Effects of Organic Manure on Wheat Yield and Accumulation of Heavy Metals in a Soil—Wheat System

Abstract

The application of organic manure is an effective way to develop sustainable agriculture. However, the application of organic manure may be associated with a potential risk of heavy metal pollution for soil and crops. In this study, the effects of organic cow manure (T1) (as base fertilizer), organic pig manure (T2) (as base fertilizer) and chemical fertilizer (T3) on winter wheat grain yields, grain quality, heavy metal concentrations and heavy metal bioconcentration factors (BCFs) in a soil–wheat system were studied from November 2021 to June 2023. The results showed that the winter wheat grain yields in the T1 and T2 treatments were lower than those in the T3 treatment by 2.57–38.0% and 10.5–25%, respectively. There were no significant differences in quality indexes of winter wheat grain among different fertilizer treatments. The concentrations of heavy metals in topsoil and winter wheat were 0.12–76.11 μg/g and 0.01–43.25 μg/g, respectively. The BCFs of heavy mental in the soil–wheat grain system was 0–2.92. In general, there were no significant differences in heavy metals’ concentrations in topsoil and wheat grain among different fertilizer treatments. In summary, compared with chemical fertilizer, the short-term application of organic manures had no significant effect on heavy metals concentrations in topsoil and wheat.

1. Introduction

China is the world’s largest consumer of chemical fertilizers. The chemical fertilizer consumption in China account for 22.8% of the world’s total [1]. While there is no significant increase in grain yield when excessive chemical fertilizer is applied, and the effective utilization rate of chemical fertilizer is at a low level [2], the excessive application of chemical fertilizers in long-term would cause a series of problems, such as agricultural non-point source pollution, soil compaction and soil acidification [3,4,5]. Therefore, applying fertilizer reasonably is one of the main tasks in China’s agriculture under the premise of ensuring crop yield and reducing environmental pollution. At present, the output of livestock and poultry manure is increasing rapidly with the development of modern agriculture and large-scale agriculture. The average annual output of livestock and poultry manure is 300 million tons in China [6]. The use of organic manure plays an important role in reducing the application of chemical fertilizers and alleviating the agricultural environment [7]. Due to the rich nutrients (such as N, P and K) in organic manures [8], the application of organic manure could increase the organic matter concentration in soil, improve soil physical and chemical properties, balance soil microorganisms [9] and improve soil fertility, thereby improving crop quality and yield [10]. The combined application of organic fertilizer and chemical fertilizer can promote fertilizer utilization efficiency, crop growth and crop yield [11]. Therefore, organic manure is often used as base fertilizer to improve soil physical and chemical properties and increase crop yield.

