3.1 Heavy metal contents in soil and grains
Since the heavy metal contents from food waste were much lower than the soil background value (Table 1), all the heavy metals in this experiment did not exceed the national standards of China (Agricultural Land Soil Pollution Risk Control Standard of China, GB 15618 − 2018). The effects of different treatments on the contents of soil heavy metals are displayed in Fig. 1. In 2020, FM3 had the highest Cu content, 24.08 mg kg− 1, however, there was no significant difference between FM4 and CF, both having significantly higher Cu than FM0 (Fig. 1A, P < 0.05). Among FM0, FM1, FM2 and CK, there were also no significant differences. In 2021, FM2 had the highest Cu content, which was significantly higher than CK and FM0 (P < 0.05), again there was no significant difference among FM1, FM3, FM4 and CF treatments. Compared with 2020, the soil Cu content from CK, FM3, FM4 and CF decreased significantly in 2021 (P < 0.05). Among all the treatments, CK had the highest Cu content in winter wheat grains (Table 2). Nevertheless, there was no significant difference in the Cu contents of summer maize grains among all treatments. The application of organic and chemical fertilizers had no impact on Cu contents of crop grains.
In 2020, FM2 had the highest soil Zn content, 70.39 mg kg− 1, however there were no significant differences among FM1, FM3, FM4 and CF (Fig. 1B) (P < 0.05). There were also no significant differences among FM0, FM1 and CK. In 2021, FM2 was found to have the highest Zn, but not significantly different from CF, both having higher Zn than other treatments (P < 0.05). Compared with 2020, FM0 remarkably declined its soil Zn in 2021(P < 0.01). For the Zn contents of winter wheat grains with different treatments, CK had the highest value (Table 2). Against 2020, FM2 and FM4 treatments increased remarkably in 2021 (P < 0.01), with CF treatments being significantly increased (P < 0.05). For the Zn content in grains of summer maize, FM0 and FM2 had the highest value, respectively in 2020 and 2021. Compared with CK, the application of organic and chemical fertilizers did not increase the Zn content of crop grains (Table 2).
FM2 had the highest Cr content, 104.86 mg kg− 1, which was significantly higher than other treatments (P < 0.05). FM0, FM3 and FM4 had significantly higher soil Cr than CF (P < 0.05). However, there was no significant difference between CK and CF. Among FM0, CF and CK, there were also no significant difference in soil Cr. Compared with 2020, the soil Cr content from CK and FM0 decreased significantly in 2021 (P < 0.05). For the Cr content of winter wheat grains with different treatments, FM0 had the highest value during the two experimental years (Table 2). At summer maize grains, there were also no significant difference in soil Zn contents among all the treatments in both 2020 and 2021, indicating soil Zn was not affected by application of food waste. The application of cattle manure increased the Cr content of winter wheat grains, while the application of chemical fertilizers increased the Cr content of summer maize grains. Even thorough, the contents of Cr were within the limitation of National standards.
Table 2 Heavy metals content in grains of winter wheat and summer maize in different treatments.
