Application of Biogas Residues in Circular Agricultural Ecological Parks: Food Security and Soil Health

Abstract

Circular agriculture is an inevitable trend in the sustainable development of global agriculture. At present, there are issues such as insufficient utilization of resources, serious land pollution, and lack of technical support in circular agricultural ecological parks. This study explores the safe application of organic fertilizer to field crops within circular agricultural ecological parks. A peanut (Arachis hypogaea L.)–wheat (Triticum aestivum L.)–maize (Zea mays L.) crop rotation system was selected to research safe application methods of biogas residues on the peanut–wheat–maize crop rotation system. In this experiment, we set up different amounts of biogas residues as the base fertilizers, with no fertilizer and only chemical fertilizer treatments serving as controls. We determined the yield, quality, heavy metal content, and nutrient uptake of wheat, maize, and peanuts, as well as soil nutrient content and heavy metal content under different biogas residue application rates. The results of the experiment are as follows: (1) Biogas residue as a base fertilizer increased the yield of peanuts, wheat, and maize. The highest yield for each crop occurred when the biogas residue amount was 67,500 kg hm⁻², with yield increases of 36.7%, 26.6%, and 14.1% for peanuts, wheat, and maize, respectively, compared to the no fertilizer treatment. (2) The application of biogas residue improved the seed quality of peanuts, wheat, and maize. The B3 treatment showed strong quality improvement potential, increasing peanut crude protein content by 3.92–7.48%, soluble sugar content by 36.99–49.70%, crude fat content by 0.95–3.27%, wheat crude protein content by 2.22–8.72%, soluble sugar content by 6.21–8.51%, maize crude protein content by 2.87–3.61%, and soluble sugar content by 21.62–28.05% compared to the control. (3) The application of biogas residue enhanced the uptake of nutrients by crops and increased the contents of effective nutrients in the soil. (4) The application of biogas residue did not cause excessive accumulation of heavy metals in crops and soil. In conclusion, the application of biogas residue positively impacts crop growth, quality, and soil health in circular agricultural ecological parks, and has potential in agricultural production systems. Future research should focus on determining the optimal ratio of organic and chemical fertilizers, their efficient use, and the mechanisms by which organic fertilizer application can increase crop yield and quality, as well as improve soil quality.

1. Introduction

“Fertilizer is the food of crops”, and the use of agricultural chemicals contributes to about 50 percent of China’s increase in food production, effectively solving the problem of food security for a large base of the Chinese population. It plays an irreplaceable role in promoting China’s agricultural development and maintaining national food security [1,2,3]. However, the excessive use of chemical fertilizers has led to environmental issues such as soil acidification, atmospheric nitrogen (N) deposition, and declining water quality [4]. In 2009, 57% of the N and 69% of the phosphorus (P) entering water bodies in China came from agriculture, and agricultural nutrient losses have been a major component of diffuse water pollution [5]. Excessive fertilizer application also affects the nitrification process in the soil and increases greenhouse gas emissions [6,7]. It may also lead to the accumulation of salts and nitrates in the soil, which can lead to nitrate contamination of vegetables [8,9]. More alarmingly, in areas with higher fertilizer losses, people develop illnesses at a younger age. An increase of one kilogram of fertilizer loss per hectare of land correlates with an earlier onset of illness (by 0.267 years), which will have long-term impacts on public health [10].

To address this critical issue, in 2005, China launched large-scale soil testing and formulation for fertilizer application, and fertilizer reduction and efficiency was rolled out across the country. In 2021, the National People’s Congress voted to adopt the Resolution on the Outline of the Fourteenth Five-Year Plan for National Economic and Social Development and the Vision for 2035; In 2022, the “Central Government’s Document No. 1” reaffirmed the importance of optimizing the use of fertilizer and pesticides while maximizing the reducing their usage. New instructions were given on China’s ecological and environmental protection, “dual-carbon” work, green agricultural production, and the battle against pollution. Thus, combining sustainable agricultural development with food security in China is urgent.

Returning livestock and poultry manure and their wastes to the land is an important method of resource utilization for organic materials in China, with greater potential and prospects. However, with the development of large-scale farming in China, large quantities of livestock and poultry manure have increased the pressure for treatment [11]. However, these wastes can be converted into eco-friendly fuels for industrial applications through integrated hydrothermal carbonization and pyrolysis [12,13]. The biogas is rich in N, P, potassium (K), organic matter, vitamins, and beneficial bacteria, making it a high-quality organic fertilizer [14]. Biogas engineering is an effective measure of resource utilization; however, there are some bottlenecks [15]. Several scholars have studied biogas residue as an alternative to chemical fertilizer and examined their effect on crop growth and quality; the results revealed that the use of biogas residue as a fertilizer improves crop growth in terms of plant height, leaf area, and number of leaves [16]. They also showed that biogas residue has a low content of dry matter because it is rich in nutrients, such as N, which are required by plants at different stages of growth, and that using biofertilizers as an alternative to chemical fertilizers helps produce high-quality and safe plants. In addition, biogas residue and biochar fertilizers positively affect soil fertility [17]. Some scholars have suggested using biogas residue as a new method of fertilizer application, thus improving the growth response of plants [18]. Given the increasingly stringent requirements of environmental regulations on the P content in the soil, coupled with the increasing application of trace elements such as copper (Cu), zinc (Zn), lead (Pb), and chromium (Cr) as feed additives in livestock and poultry farming, there is an increase in the heavy metal content of livestock and poultry manure. The heavy metal elements in livestock and poultry manure are enriched in the biogas sludge due to the biodegradation of organic matter during the biogas fermentation process, thus increasing the safety of the agricultural application of biogas residue [19]. However, the absorption capacity of land is limited. Excessive application of biogas residue can cause problems such as crop growth damage, soil fertility decline, and heavy metal pollution. Therefore, to ensure the safety of agricultural products and land protection, it is necessary to grasp the application threshold of biogas residue to reasonably control the application amount of biogas residue [20].