Due to the application of feed additives during the process of raising livestock and poultry, organic manure may contain certain heavy metals [12]. By applying organic manure, these heavy metals may be transferred to soil and plants, inhibiting the development of crops [13] and resulting in potential health risks to humans and animals. It was found that the concentrations of As, Hg, Cr, Cu, Zn and Mn in soil increased by 0.55, 0.01, 5.94, 5.40, 21.62 and 22.45 μg/g, respectively, after four years of the application of pig manure (30.50 t/ha per year) [14]. Compared with the corresponding initial values, long-term (15 years) application of high-dose chicken manure and cow manure (103.46 t/ha for two seasons) significantly increased the concentrations of Cd, Cu, Zn and Cr in the soil by 230%, 66%, 110% and 86%, respectively [15]. The Zn, Fe, Cd, Pb, Cu and Cr concentrations in wheat grain with the short-term application of cow/poultry manure (4.2 t/ha) were 17.61–18.61, 16.83–17.98, 0.95–0.98, 0.71–1.49, 3.75–5.05 and 0.80–0.97 μg/g, respectively, and all the values (except for Pb) were lower than the corresponding safe limits suggested by the FAO [16]. The concentrations of Cd and Cu in rice grain with the long-term application of pig manure increased by 100% and 6.6% compared with values in treatments with no fertilizer application [17]. The concentrations of Zn and Cu in wheat grain with the partial substitution (50%) of chemical fertilizer with manure fertilizer (PM treatment) were 47.5 and 5.47 μg/g, respectively, which increased by 23.5–67.8% compared with the values in chemical fertilizer treatment (C treatment), while the concentrations of Cd in PM treatment decreased by 47.7% compared with that in C treatment [18]. Heavy metals may migrate and transform in the soil–crop system under the action of soil physical and chemical properties (pH, organic matter, etc.) [13]. The bioconcentration factors (BCFs) of different heavy metals in different regions and different crops are different. For example, the BCF of As was 0.35–0.73 in the soil–sugarcane leaf system, in which sugarcane was planted in heavy metal contaminated farmland (the concentration of As in the soil was 30–168 µg/g, which exceeded the national agricultural soil quality standard of 25 µg/kg) [19]. The BCFs of eight heavy metals (Cr, Pb, Cd, Hg, As, Cu, Zn, Ni) in soil–grains, soil–vegetables and soil–fruits systems were 0.0013–0.413, 0.0014–0.082 and 0.0005–0.027, respectively [20]. Therefore, it is a great significance to clarify the migration of heavy metals in the soil–crop system with applying organic manure for ensuring food security and sustainable agricultural development.

Wheat is one of the most important cereal crops, and manure is usually applied as base fertilizer for wheat, so it is necessary to clarify the migration of heavy metals in the soil–wheat system with applying organic manure. According to the current research status, many studies have focused on the migration of heavy metals in soil–wheat grain system [21,22,23], while the migration of heavy metals in different organs of crops is not well studied. In this study, not only the bioconcentration factors of heavy metals in soil–winter wheat grain system but also the transformation of heavy metals from root to shoot and shoot to grain were analyzed.

2. Materials and Methods

2.1. Site Information and Experimental Design

The experiment station was located at the Agricultural Soil and Water Experimental Station of Yangzhou University (32°24′ N, 119°26′ E and about 10 m above sea level), and the experiment was carried out from October 2021 to June 2023. The area has a subtropical monsoon humid climate. The average annual temperature is 14.8 °C, the annual rainfall is 1065.5 mm and the rainfall is mainly concentrated in May to August. The climate parameters data are shown in Figure 1, and the rainfall in the wheat season of 2021–2022 and 2022–2023 was 344.8 mm and 357.4 mm, respectively. The concentrations of total nitrogen, total phosphorus and total potassium in the 0–20 cm soil of the experimental field were 0.71 g/kg, 0.53 g/kg and 5.12 g/kg, respectively. Other soil basic physical and chemical properties are shown in

Table 1. Soil basic physical and chemical properties.

Soil Depth (cm)	Volumetric Weight (g/cm3)	Saturated Water Content (%)	Field Capacity (%)	Soil Texture (International)
0–20	1.41	0.44	0.37	loam
20–40	1.36	0.35	0.29	sandy loam
40–60	1.54	0.40	0.34	sandy loam
60–80	1.40	0.44	0.37	silty loam
80–100	1.47	0.45	0.38	silty loam

.

The winter wheat cv. Zhenmai 9 was sown in early November using a 20 cm row spacing and 120 plant/m² seeding density. Three different types of base fertilizer were set up in this experiment, i.e., organic cow manure (T1), organic pig manure (T2) and chemical fertilizer (T3). The experiment was conducted in a randomized block design with three replicates using plots of 10 m × 7 m. The plots were harvested in late May.

The total amounts of nitrogen (N), phosphorus (P) and potassium (K) fertilizer in the three treatments were the same, i.e., base fertilizer: 120 kg N/ha, 130 kg P/ha and 160 kg K/ha; topdressing: 120 kg N/ha were applied at tillering stage and jointing stage with ratio of 50%: 50%, and urea (N: 46%) was used as the topdressing in all treatments. The concentrations of heavy metals in organic manure are shown in

Table 2. The concentration of heavy metals in organic manures and topsoil (before the application of treatments) (μg/g).

