Co-Compost Application of Magnesium Salts and Orthophosphate Adjusted Biochar and Cyanobacteria for Fixing Nitrogen, Improving Maize Quality, and Reducing Field Nutrient Loss

Abstract

To solve the problem of nitrogen loss and harmful gas emissions in the co-composting of cyanobacteria with other wastes and agricultural non-point source pollution in the Chaohu Lake Basin (CHLB), this study selected cyanobacteria as the main material, spent mushroom and rice hulls as excipients, and biochar (B) and Mg²⁺/PO₄³⁻ salt as an additive. The nitrogen fixation rate, maturity, toxicity, and fertilizer efficiency indexes were analyzed to explore the best rate of biochar application. Additionally, three different fertilization treatments (C0: No fertilization, CF: Chemical fertilizer, OF: Organic fertilizer) were applied to agricultural field experiments in the CHLB. The results showed that the cyanobacteria composting experiment with 10% B as additives had the lowest cumulative NH₃ emission (603.00 g) and the highest nitrogen fixation rate (61.54%). We used this compost product for maize cultivation in field experiments. The results show that OF treatment can reduce field nutrient loss, and the increase in the rate of soil TN fixation was as follows: OF (18.62%) > CF (10.34%) > C0 (−9.66%). Moreover, the TN and TK losses in runoff were 33.33% and 22.74% lower than CF in farmland runoff. The OF treatment increased the content of crude fat, crude protein, and crude starch by 4.40%, 8.34%, and 2.66% compared with the CF treatment, respectively. Overall, the N, P, and K source agronomic coefficients of cyanobacteria organic fertilizer were 1.424, 1.001, and 1.16, respectively. These results indicated that cyanobacterial organic fertilizer could replace chemical fertilizer to reduce agricultural non-point source pollution and improve maize quality. Meanwhile, the amendment of B and Mg/P salt additives could reduce air pollution and improve the quality of compost products. This study provides a theoretical basis and reference for nitrogen fixation and extension of multiple wastes co-composting field experiments.

1. Introduction

In agricultural modernization, the imbalance of nutrient input and output in many water bodies has resulted in the eutrophication of water and the outbreak of cyanobacteria bloom, which has become one of the most challenging environmental problems at present [1]. These nutrients may originate from both point pollution sources, such as industrial wastewater and municipal sewage, and non-point sources, such as agricultural runoffs [2]. In recent years, with the continuous improvement of supervision and law enforcement, non-point source pollution has gradually replaced the key position of point source pollution, becoming another focus of water pollution control. Among many non-point sources, agricultural non-point source pollution has attracted wider attention due to its strong concealment, wide coverage, and great difficulty in control [3]. According to the results of the Second National Census of Pollution Sources Bulletin jointly issued in June 2020, total nitrogen (TN) and total phosphorus (TP) discharged by agricultural sources are 1,414,900 and 212,000 t, respectively, accounting for 46.52% and 67.22% of the total emissions of surface water pollutants (TN and TP) in China, respectively, which has far exceeded the discharge volume of industrial and domestic sources and become the top of all pollution sources. Among them, the planting industry emits 719,500 t of TN and 76,200 t of TP, accounting for 50.85% and 35.94% of agricultural non-point sources (AGNPS), respectively. The excessive use of fertilizers and pesticides during agricultural production is the main cause of this result [4].

The above problems are very prominent in China’s Chaohu Lake basin (CHLB). The Chaohu Lake is the fifth largest freshwater lake in China, with a water area of about 770 km² [5]. After the 1980s, as China’s growing population led to conflicts between population and land, farmers began to build fields around lakes and reclaim wetlands. Moreover, the extensive use of chemical fertilizers and pesticides in production increased the runoff loss of soil and nutrients, coupled with the discharge of urban sewage into surrounding areas, causing Chaohu lake to suffer a significant amount of N and P pollution [6]. Eutrophication and cyanobacterial blooms have become the main environmental problem in Chaohu Lake [7]. There are some methods to control cyanobacterial blooms, such as the physical method (direct salvage method), the chemical method (hydrogen peroxide method) [8], and the biological method (polyculture patterns) [9]. The direct salvage method is an effective emergency control method to deal with cyanobacterial blooms. Due to cyanobacterial microcystins, if these salvaged cyanobacteria are not treated properly and in a timely fashion, they easily cause secondary pollution. However, the rich nutrients in cyanobacteria have the potential to become a fertilizer resource. Currently, treatment methods commonly include aerobic compost and anaerobic fermentation [10].