Currently, studies on the effects of the application of organic fertilizers, such as livestock manure and municipal sludge, on the accumulation of heavy metals in crops and soils are conflicting. Results obtained by various researchers do not agree regarding the evaluation of the soil contamination risks of organic fertilizers and healthy human diets [21,22,23]. Peanuts (Arachis hypogaea L.), wheat (Triticum aestivum L.), and maize (Zea mays L.) play a pivotal role in China, and the effects of heavy metal stress and N fertilizer application on their metabolism are evident. Although some scholars have studied the effect of biogas residue application on crops, fewer studies have been conducted to comprehensively assess its benefits in the peanut–wheat–maize rotation system. For these reasons, peanuts, wheat, and maize were selected as the field test crops in this study. We aimed to (1) elucidate the effects of different biogas residue application rates on crop yield and quality, and (2) explore the potential mechanisms and environmental risks of biogas residue application on soil quality. We hypothesized that some certain amounts of biogas residue input would enhance nutrient levels in soil, thereby promoting crop growth, without leading to excessive residues of heavy metals in the crop and soil. This study case could provide a reference for the ecological model of the breeding cycle in China.

2. Materials and Methods

2.1. Overview of the Experimental Area

The test site is located in the recycling agricultural park of Jinlong Company (116.02° E, 37.67° N), Longhua Town, Jing County, Hebei Province, which belongs to the central part of the Heilonggang Basin in the North China Plain and the Hai he River low plain area. The average rainfall in this area over the past 10 years is 544 mm, and the average temperature in the past 10 years is 13.1 °C. It is a typical wheat–maize biannual cultivation area. The favorable topographic and hydrothermal conditions are suitable for crop growth and are important for the control of our experimental variables and the accuracy of our experimental results. The experiment was conducted in the large field planting area of the ecological demonstration park of Jinlong Company, covering an area of 570 m², with a specification of 38 m × 15 m. Before the experiment, soil samples were collected from the 0–20 cm of the tillage layer, and the soil nutrient and heavy metal contents were determined as follows: pH, 7.83; organic matter, 9.10 mg kg⁻¹; total N (TN), 0.65 mg kg⁻¹; available P, 39.08 mg kg⁻¹; available K, 188.50 mg kg⁻¹; Cu, 22.83 mg kg⁻¹; Zn, 70.93 mg kg⁻¹; As, 9.96 mg kg⁻¹; Pb, 21.47 mg kg⁻¹; Hg, 19.33 ug kg⁻¹; Cr, 59.07 mg kg⁻¹; and Cd, 0.15 mg kg⁻¹.

2.2. Experimental Design

The experiment was conducted in Hebei Jinlong Agricultural Park (116.02° E, 37.67° N) from May 2014 to October 2015, with the peanut and wheat–maize rotation. Peanuts were sown on 21 May 2014, and harvested on 27 September; wheat was sown on 13 October of the same year at a seeding rate of 225 kg hm⁻², and harvested on 10 June 2015; maize was sown on 13 June 2015, at a row spacing of 60 cm and a plant spacing of 24 cm, and harvested on 7 October of the same year. Peanuts and wheat received bottom fertilizer before sowing, consisting of biogas residue from the normal fermentation of farm biogas digesters. Chemical fertilizer was selected from Lusi compound fertilizer (N:P₂O₅:K₂O = 15:15:15) and urea was applied as a follow-up fertilizer in the later stage. The crop varieties that were tested were as follows: peanuts: Jihua 9, wheat: Xinmai 296, and maize: Junshi 9.

There were six treatments in the experiment, including two controls, i.e., CK1 (no fertilizer) and CK2 (only chemical fertilizer); the rest of the treatments were based on the CK2 treatment with biogas residue as the base fertilizer. According to the different application rates of biogas residue, they were set up as four treatments, namely, B1, B2, B3, and B4. The amounts of biogas residue applied were as follows: CK1, no fertilizer; CK2, 300 kg hm⁻² of chemical fertilizer (Lusi compound fertilizer); B1, 22,500 kg hm⁻² of biogas residue; B2, 45,000 kg hm⁻² of biogas residue; B3, 67,500 kg hm⁻² of biogas residue; B4, 90,000 kg hm⁻² of biogas residue; and urea, 200 kg hm⁻². The moisture content of the biogas residue was 60%. Each treatment was replicated thrice and randomly arranged. Biogas residue was chosen as the test material because the test area was an agricultural industrial park with a planting and feeding cycle, where recycling is conducive to both waste treatment and resource conservation. The confirmation of biogas residue application rate was based on the results of our pre-experiment. The nutrient and heavy metal contents of biogas residue are shown in

Table 1. Nutrient and heavy metal contents of biogas residues.