Various Substances	N	P	K	Cu	Pb	Zn	Cr	Ni	Cd
Organic cattle manure	21,500	23,500	28,900	25.9	6.0	103	7.0	12.1	ND
Organic pig manure	24,600	22,600	28,400	155	6.0	240	16.8	13.7	ND
topsoil	710	530	5120	14.8	47.9	38.7	58.7	19.7	0.86

Note: ND represents this value is below the limits of detection.

, and the amount of fertilizer applied during the growth period of wheat is shown in

Table 3. The amount of fertilizer in different treatments during winter wheat growing season (kg/ha).

Treatments	Base Fertilizer		Topdressing (Urea)
	N/P/K	Fertilizer Type	N at Tillering Stage	N at Jointing Stage
T1	120/130/160	organic cattle manure (8.45 t/ha)	60	60
T2	120/130/160	organic pig manure (8.55 t/ha)	60	60
T3	120/130/160	Urea, KH2PO4 (0.26 t/ha, 0.57 t/ha)	60	60

. Other chemical materials (herbicides, insecticides, et al.) were not used in this experiment, and artificial weeding was used for weeding.

2.2. Experimental Observations and Methods

(1)

Determination of winter wheat grain yield

For each plot, five wheat grain samples were collected at maturity (the area of each sample was 1 m²) to determine grain yield, and the grains were dried under natural conditions for several days until the moisture content was 8% on average.

(2)

Determination of heavy metals in soil and winter wheat

At wheat harvest, the winter wheat plants were collected (the area of each sample was 0.1 m², and three samples were taken for each treatment) and deactivated at 105 °C for 30 min, and then dried to constant weight at 75 °C. For each plot, one topsoil (0–20 cm) sample was randomly collected using the 5-plot method. The concentrations of heavy metals (Cr, Cu, Zn, Pb, Ni, Cd) in soil and wheat samples (i.e., roots, stems–leaves, and grains) were determined by microwave digestion and inductively coupled plasma mass spectrometry (ICP-MS).

(3)

Determination of winter grain quality

Five sub-samples (0.05 m²) of wheat grains were taken from each plot and mixed into one sample; a total of 9 winter wheat grain samples were collected to determine the quality of wheat grains. The fat of wheat grain was determined by Soxhlet extraction method [24]. The crude fiber and ash of wheat grain were determined according to the Association of Official Analytical Chemists [25]. The total soluble sugar and starch in grains were determined by the anthrone method and ultraviolet spectrophotometer method [26].

2.3. Data Calculation and Analysis

(1)

Calculation of the bioconcentration factors (BCFs)

The bioconcentration factors (BCFs) of the heavy metals in the soil–winter wheat systems were calculated as follows:

$${BCF}_{\mathrm{s}-\mathrm{r}}=C_{\mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}}/C_{\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}}$$

(1)

$${BCF}_{\mathrm{r}-\mathrm{s}}=C_{\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m}}/C_{\mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}}$$

(2)

$${BCF}_{\mathrm{s}-\mathrm{g}}=C_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}}/C_{\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m}}$$

(3)

$${BCF}_{\mathrm{s}-\mathrm{g}}=C_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}}/C_{\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m}}$$

(4)

where BCF_s-r, BCF_r-s, BCF_s-g and BCF_so-g are the heavy metals bioconcentration factor values from soil to wheat roots, root to stem–leaf, stem–leaf to grain and soil to wheat grain system, respectively. C_soil, C_root, C_stem and C_grain are the concentrations of heavy metals in soil, wheat roots (μg/g), stem–leaf and grain, respectively.