years
|
treatment
|
Winter wheat
|
Summer maize
|
Cu
|
Zn
|
Cr
|
Cu
|
Zn
|
Cr
|
2020
|
CK
|
3.47 ± 0.17a
|
32.98 ± 1.78a
|
0.86 ± 0.18bc
|
0.30 ± 0.12a
|
17.31 ± 0.25a
|
0.38 ± 0.16a
|
|
FM0
|
2.40 ± 0.06bc**
|
26.27 ± 1.41b
|
1.45 ± 0.10a
|
0.09 ± 0.13a
|
17.82 ± 0.35a
|
0.45 ± 0.07a
|
|
FM1
|
2.06 ± 0.03c**
|
20.91 ± 0.69c
|
0.86 ± 0.03bc
|
0.24 ± 0.18a
|
14.67 ± 0.52b*
|
0.53 ± 0.07a
|
|
FM2
|
1.75 ± 0.06c**
|
20.95 ± 1.09c**
|
0.81 ± 0.03bc
|
0.34 ± 0.17a
|
17.43 ± 0.39a
|
0.40 ± 0.01a
|
|
FM3
|
2.17 ± 0.17bc**
|
22.09 ± 0.96c
|
0.66 ± 0.12c
|
0.02 ± 0.13a
|
15.67 ± 0.20b*
|
0.48 ± 0.11a
|
|
FM4
|
2.75 ± 0.44b
|
20.34 ± 0.73c**
|
1.14 ± 0.18ab
|
0.14 ± 0.17a
|
14.69 ± 0.14b*
|
0.45 ± 0.14a
|
|
CF
|
2.25 ± 0.13bc*
|
19.65 ± 1.02c*
|
0.77 ± 0.22bc
|
0.23 ± 0.09a
|
15.10 ± 0.30b
|
0.47 ± 0.11a
|
2021
|
CK
|
3.80 ± 0.11A
|
29.85 ± 0.05A
|
0.83 ± 0.10ABC
|
0.17 ± 0.01A
|
16.86 ± 0.05C
|
0.49 ± 0.13ABC
|
|
FM0
|
3.66 ± 0.16AB
|
26.17 ± 0.40BC
|
1.07 ± 0.02A
|
0.14 ± 0.02A
|
17.17 ± 0.04C
|
0.67 ± 0.09AB
|
|
FM1
|
3.44 ± 0.05BCD
|
26.74 ± 0.11B
|
0.70 ± 0.05BC
|
0.13 ± 0.03A
|
17.45 ± 0.04BC
|
0.31 ± 0.10C
|
|
FM2
|
3.61 ± 0.11ABC
|
28.89 ± 0.53A
|
0.73 ± 0.20BC
|
0.33 ± 0.07A
|
18.75 ± 0.23A
|
0.51 ± 0.15ABC
|
|
FM3
|
3.46 ± 0.04BCD
|
25.90 ± 0.23BC
|
0.69 ± 0.06BC
|
0.14 ± 0.05A
|
18.10 ± 0.41AB
|
0.48 ± 0.08ABC
|
|
FM4
|
3.35 ± 0.07CD
|
29.34 ± 0.68A
|
0.96 ± 0.05AB
|
0.25 ± 0.13A
|
17.36 ± 0.47BC
|
0.38 ± 0.07BC
|
|
CF
|
3.28 ± 0.04D
|
25.35 ± 0.05C
|
0.60 ± 0.04C
|
0.17 ± 0.03A
|
15.17 ± 0.17D
|
0.76 ± 0.06A
|
Note: The China National Food Safety Standard: Limits of pollutants in foods: Cu 10 mg kg− 1; Zn 50 mg kg− 1; Cr 1.0 mg kg− 1 (GB 2762 − 2017). The data were means ± standard error (n = 3). Different lower letters indicated significant differences between different treatments in 2020 (P < 0.05). Different capital letters indicated significant differences between different treatments in 2021 (P < 0.05). *: Indicating significant differences between different years at P < 0.05 level; **: Indicating significant differences between different years at P < 0.01 level. |
3.2 Total nitrogen and nitrate nitrogen
At maize harvest stage in 2020, CF treatment displayed the highest soil TN content, 1.39 g kg− 1, however there were no significant differences among CK, FM3 and FM4 (Table 3). In 2021, TN content from FM3 treatment showed the highest TN (1.44 g kg− 1), but there was no significant difference from FM4, both being significantly higher than others (P < 0.05). Compared with 2020, TN content in CK decreased significantly (P < 0.05) in 2021, with FM0 decreasing remarkably (P < 0.01). Again, there was no significant difference in the other treatments during the two experimental years. Along with the application ratio, the application of food waste significantly increased TN content of the soil.
During the maize harvest period of 2020, FM4 had the highest NO3-N content (12.35 mg kg− 1), however there was no significant difference between FM3 and CF. In 2021, NO3-N content from FM3 was noted to be the highest, however there was no significant difference between FM0 and FM4 (Table 3). Against 2020, NO3-N content from FM3, FM4 and CF in 2021 were decreased remarkably (P < 0.01). The more the amount of food waste was applied, the higher NO3-N content in the soil. Food waste was more likely to increase soil NO3-N content than the cattle manure. Nevertheless, with the extension of application time, soil NO3-N contents in the food waste treatments had been declined significantly.
