3.1 Effects of different amounts of insect feces on soil physicochemical properties
The application of insect feces as organic fertilizers increased soil pH and the contents of OM and EC (Fig. 1). This growing tendency was more evident with the increase in the amounts of insect feces applied. For example, soil pH increased from 6.46 (CK) to 7.13 (T4 treatment) in 2018 and from 6.51 (CK) to 7.14 (T4 treatment) in 2019. Meanwhile, the OM contents under T4 treatment were increased by 25.3% and 32.5% in the two years, respectively, compared with the control. On the other hand, the contents of ammonium nitrogen, available phosphorus and available potassium in the soil obviously elevated with the increase of the application amounts of insect feces (Fig. 1). The contents of ammonium nitrogen, available phosphorus and available potassium under T4 treatment were the highest, which were increased by 195.4%, 288.2% and 1869.7% in 2018 and 90.2%, 312.0% and 2014.5% in 2019, respectively, compared with the control. Due to the nutrients absorption for rice growth, the contents of various soil nutrients in 2019 reduced than those in 2018. For example, the contents of ammonium nitrogen, available phosphorus and available potassium in the second year were decreased by 6.6%-29.7%, 27.6%-34.5% and 29.1%-61.0%, respectively, compared with those in the first year.
3.2 Effects of insect feces on Cd and Pb morphological changes in soil
The concentrations of Cd and Pb in soil after application of insect feces are shown in Table 1. No significant difference was observed in metal concentrations under different treatments. Meanwhile, for the same heavy metal, there was no significant difference in the concentration between two years. Figure 2 reveals the distribution percentage of different Cd speciation in soil before and after application of insect feces. Cd mainly existed as weak acid soluble state (F1) in soil (44.2–69.8%) no matter whether the soil was fertilized or not. After the application of insect feces in 2018, the weak acid soluble fraction of Cd was reduced by 8.3%-24.2% compared with CK, while the percentages of reducible (F2), oxidizable (F3) and residual (F4) states of Cd in fertilized soil were enhanced by 2.2%-10.0%, 52.4%-165.7% and 46.9%-118.6%, respectively. In 2019, the weak acid soluble state of Cd in fertilized soil was reduced by 18.0%-33.4% than CK, while the percentages of reducible, oxidizable and residual states of Cd in fertilized soil were increased by 0.6%-7.8%, 52.7%-99.2% and 135.3%-225.6%, respectively. Compared with 2018, the weak acid soluble state of Cd in 2019 diminished by 5.0%-17.7% but the oxidizable state increased by 47.5%-96.9%. In both of the two years, the ratio of weak acid-soluble Cd under T4 treatment was the lowest, while the contents of oxidizable and residual Cd were the highest.
Table 1
Concentrations of Cd and Pb in soil before and after application of insect feces
Treatment
|
Cd
|
Pb
|
2018
|
2019
|
2018
|
2019
|
CK
|
10.85 ± 0.34 a
|
10.70 ± 0.31 a
|
1346.15 ± 34.5 a
|
1333.46 ± 44.3 a
|
T1
|
10.75 ± 0.21 a
|
10.90 ± 0.34 a
|
1330.31 ± 33.2 a
|
1320.70 ± 34.3 a
|
T2
|
10.69 ± 0.32 a
|
10.80 ± 0.23 a
|
1328.27 ± 46.3 a
|
1323.83 ± 48.2 a
|
T3
|
10.55 ± 0.24 a
|
10.70 ± 0.26 a
|
1320.21 ± 38.3 a
|
1319.93 ± 43.1 a
|
T4
|
10.65 ± 0.21 a
|
10.80 ± 0.24 a
|
1329.67 ± 26.8 a
|
1324.14 ± 32.3 a
|
As shown in Fig. 3, Pb mainly existed in the reducible state (65.5%-70.3%) in soil. After the application of insect feces in 2018, the proportion of weak acid soluble Pb decreased by 17.4%-54.1% compared with CK, while the percentages of reducible, oxidizable and residual Pb in fertilized soil were increased by 1.7%-2.8%, 22.4%-78.0% and 1.8%-14.7%, respectively. In 2019, the proportion of weak acid soluble Pb in fertilized soil was reduced by 22.2%-56.8% than CK, while the percentages of oxidizable and residual Pb in fertilized soil were increased by 40.0%-66.6% and 9.3%-18.7%, respectively. For reducible Pb, their proportions in fertilized soil in 2019 generally increased by 0.19%-1.66% than those in CK, except for T1 treatment (slightly decreased by 0.83%). Compared with 2018, the oxidizable and residual Pb in 2019 increased by 12.1%-49.3% and 9.1%-19.3%, respectively. In both of the two years, the ratio of weak acid-soluble Pb under T4 treatment was the lowest, while the contents of reducible, oxidizable and residual Cd were the highest. Totally, the application of insect feces fertilizers reduced the bioavailability of Cd and Pb in the paddy soil.