(2)

Data analysis

The data were analyzed using SPSS 20.0 software, and the LSD (Least-Significant Difference) test was used for analyzing the significance of differences (p = 0.05) on the concentration of heavy metals in topsoil and in different parts of winter wheat and BCFs of heavy metals in soil–winter wheat system among fertilizer treatments. Microsoft Excel 2010 software was used to create graphs.

3. Results

3.1. Wheat Grain Yields and Yield Components in Different Fertilizer Treatments

The winter wheat grain yields and yield components under different fertilizer treatments in 2021–2023 are shown in

Table 4. Grain yield and yield components of wheat under different fertilizer treatments in 2021–2023.

Year	Treatments	Yield (t/ha)	Spike Length (cm)	Spike Number (per Plant)	Grain Number (per Spike)	1000-Grain Weight (g)
2021–2022	T1	3.87 ± 0.61 a	6.50 ± 0.37 b	14.5 ± 0.82 b	19.2 ± 3.30 b	53.3 ± 1.25 a
	T2	4.68 ± 0.28 a	6.61 ± 0.41 b	15.3 ± 1.00 b	20.3 ± 2.59 b	55.6 ± 1.27 a
	T3	6.24 ± 1.07 a	7.83 ± 0.30 a	18.2 ± 0.86 a	31.8 ± 4.93 a	55.6 ± 0.25 a
2022–2023	T1	5.69 ± 0.77 a	7.50 ± 0.39 a	16.0 ± 0.87 b	33.6 ± 3.39 ab	54.7 ± 0.97 a
	T2	5.22 ± 0.84 a	6.97 ± 0.28 b	16.2 ± 0.81 b	30.6 ± 3.22 b	55.9 ± 0.90 a
	T3	5.84 ± 0.67 a	7.59 ± 0.42 a	17.3 ± 0.84 a	35.0 ± 4.93 a	53.3 ± 1.80 a
Year		ns	*	ns	*	ns
Treatment		ns	*	*	*	ns
Year × Treatment		ns	*	*	*	ns

Note: The data are means ± SD. The different lowercase letters in the same column of this table indicate there was a significant difference among the three treatments for each year (p < 0.05). ns represents no significant difference on yields and yield components (p > 0.05). * represents a significant difference on yields and yield components (p < 0.05).

. The winter wheat grain yields in the T1, T2 and T3 treatments were 3.87–5.69 t/ha, 4.68–5.22 t/ha and 5.84–6.24 t/ha, respectively. The significance analysis in this paper showed that there was no significant difference in grain yields among different treatment for both years, and there was no interaction effect of year and treatment on grain yields and yield components. The grain yields of the T1 and T2 treatments were lower than those of T3 treatment by 25.0–38.0% in 2021–2022 and 2.57–10.5% in 2022–2023, respectively. The spike length, spike number, grain number and 1000-grain weight are important yield components for wheat grain yield [27]. There were significant differences in yield components among different fertilizer treatments in this study. In 2021–2022, the spike length, spikelet number and grain number per spike of the T1 and T2 treatments were significantly lower than those of the T3 treatment by 15.60–17.01%, 15.91–20.67% and 36.28–39.66%, respectively. In 2022–2023, the spike length, spikelet number and grain number per spike of the T2 treatment were significantly lower than those of the T3 treatment by 8.15%, 6.50% and 12.44%, respectively, and the spike number of the T1 treatment was significantly lower than that of the T3 treatment by 7.53%. The spike length, spike number and grain number of T1 and T2 in 2021–2022 were lower than these in 2022–2023 by 5.54–15.38%, 5.88–10.34% and 50.74–75%, respectively. There was no significant difference in 1000-grain weight among different fertilizer treatments for both years. There was no significant difference in the 1000-grain weight between the two years, and there were significant differences in spike length, spike number and grain number between the two years, indicating spike length, spike number and grain number could increase with the application of organic manure over time.