Table 3
The soil total nitrogen and nitrate nitrogen in different treatments.
Treatment
|
TN (g kg− 1)
|
NO3-N (mg kg− 1)
|
|
2020.9
|
2021.9
|
2020.9
|
2021.9
|
CK
|
1.30 ± 0.05ab*
|
1.12 ± 0.01C
|
5.56 ± 1.34bc
|
3.05 ± 0.08B
|
FM0
|
1.18 ± 0.01bc**
|
0.94 ± 0.02D
|
4.49 ± 0.34c
|
4.98 ± 0.16A
|
FM1
|
1.14 ± 0.06c
|
1.19 ± 0.02C
|
7.97 ± 1.11b*
|
2.80 ± 0.27B
|
FM2
|
1.24 ± 0.04bc
|
1.26 ± 0.05B
|
6.82 ± 0.80bc*
|
2.89 ± 0.55B
|
FM3
|
1.29 ± 0.06ab
|
1.44 ± 0.01A
|
11.80 ± 0.54a**
|
5.00 ± 0.23A
|
FM4
|
1.30 ± 0.04ab
|
1.39 ± 0.00A
|
12.35 ± 0.56a**
|
4.21 ± 0.90AB
|
CF
|
1.39 ± 0.03a
|
1.31 ± 0.01B
|
11.42 ± 1.23a**
|
3.28 ± 0.08B
|
Note: The data were means ± standard error (n = 3). Different lower letters indicated significant differences between different treatments in 2020 (P < 0.05). Different capital letters indicated significant differences between different treatments in 2021 (P < 0.05). *: Indicating significant differences between different years at P < 0.05 level; **: Indicating significant differences between different years at P < 0.01 level. |
3.3 Total phosphorus and potassium
In 2020, the highest TP content, was found in CF treatment (1.17 g kg− 1) in 2020. However, among FM1, FM2, FM3 and FM4, there were no significant difference. All the mentioned above treatments had significantly higher TP than CK and FM0 (P < 0.05). In 2021, FM3 had the highest TP content, but it had no significant difference with CF treatment, both having significantly higher TP than others (P < 0.05). Compared with 2020, TP content from FM0 and FM2 increased significantly (P < 0.01), while FM4 increased extremely (P < 0.05). The application of food waste remarkably increased the TP of the soil, with the content being elevated with the increase of the application ratio.
In 2020, FM2 had the highest TK content (19.52 g kg− 1). However, there was no significant difference between FM3 and FM4. FM2 and FM4 had significantly higher TK than other treatments (P < 0.05) (Fig. 2B). In 2021, FM2 had the highest TK, which was significantly higher than others (P < 0.05). The application of food waste and chemical fertilizer significantly increased TP (P < 0.05), with FM0 being the lowest. Compared with 2020, FM1, FM3 and FM4 treatments remarkably increased soil TK in the year of 2021 (P < 0.01).
3.4 Crop yield
In 2020, the winter wheat yields from CK, FM0, FM1, FM2, FM3, FM4 and CF were 3505, 5385, 6403, 7956, 7634, 9758 and 8719 kg ha− 1, respectively (Fig. 3). Among them, FM4 had the highest yield, however, there were no significant differences among FM2, FM3 and CF, which had significantly higher yields than FM0 and FM1 (P < 0.05). There was also no significant difference between FM0 and FM1. The changes of grain yields in 2021 and 2020 was in a better consistent trend. The application of food waste treatment had significantly higher effect than that of CK (P < 0.05), with FM4 being 2.57 and 1.90 times that of CK and FM0, respectively. Although the grain yield of FM2 treatment in 2021 decreased a little bit compared with 2020, other treatments were not found to be significantly different in the two experimental years.
The summer maize yields from CK, FM0, FM1, FM2, FM3, FM4 and CF in 2021 were 4220, 6375, 6616, 7327, 8672, 8470 and 8722 kg ha− 1, respectively. Among them, CF had the highest yield. However, there was no significant difference between FM3 and FM4, both having significantly higher grain yields than others except CF (P < 0.05). The changes of 2021 and 2020 was in a consistent trend. The application of food waste treatment has significantly increased the grain yields compared with CK (P < 0.05).