3.3 Effects of insect feces on rice yield
In 2018, the rice yield under T2 treatment was the highest (36.0 g pot− 1), which was 43.7% higher than that of CK, followed by T1 and T3 treatment (increased by 28.0% and 19.9%, respectively) (Fig. 4). On the contrary, the rice yield of T4 treatment was the lowest, which was reduced by 20.0% compared with CK. In 2019, the order of rice yield under different treatments was T4 > T3 > T2 > T1 > CK, and the output of T4 treatment was 63.2 g pot− 1, which was enhanced by 195.5% than that of CK. Compared with 2018, the rice yield of T3 and T4 treatments in 2019 increased by 85.3% and 214.7%, respectively; while the rice yield of CK and T1 treatment decreased by 14.7% and 10.8%, respectively. There was no significant difference in the yield under T2 treatment between two years. This suggested that the amount of fertilizer applied under T2 treatment in 2018 was reasonable, but the amounts under T3 and T4 treatments were excessive. In 2019, the amount of fertilizer applied under T4 treatment was reasonable. The results indicated that the effect of high amount of insect feces fertilizers was more obvious with the extension of time
3.4 Effects of insect feces on Cd and Pb contents in various parts of rice plant
As seen in Fig. 5, the application of insect feces as organic fertilizers reduced the contents of Cd in various parts of rice plant. For CK treatment, the contents of Cd in rice grain within two years were 1.82 mg kg− 1 and 1.49 mg kg− 1, respectively. After the application of insect feces, Cd concentrations in grain decreased by 27.1%-63.9% in 2018 and by 37.9%-66.7% in 2019. During the two-year experiment, Cd contents in the root were the highest among different parts, which were 20.8 mg kg− 1 and 18.2 mg kg− 1. After the application of insect feces, Cd contents in various plant parts considerably decreased by different extents in two years, such as husk (8.3%-19.9% and 29.7%-33.6%, respectively, for years 2018 and 2019), leaf (15.9%-34.4% and 38.3%-49.8%), stem (15.0%-36.0% and 29.6%-43.1%) and root (39.7%-57.5% and 39.9%-55.1%). In 2018, the overall order of Cd content in the above-ground and underground parts of rice under different fertilization treatments was T1 > T4 > T3 > T2; while in 2019, the overall order was changed as T1 > T2 > T3 > T4. Under the same treatment, Cd content in each part of rice in 2019 was lower than that in 2018, especially for the T4 treatment with the largest decline in Cd concentration in each part. Compared with 2018, the contents of Cd in grain, husk, leaf, stem and root under T4 treatment in 2019 were decreased by 51.5%, 32.5%, 44.5%, 29.4% and 34.5%, respectively.
The effect of insect feces on Pb contents in each part of rice plant is shown in Fig. 6. The concentration ranges of Pb in different plant parts were as follows: 0.56–1.44 mg kg− 1 in grain, 0.63–1.67 mg kg− 1 in husk, 10.0-19.2 mg kg− 1 in leaf, 0.78–2.58 mg kg− 1 in stem and 559.6-1013.7 mg kg− 1 in root. Pb contents of different parts under CK treatment were the highest. After applying insect feces, Pb concentrations were decreased by 14.6%-53.1% in grain, 16.4%-53.5% in husk, 6.4%-36.8% in leaf, 24.7%-61.8% in stem and 24.4%-38.7% in root. Among all fertilization treatments, Pb contents in all parts of rice plant under T2 treatment in 2018 were the lowest, while those under T4 treatment in 2019 were the lowest. Compared with 2018, Pb contents under the same fertilization treatment in 2019 were decreased by 4.1%-51.5% in grain, 20.8%-45.6% in husk, 20.1%-42.0% in leaf, 26.0%-54.6% in stem and 2.5%-13.7% in root. This indicated that the application of insect feces could reduce Pb accumulation in rice.