The quality indexes of the winter wheat grains under different fertilizer treatments in 2021–2022 are shown in Figure 2. The concentrations of crude fiber, crude fat, ash, soluble sugar and starch in winter wheat grains for the T1, T2 and T3 treatments were 1.69–2.95%, 1.83–1.92%, 1.53–1.93%, 4.26–4.77% and 65.1–65.4%, respectively. The significant analysis showed that there was no significant difference in the quality indexes among different fertilizer treatments, indicating there was no significant effect of organic manure on grain quality indexes compared with chemical fertilizer.

3.2. Heavy Metals’ Concentrations in Topsoil in Different Fertilizer Treatments

The heavy metals’ concentrations in the topsoil (0–20 cm) of different fertilizer treatments at the winter wheat harvest is shown in

Table 5. The concentrations of heavy metals in topsoil at the winter wheat harvest under different fertilizer treatments in 2021–2023.

Year	Treatment	Cr	Cu	Zn	Cd	Pb	Ni
2021–2022	T1	43.54 ± 0.76 a	13.37 ± 0.20 a	65.74 ± 4.31 a	0.14 ± 0.01 a	21.93 ± 0.37 a	33.23 ± 0.51 a
	T2	42.50 ± 0.76 a	12.82 ± 0.23 a	76.11 ± 6.53 a	0.12 ± 0.01 a	19.93 ± 0.60 a	32.60 ± 0.27 a
	T3	41.54 ± 0.57 a	12.91 ± 0.36 a	58.84 ± 1.08 a	0.12 ± 0.00 a	20.76 ± 0.26 a	31.89 ± 0.12 a
2022–2023	T1	43.78 ± 0.11 a	11.16 ± 0.38 a	45.51 ± 1.25 b	0.12 ± 0.00 a	14.83 ± 0.56 a	19.00 ± 0.36 a
	T2	43.84 ± 0.77 a	12.72 ± 0.80 a	51.66 ± 1.63 a	0.13 ± 0.01 a	17.42 ± 1.85 a	18.61 ± 0.15 a
	T3	43.13 ± 1.35 a	11.66 ± 0.55 a	49.96 ± 1.24 a b	0.12 ± 0.01 a	15.12 ± 0.60 a	18.54 ± 0.44 a
Year		ns	*	*	ns	*	*
Treatment		ns	ns	ns	ns	ns	ns
Year × Treatment		ns	ns	ns	ns	ns	ns

Note: Different lowercase letters indicate significant differences among different fertilizer treatments (p < 0.05) for each heavy metal. ns represents no significant difference among different fertilizer treatments and different years for each heavy metal, * represents significant differences among differences among different fertilizer treatments and different years for each heavy metal.

. The concentrations of Cr, Cu, Zn, Pb, Ni and Cd in topsoil of the T1, T2 and T3 treatments were 41.54–43.84 μg/g, 11.16–13.37 μg/g, 45.51–76.11 μg/g, 14.83–21.93 μg/g, 18.54–33.23 μg/g and 0.12–0.14 μg/g, respectively. The heavy metal concentrations in the soil in this study were less than the agricultural land soil pollution risk screening values (such as Cr: 250 μg/g, Cu:100 μg/g, Zn: 300 μg/g, Pb: 170 μg/g, Ni: 190 μg/g, Cd: 0.6 μg/g) in the Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard [28]. The significant analysis showed that no significant difference was found in the heavy metals’ concentrations of the topsoil among different fertilizer treatments at the winter wheat harvest, and there was no interaction effect of year and treatment on the heavy metal concentrations of topsoil, indicating that, compared with chemical treatment, the short-term application of organic manure had no significant effect on the heavy metals’ concentrations in topsoil.