Aerobic co-composting is an aerobic, high-temperature, microbial-mediated solid-state fermentation process of different mixtures. Co-composting not only simultaneously disposes of several organic wastes but can also enhance compost quality by comprehensively using diversified waste properties [11]. Compared to anaerobic fermentation, the high-temperature phase of aerobic composting helps eliminate pathogens, pests, and weed seeds, making the resulting compost safer and without biogas slurry [12]. However, numerous studies have shown that nitrogen losses are heavy via NH₃ during the heating and high-temperature stages in aerobic composting [13,14]. For example, Jiang et al. (2011) studied animal manure composting and found that 9.6~46% of the initial nitrogen content was lost as NH₃ [15]. Eklind et al. (2000) found lost nitrogen content of about 50%~60% during urban sludge composting treatment [16]. For the above reasons, the current common methods to control NH₃ emissions can be divided into three categories: Physical methods mainly include (1) adding materials with physical adsorption properties, such as zeolite [17], biochar [18], and wood vinegar [19], (2) adjusting the properties of the compost material itself, such as reducing the pH by pickling the composting material [20], and (3) improving compost processing conditions, such as implementing suitable temperature control, ventilation, and humidification [21]. Chemical methods mainly change the essential properties of compost materials or react with NH3 by adding chemicals, such as calcium superphosphate, aluminum sulfate, and calcium chloride [22]. Bio-methods mainly include increasing exogenous microorganisms to improve nitrogen consumption rates during composting and reduce NH₃ emissions while fixing more nitrogen [23]. Among them, the precipitation of struvite MgNH₄PO₄·6H₂O method (MAP) was previously widely used in the recycling and reuse of P and N in wastewater treatment. Jeong et al. (2001) first applied MAP to aerobic composting, providing a new idea for N loss control during composting [24]. Lee et al. (2009) precipitated NH₃ released from composting as crystalline magnesium phosphate by adding MgCl₂ and orthophosphate, effectively reducing the volatilization of nitrogen. In addition, the resulting compost product is an excellent organic and inorganic compound slow-release fertilizer with a high fertilizer effect [25]. Many studies have confirmed that MAP has positive effects on reducing NH₃ emissions during the composting of food waste [26], poultry [25], and pig manure [27]. Biochar is environmentally friendly and has good stability and high porosity [28]. Not only does biochar performs well in soil-improvement applications, but it also holds promising prospects in composting [29]. Biochar adds compost as a beneficial amendment to increase organic degradation rates [30] and reduce greenhouse gas emissions losses [31]. Wang et al. (2018) showed that adding the biochar compound zeolite and wood vinegar has a positive role in controlling the greenhouse gas and NH₃ emissions produced by the compost while also improving the composting quality [19]. The increases in temperature, pH, aeration, and organic mineralization during composting may lead to increased NH₃ emissions. However, biochar relies on its pores or surface adsorption of NH₃ and NH₄⁺ to reduce the NH₃ release and increase the fixation rate of nitrogen [32]. Steiner et al. (2010) reported that in poultry litter composting containing 20% (w/w) biochar, NH₃ concentrations in emissions decreased by 64%, and there was a 52% reduction in total nitrogen loss [33]. Research by Dias et al. (2010) showed that acid treatment on the biochar surface could capture the NH₄⁺, thus preventing the volatilization of NH₃ during aerobic composting [34]. Furthermore, biochar-treated composting has more enzyme activity, so the soluble organic carbon produced by cellulose decomposition may be combined with microbial utilization of NH₄⁺, thus ultimately reducing the emission of NH₃ [32].