3.5 Effects of insect feces on Cd and Pb absorption and transport coefficients of rice
The values of absorption coefficients and transport coefficients were used to characterize the ability of rice to enrich and transport heavy metals (McGrath and Zhao 2003). Higher absorption coefficients suggest stronger ability of plant to absorb heavy metals, and higher transport coefficients suggest stronger ability of plant root to transport heavy metals to the above-ground parts. The effect of insect feces application on the absorption and transport coefficients of Cd and Pb is shown in Table 2. Without insect feces application, the Cd absorption coefficients were 1.959 and 1.712 in two years, while the Pb absorption coefficients were 0.739 and 0.624 in two years. The Cd absorption coefficients were 2.65 and 2.74 times higher than those of Pb within two years, indicating that rice could absorb Cd more readily than Pb. The application of insect feces significantly reduced the absorption coefficients of the two heavy metals in rice (Cd and Pb decreased by 39.5%-57.5% and 24.4%-38.7%, respectively). For CK treatment, the primary transport coefficients of Cd were 0.127 and 0.120 within two years, which were 16.1 and 12.2 times higher than those of Pb, respectively. The application of insect feces increased the primary transport coefficients of Cd and Pb. Moreover, the secondary transport coefficients of Cd under CK treatment were 0.690 and 0.684 within two years, and those of Pb were 0.180 and 0.143, respectively. Application of insect feces reduced the secondary transport coefficients of Cd by 7.8–44.1% and reduced those of Pb by 3.6–21.6%. Among different treatments, the absorption coefficients of Cd and Pb and the secondary transport coefficients of Cd under T2 and T3 treatments in 2018 were significantly lower than those under T1 treatment; while in 2019, the absorption coefficients and secondary transport coefficients of Cd and Pb under T4 treatment were significantly lower than those under T1 treatment. This indicated that the insect feces application reduced the uptake of Cd and Pb from soil by rice plant and decreased the metal migration from stem and leaf to grain, but enhanced the migration of Cd and Pb from root to stem and leaf.
The absorption coefficients of Cd and Pb were the highest, followed by the secondary transport coefficients, and the primary transport coefficients were the lowest (Table 2). For example, the absorption coefficients of Cd were 2.1 times and 7.4 times of the secondary and primary transport coefficients on average, respectively, and the absorption coefficients of Pb were 3.4 times and 52.7 times of the secondary and primary transport coefficients, respectively. This suggested that the absorption and accumulation capacities of soil heavy metals by rice root were much greater than those of above-ground part.
Table 2
Cd and Pb absorption and transport coefficients of rice before and after application of insect feces
|
Treatment
|
Absorption coefficients
|
Primary transport coefficients
|
Secondary transport coefficients
|
|
2018
|
2019
|
2018
|
2019
|
2018
|
2019
|
Cd
|
CK
|
1.959 ± 0.044 a
|
1.712 ± 0.066 a
|
0.127 ± 0.001 c
|
0.120 ± 0.002 c
|
0.690 ± 0.029 a
|
0.684 ± 0.040 a
|
T1
|
1.182 ± 0.092 b
|
1.029 ± 0.013 b
|
0.179 ± 0.019 a
|
0.135 ± 0.008 b
|
0.594 ± 0.048 b
|
0.631 ± 0.036 a
|
T2
|
0.832 ± 0.009 c
|
0.913 ± 0.010 c
|
0.192 ± 0.006 a
|
0.139 ± 0.000 ab
|
0.386 ± 0.036 c
|
0.469 ± 0.025 b
|
T3
|
0.909 ± 0.027 c
|
0.789 ± 0.025 d
|
0.183 ± 0.003 a
|
0.145 ± 0.002 a
|
0.446 ± 0.018 c
|
0.447 ± 0.026 b
|
T4
|
1.174 ± 0.071 b
|
0.769 ± 0.018 d
|
0.150 ± 0.011 b
|
0.145 ± 0.003 a
|
0.551 ± 0.051 b
|
0.421 ± 0.015 b
|
Pb
|
CK
|
0.739 ± 0.040 a
|
0.624 ± 0.021 a
|
0.0079 ± 0.0005 c
|
0.0098 ± 0.0005 b
|
0.180 ± 0.014 a
|
0.143 ± 0.010 a
|
T1
|
0.525 ± 0.034 b
|
0.472 ± 0.020 b
|
0.0099 ± 0.0008 ab
|
0.0105 ± 0.0004 a
|
0.174 ± 0.013 ab
|
0.134 ± 0.006 a
|
T2
|
0.453 ± 0.031 c
|
0.442 ± 0.015 bc
|
0.0091 ± 0.0001 b
|
0.0098 ± 0.0000 ab
|
0.153 ± 0.006 b
|
0.126 ± 0.018 a
|
T3
|
0.465 ± 0.011 c
|
0.425 ± 0.038 bc
|
0.0089 ± 0.0003 b
|
0.0099 ± 0.0012 ab
|
0.164 ± 0.013 ab
|
0.133 ± 0.009 ab
|
T4
|
0.473 ± 0.017 bc
|
0.408 ± 0.024 c
|
0.0106 ± 0.0005 a
|
0.0090 ± 0.0004 b
|
0.154 ± 0.010 b
|
0.112 ± 0.008 b
|