3.3. Heavy Metals’ Concentrations in Different Parts of Winter Wheat in Different Fertilizer Treatments

The concentrations of heavy metals in different organs of winter wheat under different fertilizer treatments are shown in Figure 3 The heavy metals’ (Cu, Zn, Pb, Cd, Ni, Cr) concentrations in wheat roots, stems–leaves and grains in all treatments were 0.12–43.25 μg/g, 0.03–20.12 μg/g and 0.01–32.98 μg/g, respectively. The six kinds of heavy metals concentrations in winter wheat grain were all less than the standard threshold limits (such as Pb:0.4 μg/g, Cd: 0.1 μg/g, Ni:1.0 μg/g, Cr:1.0 μg/g, Zn:50 μg/g, Cu:10 μg/g) [29]. The significant analysis showed that there was no significant difference in the heavy metals’ concentrations in each organ of winter wheat among different fertilizer treatments overall, indicating that the short-term application of organic manure did not affect the ability of wheat to absorb heavy metals. The result is consistent with the conclusion of Ugulu et al. (2021) [30], who found that the short-term (one season) application of farmyard manure and poultry manure had no significant effect on the average concentration of heavy metals in crops.

3.4. Heavy Metals’ Bioconcentration Factors in the Soil–Winter Wheat System in Different Fertilizer Treatments

The BCFs of heavy metals in the soil–wheat root, root–stem/leaf and stem/leaf–grain systems in the T1, T2 and T3 treatments in 2021–2022 were 0.24–1.19, 0.07–0.49 and 0.06–2.36, respectively (

Table 6. The bioconcentration factors of heavy metals in 2021–2022.

	Treatment	Cr	Cu	Zn	Cd	Pb	Ni
BCFs-r	T1	0.64 b	0.79 b	0.46 a	1.08 ab	0.30 a	0.26 a
	T2	0.98 a	0.88 ab	0.57 a	0.99 b	0.31 a	0.29 a
	T3	0.48 b	1.01 a	0.61 a	1.19 a	0.32 a	0.24 a
BCFr-s	T1	0.09 a	0.21 a	0.41 a	0.27 a	0.08 a	0.05 a
	T2	0.07 a	0.20 a	0.49 a	0.26 a	0.11 a	0.09 a
	T3	0.13 a	0.20 a	0.35 a	0.25 a	0.08 a	0.06 a
BCFs-g	T1	0.13 a	2.08 a	2.36 a	0.44 a	0.07 a	0.93 a
	T2	0.09 a	1.92 a	1.40 a	0.46 a	0.06 a	0.35 a
	T3	0.18 a	1.91 a	2.29 a	0.46 a	0.26 a	0.88 a

Note: The different lowercase letters in the same column of this table indicate there were significant differences among three treatments for each year (p < 0.05).

). The significant analysis showed that no significant difference was found in the BCFs of heavy metals among different fertilizer treatments (except BCF_s-r of Cr, Cu and Cd), indicating there was no significant difference in the BCFs of heavy metals in the soil–wheat grain system among different fertilizer treatments overall. So, the BCFs of each heavy metal in all fertilizer treatments were combined to analyze the migration of heavy metals (shown in Figure 4). Figure 4 shows that the BCF_r-s of heavy metals were lower than the BCF_s-r and BCF_s-g of heavy metals. The BCF_s-r, BCF_r-s and BCF_s-g of heavy metals were in the following order: Cd > Cu > Cr > Zn > Pb > Ni, Zn > Cd > Cu > Cr > Pb > Ni, Zn > Cu > Ni > Cd > Cr > Pb, respectively.

The BCF_so-g values of heavy metals for the T1, T2, and T3 treatments were in the range of ND-0.64 (

Table 7. The bioconcentration factors of heavy metals in the soil–grain system in 2021–2023.