Nutrient	Organic Matter (g kg−1)	TN (g kg−1)	TP (g kg−1)	TK (g kg−1)	Available N (g kg−1)	Available P (mg kg−1)	Available K (mg kg−1)
	182.81	8.46	4.23	9.13	1.16	201.23	345.50
Heavy metal	Cu (mg kg−1)	Zn (mg kg−1)	As (mg kg−1)	Pb (mg kg−1)	Hg (μg kg−1)	Cr (mg kg−1)	Cd (mg kg−1)
	267.33	562.67	4.19	10.97	25.33	29.23	0.19

.

2.3. Indicator Measurements and Methods

2.3.1. Physicochemical Properties of Foundation Soil

Basal soil samples were taken before planting, air dried in their natural state, ground, and sieved to obtain powdered soil samples, which in turn were used to determine the physicochemical properties of the soil. Soil pH was determined by mixing soil and water at a ratio of 1:5, oscillating for 5 min; a pH meter was then used to determine the pH. Soil organic matter was determined by using the volumetric method with K dichromate and weighing air-dried soil samples in an oil bath. A certain amount of K₂Cr₂O₇ standard solution was added with concentrated sulfuric acid for 5 min. The reaction was then transferred into a conical flask and 2–3 drops of O-phorpholine indicator were added and titrated with FeSO₄. Soil TN was determined through combined digestion with concentrated sulfuric acid–mixed catalyst and a flow analyzer. Available P was determined using the molybdenum–antimony colorimetric method after the air-dried soil samples were first leached with 0.5 mol L⁻¹ of sodium bicarbonate. Fast K was determined as follows: air-dried soil samples were first extracted with 1 mol L⁻¹ of ammonium acetate; after which, available K was determined using a flame spectrophotometer (FP6410, AOYI Instruments Shanghai Co., Ltd., Shanghai, China). Fresh soil samples were leached with superior pure K chloride, and then soil nitrate N and ammonium N were measured using a flow analyzer [24].

2.3.2. Soil Nutrients and Water Content

The samples were collected immediately after harvesting, and soil samples in the 0–20, 20–40, and 40–60 cm soil layers were collected using a soil auger in each plot, respectively. The 0–60 cm soil depth range encompasses the root length density distribution and soil nutrient change processes of crops [25]. Fresh soil samples were extracted with superior pure K chloride to determine soil inorganic N (nitrate N and ammonium N) using a flow analyzer [24].

2.3.3. Crop Yield

Determination of peanut yield: Theoretical yield (kg m⁻²) = number of plants per hectare × number of fruits per plant × (100-fruit weight 100⁻¹).

Measurement of wheat yield: Theoretical yield (kg hm⁻²) = number of spikes per hectare × number of grains per spike × (1000-grain weight 100⁻¹).

Determination of maize yield: Theoretical yield (kg hm⁻²) = number of ears per hectare × number of grains per ear × (100-grain weight 100⁻¹).

Peanut hundred-kernel weight, wheat thousand-kernel weight, and maize hundred-kernel weight are measured as the dry weight.

2.3.4. Nitrogen, Phosphorus, and Potassium of Crop Plants

After harvesting, the fruits (seeds), plant stems, and leaves were dried, crushed, and ground into powder and then digested with concentrated sulfuric acid and hydrogen peroxide to obtain the digested liquid to be used. The decoction is analyzed using a flow analyzer to obtain the TN and total P (TP) of the plant, and total K (TK) was determined using a flame spectrophotometer [24].

2.3.5. Determination of Crop Quality Indicators

Crop products from each plot were sampled after harvest to determine soluble sugar, crude protein, and crude fat content. Soluble sugar was measured using the anthrone colorimetry method. Crude protein was calculated by measuring the TN content and crude fat was determined using the Soxhlet extraction method [24].

2.3.6. Heavy Metals in Plant and Soil Samples

Plant and soil samples were used to determine the heavy metal indicators, including Cu, Zn, As, Pb, Hg, Cd, and Cr, using an atomic absorption spectrophotometer [24].

2.3.7. Data Analysis

IBM SPSS Statistics v.26 and Excel 2019 were used to process and perform statistical analyses on all data. Significant differences between the control and treatment groups were identified using one-way analysis of variance and mean comparison tests (LSD). Origin 2024 was used to plot graphs. A comprehensive evaluation of experimental data was conducted using the fuzzy mathematical affiliation function method [26]. The value of the affiliation function is calculated using Equation (1), as follows:

U(X_i) = (X − X_min)/(X_max − X_min), i = 1, 2, 3, …, n,

(1)

where X is the measured value of an index of the test material, X_max is the maximum value of the index, and X_min is the minimum value of the index. If an index and nutritional quality are negatively correlated, we use the inverse affiliation function for conversion. The inverse affiliation function is calculated using Equation (2), as follows:

U(X_i) = 1 − (X − X_min)(X_max − X_min), i = 1, 2, 3, …, n.