In 2020, FM4 had the highest annual yield (wheat + maize), which was significantly higher than other treatments (P < 0.05). Nevertheless, there was no significant difference between FM3 and CF. FM2 was noted to be significantly higher annul yield than CK, FM0 and FM1 (P < 0.05) (Fig. 3C). The changes of 2021 and 2020 were in a consistent trend. Food waste treatments had better effect than those of CK and FM0 (P < 0.05), as there was no significant difference with CF, indicating food waste could replace chemical fertilizer without yield losses. Compared with 2020, the annual yield of FM1 increased significantly in 2021 (P < 0.05), nevertheless there was no significant difference in other treatments during the two years. The use of food waste as organic fertilizer was more conducive to increasing the annual yield of winter wheat and summer maize than cattle manure.
3.5 Economic benefits
CF treatment had the highest inputs, FM4 and CK the lowest, with others being somewhere in between (Table 4). For the outputs of different treatments, FM4 had the highest value, $19219.6 ha− 1, in 2020, while FM3 had the highest in 2021, $20045.9 ha− 1. CF was found to have the lowest output. Above all, the outputs of organic fertilizer treatments were much higher than CF treatment (Table 4). FM3 and FM4 had the highest net outputs among all the treatments. CF had the lowest net outputs, which were 3074.6 and $3367.4 ha− 1 in 2020 and 2021, respectively. The net outputs of organic fertilizer and CK were higher than that of CF treatment.
Table 4 Comprehensive economic benefits under different fertilizer treatments. Unit: $ ha− 1
|
|
Details
|
CK
|
FM0
|
FM1
|
FM2
|
FM3
|
FM4
|
CF
|
Input
|
Materials
|
Fertilizer
|
0.0
|
490.5
|
367.9
|
245.2
|
122.6
|
0.0
|
1441.6
|
|
|
Seed
|
206.3
|
206.3
|
206.3
|
206.3
|
206.3
|
206.3
|
206.3
|
|
Equipment
|
Ploughing
|
182.6
|
182.6
|
182.6
|
182.6
|
182.6
|
182.6
|
182.6
|
|
|
Seeding
|
136.9
|
136.9
|
136.9
|
136.9
|
136.9
|
136.9
|
136.9
|
|
|
Harvesting
|
365.2
|
365.2
|
365.2
|
365.2
|
365.2
|
365.2
|
365.2
|
|
Labor
|
transportation
|
0.0
|
245.2
|
235.0
|
224.7
|
214.5
|
204.2
|
0.0
|
|
|
Fertilization
|
0.0
|
91.3
|
91.3
|
91.3
|
91.3
|
91.3
|
7.6
|
|
|
Irrigation
|
273.9
|
273.9
|
273.9
|
273.9
|
273.9
|
273.9
|
273.9
|
|
|
Weed control
|
547.8
|
547.8
|
547.8
|
547.8
|
547.8
|
547.8
|
547.8
|
Annual average input
|
|
1712.7
|
2539.8
|
2406.9
|
2274.0
|
2141.1
|
2008.2
|
3161.9
|
Output
|
|
2020
|
8337.5
|
12677.2
|
13899.4
|
16183.0
|
17525.4
|
19219.6
|
6236.5
|
|
|
2021
|
9975.2
|
15061.4
|
16240.1
|
17897.9
|
20045.9
|
19560.7
|
6529.4
|
Annual average net income
|
2020
|
6624.7
|
10137.4
|
11492.5
|
13909.0
|
15384.3
|
17211.3
|
3074.6
|
|
|
2021
|
8262.5
|
12521.6
|
13833.2
|
15623.9
|
17904.7
|
17552.4
|
3367.4
|
Note: $1.0 = 6.5719 Chinese Yuan. The organic fertilizer treatment of wheat and maize were calculated according to the organic price, which were sold online for $ 0.91/kg and $ 1.22/kg, respectively. According to market prices, the output values of chemical fertilizer-treated wheat and maize were respectively $ 0.33/kg and $ 0.38/kg in 2020; were $ 0.33/kg and $ 0.41/kg in 2021. |