Year	Treatment	Cr	Cu	Zn	Cd	Pb	Ni
2021–2022	T1	0.01 a	0.35 a	0.41 ab	0.13 a	0.00 a	0.01 a
	T2	0.00 a	0.32 a	0.34 b	0.11 a	0.00 a	0.01 a
	T3	0.01 a	0.38 a	0.47 a	0.13 a	0.01 a	0.01 a
2022–2023	T1	0.01 a	0.47 a	0.63 ab	0.23 a	0.00 a	0.04 a
	T2	0.01 a	0.40 a	0.64 a	0.18 b	0.00 a	0.02 a
	T3	0.01 a	0.44 a	0.57 b	0.22 a	0.00 a	0.01 a
Year		ns	*	*	*	ns	ns
Treatment		ns	ns	ns	*	ns	ns
Year × Treatment		ns	ns	ns	ns	ns	ns

Note: The different lowercase letters in the same column of this table indicate there was a significant difference among the three treatments for each year (p < 0.05). ns represents no significant difference among different years and different treatments for each heavy metal, * represent there was a significant difference among different years and different treatments for each heavy metal.

). The significant analysis showed no significant difference in the BCF_so-g of heavy metals in the soil–wheat grain system among different fertilizer treatments during the same year, and there was no significant difference in the BCF_so-g of Cr, Pb or Ni in the soil–wheat grain system between different years. There was no significant difference in the BCF_so-g of heavy metals in the soil–grain system fertilizer between two factors (different years and different fertilizer treatments). So, the BCF_so-g of each heavy metal in all treatments was combined to analyze the migration of heavy metals (shown in Figure 5). The BCF_so-g of Cu, Zn and Cd were 0.32–0.47, 0.34–0.64 and 0.11–0.23, respectively, and the BCF_so-g of Cr, Pb and Ni were less than 0.1.

4. Discussion

4.1. Effects of Different Fertilizer Treatments on Wheat Grain Yields and Wheat Grain Qualities

In this study, the applied organic manure had the same amount of nutrients (N, P, K) as the applied chemical fertilizer, while the nutrient releasing rate in manure was slow [31], so the grain yields and yield components of the T1 and T2 treatments were lower than those of the T3 treatment. Continuous application of organic manure can increase grain yield and yield components. For example, Han et al. (2020) [32] showed that wheat grain yield with long-term (34 years) application of manure and chemical fertilizer was significantly higher than that with application of chemical fertilizer, and the yield applied with organic and chemical fertilizer increased with the increasing in application years. Cisse et al. (2019) [27] reported that compared with the application of chemical fertilizer, the spike number of wheat significantly reduced by 1.45% when 50% of total fertilizer was replaced by organic manure in the first year, but the spike number of wheat significantly increased by 11.51% when 50% of total fertilizer was replaced by organic manure in the second year. In this study, the wheat grain yields of manure treatments (T1 and T2) were lower than that of chemical treatment by 25–38% and 2.57–10.5% in the first and second experiment years, respectively, and the yields in manure treatments in the second year were higher than these in the first year by 11.5–47.0%; the results are similar with those reported by Cisse et al. (2019) [27]. The values of total soluble sugar concentration and starch concentration in this paper were similar to that result of Mohammad et al. [33], who found that total soluble sugar concentration and starch concentration of wheat grain were 1–1.5% and 60–75%, respectively, when 50% of total fertilizer was replaced by bio-organic fertilizer. The crude fiber concentration (2.39%) and ash concentration (1.52%) of wheat grains in this study were similar to those of wheat grains with application of urea [34].

4.2. Effects of Different Fertilizer Treatments on Heavy Metals Concentrations in Topsoil

Heavy metals in organic manure fertilizers and chemical fertilizers could migrate to the soil, and the progress of migration is influenced by various soil parameters (pH, SOC, etc.) [18]. In this paper, the concentration of Zn in soil was higher than other heavy metals. The reasons were as the follows; firstly, the background value of Zn (38.7 μg/g) in soil was relatively high. Secondly, the concentrations of Zn in organic manures (T1: 103 μg/g, T2: 240 μg/g) were also relatively high. The heavy metals’ concentrations of topsoil for the three treatments in this paper were in the following order: Zn > Cr > Ni > Pb > Cu > Cd. Zhu et al. (2024) [35] found that the heavy metal concentrations in soil collected from coal mining area were in the following order: Zn > Cr > Cu > Cd. Ugulu et al. (2021) [16] found that the heavy metal concentrations in soil were in the following order: Zn > Cu > Cd when 40% of chemical fertilizer was replaced by organic cow manure. The results of this paper are consistent with the trend of the results reported by Zhu et al. [35] and Ugulu et al. [16].