(2)

The values of the specific characteristic affiliation function for each indicator were summed and averaged for each test crop under the different treatments.

3. Results

3.1. Effects of Different Amounts of Biogas Residue on Crop Yield

Using biogas residue as a base fertilizer can increase yields, subsequently decreasing with higher application rates (Figure 1). When peanuts were used, the B3 treatment had the highest yield of 4918.4 kg hm⁻², considerably higher than the yields of CK1 and CK2, with a 14.1% increase relative to CK2 and a 36.7% increase relative to CK1. The yields of B1, B2, and B4 treatments were higher than those of the control. When wheat was used, the yield of the B3 treatment was higher than the other treatments, which was 7616.7 kg hm⁻², with a 26.6% increase relative to CK1 and a 6.7% increase relative to CK2. The yields of B1, B2, and B4 treatments were somewhat higher than CK2 but the difference was not significant, whereas there was a significant increase in the yields of B1, B2, and B4 treatments compared with that of CK1. When maize was used, the yields of B1, B2, B3, and B4 treatments were higher than those of controls, CK1 and CK2, and the trend of the treatments was the same as that of wheat; the yield of B3 treatment was the highest at 10,497.4 kg hm⁻². In conclusion, in the peanut and wheat–maize continuous cropping system, a very high application rate could reduce the crop yield, and the appropriate application rate is 67,500 kg hm⁻².

3.2. Effects of Different Amounts of Biogas Residue on Soil Nutrient Content after Crop Harvest

After crop harvesting, the trend of inorganic N content remained the same in the soil layers, and the difference between each treatment and the control in the 0–20 cm soil layer reached a significant level. In addition, the inorganic N content in other soil layers increased significantly compared with the control, and with increasing biogas residue application, inorganic N content in each soil layer increased, indicating that the application of biogas residue and other substances was beneficial for the transformation of organic N to inorganic N in the soil. This may be related to the bioactive substances present in the biogas residue. The root system of crops is mainly distributed in the 10–30 cm soil layer, and the demand for effective N in the soil in the upper and lower parts of this layer is higher. This causes the inorganic N content in the 20–40 cm soil layer to be lower than that in the 40–60 cm soil layer after the harvesting of peanuts; simultaneously, the accumulation of inorganic N in the <40 cm soil layer cannot be fully utilized by the crop, leading to the wastage of resources and an increase in environmental pollution risk. Available P and K contents in all soil layers of each treatment after crop harvest increased with the application of bottom fertilizer and were significantly different compared to the control (

Table 2. Effects of different biogas residue dosages on postharvest soil nutrient content of crops.

	Crops	Treatments	Nutrient Content (mg kg−1)
Nutrient			Inorganic N			Available P			Available K
soil layer (cm)			0–20	20–40	40–60	0–20	20–40	40–60	0–20	20–40	40–60
	Peanut	CK1	118.64f	99.13e	105.60c	20.77e	1.68d	9.41f	135.17f	49.00e	49.50d
		CK2	135.21e	104.19d	109.06d	36.20d	2.15cd	10.23e	142.50e	52.83d	51.50c
		B1	149.54d	105.47d	110.28c	47.43c	2.64c	11.21d	148.17c	53.50d	52.28c
		B2	155.22c	119.92c	126.50b	59.67b	3.28b	13.42c	155.00b	56.00c	56.33b
		B3	165.80b	127.09b	130.95b	61.57b	3.55b	14.36b	168.50a	59.50b	58.83a
		B4	171.96a	136.65a	140.72a	68.50a	4.71a	16.16a	171.17a	62.50a	60.17a
	Wheat	CK1	29.65e	31.02c	35.46c	56.43f	4.68f	18.08f	118.50f	42.67f	43.50f
		CK2	33.71de	34.49c	38.44c	84.87e	6.15e	21.23e	147.50e	53.50e	52.17e
		B1	35.82d	31.59c	38.90c	107.43d	8.64d	23.88d	188.17d	63.50d	62.28d
		B2	42.58c	41.13b	50.24b	134.00c	13.28c	29.08c	235.00c	71.00c	68.67c
		B3	58.70b	38.98b	52.07b	153.23b	15.88b	34.03b	248.50b	79.83b	78.83b
		B4	67.85a	54.23a	64.07a	169.83a	18.71a	38.83a	271.17a	83.17a	84.00a
	Maize	CK1	19.76e	18.25c	22.16c	17.77f	1.61e	8.08e	114.17f	39.33f	40.50e
		CK2	22.47de	20.29c	24.03c	23.93e	2.12d	9.57d	124.17e	43.50e	45.50c
		B1	23.88d	18.58c	24.31c	48.43d	2.04d	12.21c	138.17d	53.50d	42.28d
		B2	28.39c	24.19b	31.40c	52.00c	3.35c	13.08c	145.00c	55.33c	46.67c
		B3	39.13b	22.93b	32.55b	64.23b	3.75b	14.36b	156.17b	59.83b	58.17b
		B4	45.24a	31.90a	40.05a	69.83a	4.71a	15.8a	161.17a	63.17a	62.00a

Note: Lowercase letters indicate significance among treatments at the specific soil layer.