Amending a light-textured low-fertile soil with co-composted biochar significantly enhanced plant growth parameters and improved soil characteristics. Fetjah et al. (2022) showed that co-compost with biochar had led to increased biomass production of seashore paspalum by improving plant water status, photosynthesis, and chlorophyll pigment [35]. Farid et al. (2022) reported that co-composted biochar significantly increased seed vigor index, dry weight, total chlorophyll content, root, and stem length of zucchini, and exhibited higher residual organic C than conventional composts [36].

However, most of the research has focused on the mechanism of biochar materials to reduce composting nitrogen loss, with few large-scale open-air farmland experiments and comprehensive evaluations of soil, runoff, and maize. Due to the complexity and uncertainty of the experimental field, it is necessary to study the comprehensive evaluation of new cyanobacteria organic fertilizer products. Maize (Zea mays L.) is cultivated worldwide as food, feed, or raw materials for the alcohol industry. In the hilly areas of CHLB, maize planting is popular as well. However, because of the heavy rain and extensive water networks in this region, a significant amount of N and P is lost by runoff from agroecosystems to water systems, resulting in decreasing nutrient-use efficiency and water eutrophication. The objective of this study was to (i) to investigate the nitrogen fixation effect of the optimal rate of biochar application in cyanobacteria co-composting and (ii) investigate the effects of biochar and bio-organic fertilizer on farmland soil improvement, improving maize quality, and reducing field nutrient loss. Based on previous experiments, we preferred Mg(OH)₂ and KH₂PO₄ to compound biochar. Additionally, three fertilizer measures were designed to study the influence of organic fertilizer on farmland runoff water quality and maize yield and quality. This research will provide parameter support for the engineering application of cyanobacteria co-composting and provide useful and valuable information guidelines for soil improvement, farmland runoff purification, and maize yield and quality in similar areas. Cyanobacterial biochar mixed composting technology can be used for large-scale treatment of lake cyanobacteria in the future, and its products have the potential to replace traditional chemical fertilizers as organic fertilizers in sustainable agriculture to achieve sustainable development between agriculture and the environment.

2. Materials and Methods

2.1. Experimental Materials

The cyanobacteria for the experiment were salvaged from Chaohu Lake. The spent mushrooms were provided by Heifei Lixin Seed Factory(Hefei, China). The rice hulls were from Zipeng Town Rice Factory (Hefei, China) and the biochar was provided by Anhui Yongfeng Protection Technology Co., Ltd. (Hefei, China). The basic physical and chemical properties of the above materials are shown in

Table 1. Basic characteristics of composting materials.

Material	Moisture Content (%)	pH	Organic Matter (DM) (g·kg−1)	Total Nitrogen (DM) (g·kg−1)	Total Phosphorus (DM) (g·kg−1)	Total Potassium (DM) (g·kg−1)
Cyanobacteria	90.23 ± 0.158	8.35 ± 0.025	634 ± 4.041	73.32 ± 0.038	13.80 ± 0.015	10.53 ± 0.02
Spent mushroom	15.68 ± 0.784	7.58 ± 0.015	655 ± 2.52	16.50 ± 0.015	12.26 ± 0.035	8.45 ± 0.025
Rice hulls	12.83 ± 0.041	7.73 ± 0.021	552 ± 3.21	5.25 ± 0.025	3.42 ± 0.026	2.68 ± 0.021
Biochar	7.22 ± 0.19	10.65 ± 0.03	64.98 ± 1.14	5.65 ± 0.68	4.23 ± 0.41	70.89 ± 4.72

DM means dry matter. All the measured data are shown as mean ± standard deviation (SD). N = 3.

. Effective Microorganisms (EM) bacteria strains were purchased from Zhengzhou Nongshengle Biotechnology Co., Ltd. (Hefei, China). The maize (Zea mays L.) varieties in the planting experiment were provided by Fengle Seed Industry Co., Ltd. (Hefei, China).

In addition, Mg(OH)₂ and KH₂PO₄ added in the composting process were at chemical purities. The chemical fertilizer included urea (including 46% N) as a nitrogen source, calcium superphosphate (including 46% P₂O₅) as a phosphorus source, and potassium chloride (including 60% K₂O) as a potassium source.

