indexes of Cd after SO treatments were slightly lower than those of CK, but the TF indexes increased slightly, indicating that when the plants absorbed Cd and As at the same time, the As content in rice decreased due to the antagonism of the two metals, the transport of Cd in rice might be increased accordingly(Sun et al. 2009).
Plant growth and biomass
Improving plant growth and biomass is the fundamental attribute of organic fertilizer. In this study, after rice harvesting at the tillering stage, the conditions of plant growth and biomass (plant height, root length, fresh weight, and dry weight) of rice grown after different treatments were analyzed (Fig. 7). This may be because the low concentration of heavy metal stress in contaminated soil in this study did not inhibit the plant height of rice. Application of several organic fertilizers might help plant height increase, but there was no significant difference compared with CK. The medium and high application of HO and all MO treatments promoted root length (P < 0.05). The application of HO, MO, and SO increased the fresh weight of plants by 18.48-24.72%, 9.82-13.41%, and 18.05-21.18%, increasing the dry weight of plants by 12.48-15.53%, 5.98-8.86%, and 13.57-18.98%, respectively, which was proportional to the application rates of organic fertilizer.
Redundancy analysis
Figure 8 presented the results of redundancy analysis between soil properties and TCLP-Cd/As, accumulation factors (BCF and TF), and heavy metal concentrations in rice plants. The results of RDA revealed that 40.09% of total plant variation was related to the soil properties (pH, OM, and CEC), 44.74% of total plant variation was related to the TCLP extractable heavy metals concentration of soil (TCLP-Cd and TCLP-As). Fig. 7a showed that the Shoot-Cd and accumulation factors were not significantly correlated with OM and CEC content but negatively correlated with soil pH. The Shoot-As and accumulation factors were negatively correlated with soil properties. However, for Root-As, the increase in OM and CEC content might promote the uptake of As by rice roots due to rhizosphere reaction. It was seen from Fig. 7b that the Shoot-Cd/As and corresponding accumulation factors were strongly positive-correlated with TCLP-Cd/As, while the Root-Cd/As were inversely related to them. These results indicated a strong relationship between the application of organic fertilizers and metal concentrations in rice plants. After the application of organic fertilizers, the improvement of soil properties and the adsorption or complex mechanism between heavy metals and organic fertilizers might reduce the availability of heavy metals in the soil, thus resulting in reduced metal absorption and bioaccumulation by rice.
Safe utilization of organic fertilizer
Among the various methods available for evaluating the potential environmental risks of heavy metal accumulation, the single pollution index and Nemerow comprehensive pollution index (NIPI) were widely used(Hu et al. 2018, Krcmar et al. 2018). The advantage of the NIPI lied in its considerations of the comprehensive pollution risks caused by all heavy metals existed (Yari et al. 2020). This method of organic fertilizer evaluation was also used to determine the safety of organic fertilizer application in the soil to a certain extent. And the single pollution index method and Nemerow comprehensive pollution index method were used for evaluation of the heavy metal’s safety assessment of organic fertilizers as shown in supporting information. It was implied from Table S3 that the single pollution index values of heavy metals in HO were ranked in the order of Cu>Zn>Pb>Cr>Ni>Cd>As. While for MO, the index values were ranked as Ni>Cr>Cu>Zn>Cd>Pb>As. The ranked order of the pollution index values for SO was Cd>As>Pb>Cr>Ni>Cu>Zn. The Nemerov comprehensive pollution index for heavy metals in the three organic fertilizers was ranked in MO>HO>SO, with all of them at a safe level. Therefore, it might be speculated that these organic fertilizers could be safely applied to soils as fertilizers and treatment agents.
In daily agricultural activities, organic fertilizer replacing chemical fertilizer seems to be a feasible idea. We found that the application of specific organic fertilizer under low heavy metal pollution can not only improve the growth of rice but also reduce the accumulation of heavy metals in rice. This method can realize the simultaneous restoration and production of contaminated agricultural land, ensure food safety, and improve economic benefits. The medium-severe polluted soil should seek other pollution control measures, such as planting low-accumulation economic crops, or production after remediation. The selection of organic fertilizer types is extremely important. As this study found, the organic fertilizer prepared from human manure can significantly increase the biomass and reduce the accumulation of Cd in rice, but it will promote As accumulation in roots due to antagonism. The organic fertilizer prepared from soybean meal waste is rich in carboxylic acids after amino acid decomposition, which provides a large number of active adsorption sites for As, thus reducing the migration of As to plants, but has no significant effect on Cd. Organic fertilizer prepared with mushroom residue as raw material is a rare multi-effect and low-cost soil remediation material because its surface is rich in -OH, C-H, and C=O groups, and humic-like substances which can exchange ions and complexes with heavy metals (both Cd and As), to achieve the stabilization of two types of heavy metals at the same time. Therefore, organic fertilizer should be selected according to the actual situation.