).

3.3. Effects of Different Amounts of Biogas Residue on Crop Quality

The crude protein content of the peanut kernel was significantly higher than that of CK1 in all the treatments; however, the B3 and B4 treatments showed significantly higher contents than CK2, and the B3 content was the highest at 30.46%. Peanut kernel soluble sugar content was in the order of B1 < B2 < B3 > B4, and the soluble sugar content of each treatment was significantly higher than the controls CK1 and CK2, of which, the content of B3 was the highest (22.59%) compared to B1, B2, and B4 treatments. The crude fat content of the B3 treatment was the highest, and that of all treatments was significantly higher than that of CK1, but the differences between the other treatments were not significant (Figure 2).

There was no significant difference in crude protein between the biogas treatments and the control. The soluble sugar content in wheat grains in each treatment increased first and then decreased with the increasing biogas residue application. The highest content was observed in the B1 treatment, whereas the content in other treatments was higher than that of the control. The B1 treatment showed significant differences from the control, whereas B2, B3, B4, and the control showed no significant differences (Figure 3).

The crude protein content of maize kernels did not differ significantly between the treatment and control groups, and in general, showed a tendency to increase and then decrease with the increasing application of biogas residues. The highest crude protein content was recorded in the B3 treatment, whereas the crude protein content decreased in the B4 treatment. The content of soluble sugars in maize kernels treated with B1 was significantly increased compared with the control group, and there was a significant difference between the treatment and control groups (Figure 4). In addition, different amounts of biogas residue affected the nutrient uptake of the crop. The contents of TN, TP, and TK in different plant organs in each treatment after harvesting are shown in Figures S1–S3.

3.4. Effects of Different Amounts of Biogas Residue on Heavy Metal Content in Crops

The Pb and Cr contents absorbed by peanut plants are mainly enriched and distributed in the stems and leaves; Cd is mainly enriched in the stems and leaves and Hg is mainly enriched in the fruit kernel. As is mainly distributed in the stems and leaves and the fruit shell, Zn content is the highest in the kernel, and Cu is mainly enriched in the stems and leaves. The Pb content in peanut kernels under each treatment meets the permissible limit of ≤0.4 mg kg⁻¹ for pollution-free peanuts set by the Ministry of Agriculture of China. The Cd content in CK1 and B4 treatments (with a moisture content of 5% in the peanut shell) exceeds the permissible limit of ≤0.05 mg kg⁻¹ (fresh weight content) for pollution-free peanuts set by the Ministry of Agriculture of China, but meets the limit set by the WAO/WHO (≤0.1 mg kg⁻¹). The Cd content in other treatment kernels meets the permissible limit of ≤0.05 mg kg⁻¹ for pollution-free peanuts set by the Ministry of Agriculture of China (

Table 3. Effects of different amounts of biogas residues on heavy metal content in crops.