2.2. Indoor Experimental Design

Composting experiments were mixed with cyanobacteria, spent mushrooms, and rice hulls, with a wet weight ratio of 2:2:1 and each having a composting mass of 1250 kg. To accelerate the composting process, EM of 1% total raw materials mass was added, which was a mixture of microbial inoculum of more than 80 microorganisms from 10 genera, including photosynthetic bacteria, lactic acid bacteria, yeast, and actinomycetes [37]. The initial moisture content of the raw materials was about 55%, and the initial C/N was about 22.89 [38].

The existing results show that adding Mg/P salts during composting is equal to 15% of the initial nitrogen content (in mol), which is conducive to the formation of struvite crystallization [39,40]. Thus, the amount of Mg(OH)₂ and KH₂PO₄ in the study was in this proportion. Mg(OH)₂ and KH₂PO₄ were dissolved in water and mixed to compost material.

To compare the effects of different proportions of biochar on the control of nitrogen loss in cyanobacteria aerobic composting, five treatments were prepared: CK: Compost material (cyanobacteria, spent mushroom and rice hulls), B₀:Mg(OH)₂ + KH₂PO₄ + compost material, B₅:5% (w/w) biochar + B₀, B₁₀:10%(w/w) biochar + B₀, B₁₅:15%(w/w) biochar + B₀, B₂₀:20% (w/w) biochar + B₀. Biochar was mixed directly into the raw materials [19]. All test treatments were naturally stacked in the plastic film shed, and all stacks size were fixed as long × wide × high =1.5 × 1.5 × 0.8 m. Composting cycle was set to 42 d and overturned at 10, 20, 30 d, and 40 d, respectively. All the experiments were conducted in triplicate.

2.3. Outdoor Field Setting

As shown in Figure 1, The experimental farmland study area was located at Zipeng town, an ecological park in the Chaohu area of Hefei city, Anhui, China (116°57′25″ E, 31°44′43″ N, Figure 1). This area has a subtropical monsoon climate. The annual sunshine and precipitation are 2000 h and 964 mm, respectively, and the average temperature is 15 to 16 °C. The soil type used for the experiments was acid purple soil based on the soil classification system of China in 1992. The soil contains medium potassium and low levels of organic matter and nutrient content. The area is located in the Jianghuai watershed, with large terrain and a distinct rainy season. About 70% of the precipitation is concentrated in June, July, and August, which means frequent heavy rain in the rainy season in the basin and often significant surface runoff. The agricultural area is mainly seedlings and grain crops planting. Due to the influence of traditional business methods, the local people apply many chemical fertilizers and pesticides in agricultural production, resulting in a declining soil quality of cultivated land, gradual deterioration of the runoff water quality, and a reduction in the utilization rate of fertilizer.

The planting experiment was designed in a field with a size of 5 × 5 m. It was necessary to vertically insert tiles with a size of 80 × 80 cm around the field, and the vertical burial depth of the tiles was at least 40 cm. We covered the exposed tiles with plastic cloth from top to bottom and buried the excess plastic cloth on both sides of the tiles in the soil. A hole with a diameter of 5 cm was left on the tile downhill of the field, and the runoff from the hole was collected in a 50 L bucket through PVC pipes. In total, three treatments were set up in the experimental field: ① CF: Chemical fertilizer, ② OF: Organic fertilizer, ③ C0: No fertilizer. Each treatment was repeated twice. Maize was sown in early May and harvested in early October 2020. Chemical fertilizer and organic fertilizer were applied as base fertilizers and mixed with soil before planting. The following standard dosages were used for each treatment: N was 241 kg/hm², P₂O₅ was 150 kg/hm², and K₂O was 150 kg/hm². Compound chemical fertilizer was directly mixed according to this standard amount, and organic fertilizer was converted according to this standard amount. Insufficient nitrogen, phosphorus, and potassium were supplemented with urea, single superphosphate, and potassium chloride, respectively. The row spacing × plant spacing of maize planting was 50 × 35 cm.