4.3. Effects of Different Fertilizer Treatments on the Migration of Heavy Metals in Soil–Winter Wheat System

The BCF_so-g values of the six heavy metals in this paper were less than 1, indicating low migration ability of heavy metals from soil to wheat grain. And the BCF_so-g values of heavy metals were also lower than the corresponding BCF_s-r, the BCF_r-s, and the BCF_s-g, indicating that the transport and accumulation of heavy metals in wheat were hindered. The result could be explained by the following reasons; firstly, the lignification of roots and the presence of bacteria, fungi and compound around the root system of the crop alters the activity and mobility of heavy metals, thereby affecting their transport and accumulation in wheat [36,37]. Pb could be easy to combine with carbonate and oxide, organic matter in soil due to its high electronegativity, which hinders the migration of Pb [38]. Secondly, the organic acids released by wheat could combine with heavy metal cations to form a stable complex, which hinders the transport and accumulation of heavy metals in various tissues of wheat [39]. These are also the reasons why the concentrations of heavy metals (except Cu and Zn) in wheat root were higher than that in stem and leaf and wheat grain. This result is consistent with the conclusion of Ejaz et al. (2022) [40], who found the heavy metal concentrations in various tissues of wheat were in the following order: root > stem–leaf > grain when poultry manure (4.0 t/ha) and farmyard manure (8.0 t/ha) were applied. The concentrations of Zn and Cu were higher than the concentrations of the other heavy metals in wheat grain in this paper. The reason is that Zn and Cu easily bind with the transporters protein (HMA) in wheat, facilitating their migration to wheat grains [41,42]; in addition, Cu and Zn are essential elements in plants [43], so they are more likely to transfer to wheat grain. There were antagonistic relationships between Zn and Cu in crops. Zn could reduce the bioaccumulation factor and uptake of Cu in crops [44]; the foliar and soil Zn applications could reduce grain or stem–leaf Cd concentrations by the regulation of Cd transport genes [45], so the BCFr-s and BCFs-g values of Zn were higher than those of Cu and Cd. In this paper, the BCF_so-g values of Cu, Zn, and Cd were higher than the BCF_so-g values of other heavy metals. The result in this paper is consistent with Zhang et al. (2018) [46], who showed that the BCFs of Cu, Zn and Cd were higher than those of three other heavy metals (Pb, Ni and Cr) when organic fertilizer (225 kg/ha) was applied. Dong et al. (2023) [47] also found that wheat planted in intensive agricultural areas had a stronger bioaccumulation ability for Cu, Zn and Cd compared with other heavy metals (Pb, Ni, Cr).

5. Conclusions

The results of this study shown that, compared with chemical fertilizer, the short-term application of organic manure fertilizers (substituting all base fertilizers) decreased wheat grain yield, while it did not significantly increase heavy metals’ concentrations in the topsoil and various wheat tissues nor the bioconcentration factors of heavy metals in the soil–wheat system. This study was based on the application of organic manure instead of all base fertilizer during the two-year winter wheat season. Long-term application of organic manure has a better effect on crop growth due to the slow rate of nutrient release. However, heavy metals are difficult to degrade in soil, and the potential impact of the long-term application of organic fertilizers on soils and crops remains unclear due to the slow rate of nutrient release from organic manure fertilizers and the challenge in degrading heavy metals. Therefore, the effects of long-term application of organic manure on yield and the migration of heavy metals in the soil–crop system should be studied to safeguard the environment and guarantee food safety.