	Crops	Organs	Treatments	Heavy Metal Content (mg kg−1; Hg: μg kg−1)
Types				Cu	Zn	As	Pb	Hg	Cr	Cd
	Peanut	Stem leaf	CK1	25.113b	26.000c	0.590b	1.523d	0.025bc	2.437a	0.096bc
			CK2	25.723b	25.900c	0.637ab	1.613cd	0.026bc	2.500a	0.093c
			B1	8.073c	31.867a	0.580b	1.700abc	0.028a	2.620a	0.096bc
			B2	35.837a	28.700b	0.630ab	1.767ab	0.024c	3.287a	0.105ab
			B3	25.510b	31.033a	0.607ab	1.680bc	0.026b	2.593a	0.099bc
			B4	25.220b	28.667b	0.680a	1.800a	0.026bc	2.697a	0.113a
		Peanut kernel	CK1	12.88a	11.75b	0.64a	0.51cd	5.73b	1.64bc	0.028a
			CK2	13.27a	10.77cd	0.63a	0.68a	4.50c	1.86a	0.021c
			B1	9.57d	11.40bc	0.48c	0.47d	5.40b	1.26d	0.022c
			B2	12.40b	13.50a	0.62a	0.56bc	6.10b	1.62c	0.029a
			B3	11.83c	12.77a	0.68a	0.60b	7.40a	1.70bc	0.024bc
			B4	13.18a	10.13d	0.55b	0.57bc	5.40b	1.72b	0.026ab
		Peanut shell	CK1	13.47b	41.00a	0.049bcd	0.019cd	1.77b	0.34c	0.057ab
			CK2	12.23c	38.97b	0.043cd	0.023bc	2.10a	0.33c	0.046bc
			B1	15.30a	41.43a	0.040d	0.018d	1.57b	0.35c	0.042c
			B2	13.13b	41.23a	0.061	0.025ab	2.17a	0.42a	0.045bc
			B3	13.30b	41.27a	0.051	0.020cd	1.50b	0.40ab	0.046bc
			B4	11.24d	32.00c	0.049	0.027a	1.50b	0.36bc	0.064a
	Wheat	Stem leaf	CK1	5.470a	24.000a	0.560a	1.327a	0.023a	1.273a	0.053a
			CK2	4.800a	25.100a	0.573a	1.223a	0.021ab	1.283a	0.053a
			B1	5.003a	25.067a	0.530a	1.107a	0.019bc	1.110a	0.055a
			B2	4.560a	25.800a	0.497a	1.067a	0.018c	0.933a	0.050a
			B3	4.603a	22.800a	0.503a	1.097a	0.019bc	0.997a	0.060a
			B4	6.107a	25.800a	0.547a	1.113a	0.019c	1.240a	0.054a
		Seed	CK1	4.547b	23.400b	0.057bc	0.051b	0.603a	0.095a	7.300a
			CK2	4.750b	24.233b	0.052c	0.056ab	0.573a	0.099a	7.000a
			B1	5.433ab	31.767a	0.062abc	0.052b	0.680a	0.089a	8.833a
			B2	5.707a	35.067a	0.065ab	0.061ab	0.433a	0.098a	9.267a
			B3	5.427ab	35.467a	0.073a	0.063ab	0.467a	0.091a	10.400a
			B4	5.407ab	30.933a	0.068ab	0.072a	0.733a	0.097a	7.700a
	Maize	Stem leaf	CK1	9.633a	25.967a	0.380a	1.153a	0.012a	0.747a	0.061a
			CK2	8.557ab	26.467a	0.310ab	0.873bc	0.009bc	0.647a	0.055ab
			B1	7.263b	23.867a	0.307ab	0.950abc	0.008c	0.647a	0.051ab
			B2	8.300ab	24.600a	0.333ab	1.013ab	0.009bc	0.693a	0.060a
			B3	7.327b	22.300a	0.293b	0.720c	0.007c	0.627a	0.043b
			B4	9.980a	27.033a	0.373ab	1.153a	0.012ab	0.683a	0.063a
		Seed	CK1	1.257a	17.767b	0.020a	0.007a	0.497a	0.056a	1.183a
			CK2	1.590a	19.367ab	0.017a	0.007a	0.513a	0.071a	1.833a
			B1	1.200a	19.800ab	0.018a	0.005a	0.473a	0.061a	1.153a
			B2	1.480a	22.467a	0.019a	0.004a	0.437a	0.090a	1.220a
			B3	1.393a	17.033b	0.017a	0.004a	0.523a	0.084a	1.253a
			B4	1.260a	18.533b	0.015a	0.005a	0.583a	0.099a	1.193a

Note: Lowercase letters indicate significance among treatments at the specific soil layer.

).

After harvest, Cu and Zn contents in wheat grains were equivalent to those in stems and leaves, whereas the overall contents of As, Pb, Hg, Cr, and Cd in grains were lower than those in stems and leaves. Moreover, the contents of various heavy metals in grains met the seven-element limit standards for grain and products in China (Cu ≤ 10 mg kg⁻¹, Zn ≤ 50 mg kg⁻¹, As ≤ 0.7 mg kg⁻¹, Pb ≤ 0.4 mg kg⁻¹, Hg ≤ 0.02 mg kg⁻¹, Cr ≤ 1.0 mg kg⁻¹, and Cd ≤ 0.1 mg kg⁻¹). Cu, Zn, and As contents in the grains of the wheat postharvest treatment group increased compared with those in the control group. There was a significant difference in Pb content between B4 and CK1 and B1, whereas there were no significant differences in the contents of various heavy metal indicators (Pb, Hg, Cr, and Cd) in the grains of other treatments (Table 3).

After harvest, the Cu, Zn, As, Pb, Hg, Cr, and Cd contents in maize kernels were lower than those in stems and leaves, and the contents of various heavy metals in the kernels met the standards for seven elements, including Pb, Cr, Cd, Hg, As, Cu, and Zn in grain and products in China (Cu ≤ 10 mg kg⁻¹, Zn ≤ 50 mg kg⁻¹, As ≤ 0.7 mg kg⁻¹, Pb ≤ 0.4 mg kg⁻¹, Hg ≤ 0.02 mg kg⁻¹, Cr ≤ 1.0 mg kg⁻¹, and Cd ≤ 0.05 mg kg⁻¹; Table 3).

3.5. Effects of Different Fertilization Treatments on Heavy Metal Content in Soil

The heavy metal content in the soil after harvest is shown in (

Table 4. Effects of different fertilization treatments on heavy metal content in soil after crop harvest.