2.4. Sample Collection and Monitoring

2.4.1. Sample Collection

The composting sample was collected on days 0, 5, 10, 16, 22, 32, and 42 and fully mixed with about 200 g of material from the upper, middle, and bottom parts of the pile [41]. The sample was divided into two parts. One part was stored in a 4 °C refrigerator as fresh sample, which was used to determine the pH value, ammonium nitrogen (NH₄⁺−N), nitrate nitrogen (NO₃⁻−N), seed germination index (GI), microcystins (MC), and other indexes. The other part was air-dried, ground, and screened for the determination of organic matter (OM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), and other indexes.

NH₃ samples were collected by ventilation [41], which used a sampler made of PC plastic pipe with an internal diameter of 16 and 15 cm high. A total of 15 mL phosphate glycerin solution was sprayed evenly into the sampler. The upper sponge was placed on the top of the pipe to isolate the external gas interference. Additionally, the lower sponge was 5 cm from the bottom surface. The device was placed at the sampling point for about 1 h to collect NH₃ and set 3 sampling points evenly for each test treatment.

The composting temperature was measured at 9:30 a.m. and 17:30 p.m. by infrared temperature detector, and pH, OM, NH₄⁺−N, NO₃⁻−N, TN, TP, and TK were measured by oven, potential, potassium dichromate, UV spectrophotometry, KelNitrogen, molybdenum, antimony, and flame atomic spectrophotometry, respectively [14].

Before sowing and after harvest, field soil samples were collected using five-point mixed sampling method to determine soil OM, TN, TP, TK, and other indexes in each community [13].

After the maturity of maize, five plants were randomly collected to measure plant height (cm), quality per plant (g), leaf area (cm²), ear length (cm), ear diameter (cm), number of grains per spike (grain), and 100-grain weight (g) from each block on 7 October. Additionally, the cobs and straws were hand-harvested separately. Grain was placed in the sun for 1 month, adjusted to seed moisture content less than 10%, and then weighted to determine the maize yield. The grain and straw samples were dried in an oven at 80 °C to a constant weight and ground to powder to measure the TN and TP content [41].

The volume of the water in the runoff collection bucket was recorded after each rainfall, and 500 mL water samples were taken to analyze TN, TP, TK, NH₄⁺−N, NO₃⁻−N, and other indicators.

2.4.2. Monitoring Methods

Composting temperature measurement was taken daily at 9:30 a.m. and 17:30 p.m. at three points along the length of the middle of composting mixtures using infrared temperature detector. After the soil, maize, and water samples were collected, they were taken to the laboratory for measurement. In the water samples, the TN content was determined using a standard alkaline potassium persulfate digestion-UV spectrophotometric method [42], and the TP content was determined using an ammonium molybdate spectrophotometric method [43]. In the soil and maize samples, the TN was digested and measured using the Kjeldahl method [44], and the TP was digested with H₂SO₄-HClO₄ and measured using the molybdenum blue colorimetric method [45]. The soil OM was measured using the potassium dichromate volumetric method [45]. The suspended solids (SS) were measured using the centrifugal-filter paper filtration method [45].

We took maize seed samples to determine crude fat, soluble protein, and coarse starch. The crude fat and coarse starch contents were determined in the grains according to the method of [46]. Soluble protein contents were measured in the grains according to the method portrayed by Bradford [47].

GI (%) = 100 (mean number of germinated seeds per dish × mean root length per dish)/(mean number of germinated seeds in the control × average root length of the control) [41].

NH₃ emission flux measurement method: Remove bottom sponge of sampler, soak in 1 mol·L⁻¹ potassium chloride solution, then oscillation for 1 h to determine ammonia nitrogen concentration in infiltration solution by distillation nitrogen method for 1 h, and ammonia nitrogen concentration in potassium chloride solution soaked in 1 mol/L of ammonia nitrogen (mg/L) by distillation nitrogen method. The emission flux and cumulative emission of NH₃ were calculated according to the literature [14].