	Crops	Treatments	Heavy Metal Content (mg kg−1; Hg: μg kg−1)
Types			Cu	Zn	As	Pb	Hg	Cr	Cd
	Peanut	CK1	19.67a	62.03a	9.64a	19.03a	16.00a	56.87ab	0.13a
		CK2	19.47a	63.27a	9.58a	19.13a	16.33a	56.97ab	0.13a
		B1	19.07a	63.07a	9.51a	20.17a	16.33a	56.27b	0.14a
		B2	20.97a	65.97a	9.85a	19.57a	18.67a	59.10a	0.14a
		B3	20.87a	58.27a	9.60a	19.33a	19.33a	56.70ab	0.13a
		B4	21.27a	64.03a	9.52a	19.17a	16.67a	55.83b	0.14a
	Wheat	CK1	23.050a	64.800b	9.765ab	20.200a	17.500a	55.850a	0.135a
		CK2	23.067a	64.533b	9.753b	20.267a	16.667a	54.400a	0.127a
		B1	25.767a	71.733ab	9.670b	20.533a	20.333a	54.800a	0.137a
		B2	26.767a	75.233a	9.980ab	20.033a	17.333a	55.433a	0.127a
		B3	31.500a	79.100a	10.167a	20.367a	19.333a	55.300a	0.130a
		B4	29.633a	78.067a	10.030ab	20.200a	19.333a	55.933a	0.133a
	Maize	CK1	27.900a	75.267ab	9.700bcd	20.033b	16.667a	54.800c	0.130a
		CK2	26.067a	75.467ab	9.567cd	20.200ab	14.000a	55.767bc	0.137a
		B1	30.600a	76.967a	9.907abc	19.900b	17.333a	54.933bc	0.133a
		B2	25.767a	70.967ab	10.107a	20.933ab	20.000a	57.633ab	0.140a
		B3	28.400a	74.400ab	9.343d	20.967ab	15.333a	56.200bc	0.137a
		B4	25.300a	65.967b	10.047ab	21.300a	14.000a	59.267a	0.143a

Note: Lowercase letters indicate significance among treatments at the specific soil layer.

). Compared to the environmental quality standard values outlined in the national standard (Table S1), the contents of various heavy metal indicators in the soil after harvest are lower than those of the national first-level standard. After the peanut–wheat–maize rotation cycle, Cu and Zn contents in the soil increased, whereas As, Pb, Hg, Cr, and Cd contents showed a decreasing trend; however, all were lower than the heavy metal content specified in the national first-class standard.

3.6. Comprehensive Evaluation of Biogas Residue Application

The fuzzy mathematical affiliation function method was used to comprehensively evaluate the experimental treatments to specify the best treatment for each crop. The indicators measured in the experiment were classified into five broad categories: yield, quality, soil nutrient content (0–20 cm), crop heavy metal content, and soil heavy metal content. Among them, the values of the yield, quality, and soil nutrient content were positively correlated with agricultural production and calculated using Equation (1). The values of crop and soil heavy metal contents were not large; hence, they were calculated using Equation (2). The analysis of the affiliation function revealed that the application of biogas residue in peanuts, wheat, and maize cultivation was better as a result of more application, and that the B3 treatment showed a strong advantage (

Table 5. Comprehensive evaluation of different treatments for each crop.

	Crops	Treatments	Membership Mean
Indicators			Yield	Quality	Soil Nutrient	Crop Heavy Metal	Soil Heavy Metal	Comprehensive	Order
	Peanut	CK1	0.000	0.000	0.000	0.586	0.721	0.262	6
		CK2	0.616	0.447	0.280	0.590	0.775	0.541	5
		B1	0.708	0.660	0.499	0.708	0.592	0.634	3
		B2	0.971	0.754	0.685	0.292	0.124	0.565	4
		B3	1.000	1.000	0.889	0.457	0.630	0.795	1
		B4	0.872	0.638	1.000	0.479	0.561	0.710	2
	Wheat	CK1	0.000	0.000	0.000	0.459	0.616	0.215	6
		CK2	0.876	0.222	0.187	0.511	0.909	0.539	4
		B1	0.884	0.233	0.356	0.606	0.460	0.508	5
		B2	0.904	0.470	0.595	0.665	0.561	0.639	2
		B3	1.000	0.667	0.824	0.626	0.202	0.664	1
		B4	0.886	0.304	1.000	0.380	0.367	0.583	3
	Maize	CK1	0.000	0.000	0.000	0.393	0.665	0.212	6
		CK2	0.360	0.067	0.143	0.459	0.681	0.342	5
		B1	0.704	0.362	0.416	0.809	0.490	0.556	2
		B2	0.856	0.181	0.548	0.465	0.334	0.477	4
		B3	1.000	0.406	0.850	0.822	0.550	0.725	1
		B4	0.705	0.304	1.000	0.325	0.440	0.555	3

Note: Yield (yield of crops); quality (crude protein, soluble sugar, and crude fat content of the crop); SOIL nutrient (inorganic N, available P, available K); crop heavy metal (heavy metal content in post-harvest crops); Soil heavy metal (heavy metal content in post-harvest soil).

).

4. Discussion

4.1. Crop Yield Response to the Biogas Residue Application Rate

The potential of organic materials (such as manure [27,28] and straw [29,30]) to increase crop yields has been proven. Biogas residue is derived from the fermentation of a wide range of organic materials and contains various plant nutrients, making it a valuable fertilizer that increases crop yields. Studies have shown that the application of biogas residue can increase wheat yield [31]. Our study also showed that compared with the control, the application of biogas residue increased the yields of peanuts, wheat, and maize and that there was an increasing and then decreasing trend of yield (Figure 1). Sieling reported that the biogas residues that remained after anaerobic digestion provided a valuable source of nutrients and improved the efficiency of nutrient use in the field and on the farm [32]. The yield-enhancing benefits of biogas residues are closely related to N flow in the soil–plant system [33]. We examined the TN contents of different tissues of different crops and found that the application of biogas residue positively affected the crop’s N uptake (Figures S1–S3b). There is a strong correlation between crop yield increases and soil nutrients (Figure S4); Organic fertilizer application affects the intensity of nutrient uptake by the crop at a given growth period, which in turn alters nutrient retention and release [34]. The yield-enhancing mechanism of organic fertilizers is mainly attributed to their effect on soil quality, modification of crop root traits, and soil microorganisms [35].

4.2. Effect of Biogas Residue Application on Crop Quality

Organic materials modify the quality of crops in a number of ways, including increasing nutrient supply, improving soil ecology, promoting growth and development, and improving soil properties. This study showed that the application of biogas residue increased maize thousand kernel weight, crude fat, and protein contents; simultaneously, the application of biogas residues increased maize cellulose and lignin contents and their related synthase activities, significantly improving the mechanical properties of maize internodes and leading to a significant increase in the stalk resistance index [36]. It has also been shown that solid biogas residue application significantly affects sorghum and sorghum × Sudan hybrid forage quality [37]. Evidently, the application of biogas residue is important for improving the growth and nutritional quality of crops. Our study also showed that the application of biogas residue can improve the quality of peanuts, wheat, and maize (Figure 2, Figure 3 and Figure 4). However, this effect did not show a significant trend with the amount of biogas residues applied. Another important reason why organic fertilizer improves crop quality might be that organic fertilizer can increase the antioxidant activity of crops [38].

4.3. Fertilizer Benefits of Biogas Residue Application

Biogas residue, being richer in nutrients, has more potential applications compared to raw organic materials. In particular, biogas residues and compost have a higher nutrient content, stable chemical composition [39], and fewer pathogenic microorganisms [40] than animal manure used directly as fertilizer, thus conferring a better fertilizer efficiency. From a carbon-neutral perspective, biogas residues and compost promote the conservation and sequestration of soil organic carbon [41]. The results of our study were similar to those of previous studies, showing that the application of biogas residues increased the nutrient content of all the soil layers of the postharvest crop, and the postharvest soil nutrient accumulation of the crop increased with the increasing rate of biogas residue application compared with the control (Table 2). Excessive nutrient residue application is not beneficial, and we should also consider the environmental risks of nutrient loss and accurately assess the threshold between soil enrichment and environmental pollution.

4.4. Evaluation of Biogas Residue Application on Food Safety and Soil Health

Food safety and soil health have been widely used as measures of land reclamation [42,43] and sustainability [44,45,46]. Currently, studies have shown that the application of biogas residues improves the quality and heavy metal accumulation of crops [47,48]. This coincides with the findings of our study; notably, the accumulation of heavy metals did not exceed the national or other organizational limits (Table 3). The addition of organic fertilizer positively impacts soil health [49,50,51]; however, the link between biogas residue application and soil health and agronomic gains is less clear. Our research shows that the addition of biogas residues improves soil nutrients, but the increase in soil nutrients is not directly proportional to the application rate. However, the heavy metals carried by the biogas residue also increased the accumulation of heavy metals in the soil, which fortunately did not exceed China’s environmental limits (Table 2 and Table 4). Currently, some scholars are attempting to use technology to treat biogas residues by changing their structural properties, to increase nutrient migration, promote the maturation of organic matter [52], and achieve resource-efficient nutrient use with minimal residuals of harmful substances. Although animal manure can be used as an effective resource, its improper disposal can lead to agricultural nonpoint source pollution [53]. Therefore, when assessing the environmental impacts of biogas residues and composting, it is crucial to thoroughly consider the results of multiple assessments to improve the comprehensiveness and accuracy of the results. It is also important to consider the effects of additives applied in the anaerobic digestion and composting of animal manure [54]. However, as most of the nutrients from organic fertilizers are usually slowly released; thus, the long-term effects of organic fertilizers on soil health and crop yields require further research. Other studies have shown that long-term organic fertilizer application reduces the disease resistance of crops [55,56].

5. Conclusions

This study clearly shows that the optimal application rate of biogas residue in circular agricultural ecological parks is 67,500 kg hm⁻². On this basis, the application of biogas residue as a base fertilizer significantly increased crop yield and quality without causing excessive accumulation of heavy metals in the stems, leaves, and seeds of the crops. Moreover, after the crop harvest, the contents of N, P, and K in the soil increased significantly; the contents of Cu and Zn in the soil increased, and the contents of As, Pb, Hg, Cr, and Cd showed a decreasing trend. This research supports the safe use of organic fertilizers to field crops in circular agricultural ecological parks. Notably, it also provides technical support and a scientific basis for constructing a recycled agriculture technology model for an integrated park that combines planting and raising. Future research should consider the long-term synergistic mechanisms of the biogas residue–crop–soil environment to determine the optimal ratio of organic and chemical fertilizer allocation. In addition, further research is needed for the large-scale replication and demonstration of the trial results.