A total of 5 g fresh sample was taken accurately and extracted with 5% acetic acid and supplemented by ultrasonic oscillation. Crude extract was purified with C18-SPE column and 80% methanol aqueous solution. Extraction liquid was dried in a 65 °C vacuum rotator with a fixed capacity with 1 mL methanol solution for HPLC analysis after 0.22 μm filter membrane. HPLC test conditions were that the flow phase consists of A acetonitrile and B pure water (containing 0.08% trifluoroacetic acid), with a detection flow velocity of 1.0 mL/min, sample intake of 20 μL, column temperature of 35 °C, and the detection wavelength of 238 nm [48].

Analysis of struvite (MgNH₄PO₄·6H₂O) composition in compost samples was conducted using X-ray Diffraction Instrument [49].

2.5. Data Processing and Analysis

According to the nitrogen balance principle, before and after the first sampling:

W₀ × (1 − M_w0) × N₀ = (W₁ + W₁′) × (1 − M_{w 1}) × N₁ + N_{1 loss}

Before and after the second sampling:

W₁ × (1 − M_w1) × N₁ = (W₂ + W₂’) × (1 − M_w2) × N₂ + N_{2 loss}

Therefore, there is a nitrogen loss at each sampling interval. Nitrogen balance before and after the n sampling:

W_n−1 × (1 − M_{w n−1}) × N_n−1 = (W_n + W_n’) × (1 − M_{w n}) × N_n + N_{n loss}

The total nitrogen loss:

N_{T loss} = N_{1 loss} + N_{2 loss} +…+ N_{n−1 loss}

Nitrogen loss rate (R_N-loss):

R_N-loss = N_{T loss}/N₀ × 100%

NH₃-N loss rate (R_AN-loss):

R_AN-loss = N_{AT loss}/N₀ × 100%

Nitrogen fixation rate (R_NF)

R_NF = (R_{N-loss CK} − R_{N-loss T})/R_{N-loss CK} × 100%

where n is the number of samples, W₀ is the total weight of composting raw material (fresh weight) at the beginning of composting (kg), M_w0 is the composting sample water content at the beginning of composting (%), W_n is the composting weight (fresh weight) after each sample sampling (kg), W_n’ is the composting sample weight (fresh weight) (kg), M_wn is the water content of each compost sample (%), and N_n is the total nitrogen content of each sample (g/kg) [38].

N (or P and K) uptake of maize = grain yield × N (or P and K) content of grain + straw yield × N (or P and K) content of straw content.

N (or P and K) source agronomic coefficient = [The N (or P and K) content of soil and N (or P and K) uptake of maize in OF treatment]/[The N (or P and K) content of soil and N (or P and K) uptake of maize in CF treatment].

2.6. Statistical Analysis

Test data were analyzed using Microsoft Excel 2010 and Origin 2018 software for basic analysis and drawing. One-way analyses of variance (ANOVAs) were used to determine whether the properties differed among the treatments. When ANOVAs were significant, significant difference was detected through LSD-t. All statistical analyses were performed with SPSS 22.

3. Results and Discussion

3.1. Effect of Maturity Indexes in the Composting Process

During the experiment, compost mixtures followed the three typical phases of composting [48]: Mesophilic, thermophilic, and curing. At the beginning of composting, the initial temperature of each treatment was 19.5 °C, close to the environment temperature (ET) of 20 °C, which increased rapidly (Figure 2a). Additionally, the highest temperature of B₀, B₅, B₁₀, B₁₅, B₂₀, and CK in the process of composting reached 60.5, 60.9, 62.1, 63.5, 61.2 °C, and 61.5 °C, respectively. The high-temperature stage (>50 °C) lasted for 12 to 15 days, sufficient for the composting product to meet the hygiene requirements. In the middle of composting, the temperature subsequently increased slightly, mainly due to the homogenization of the organic composition and increased microbial activity, causing the secondary fermentation of the mixtures [30]. The addition of biochar extended the thermophilic phases of each treatment by 1 to 3 d with the CK and B₀. This may be due to a higher density substrate accelerating the composting process: The biochar amendment fills the stack’s pore space, thus reducing the heat loss due to increased air space [11].

As shown in Figure 2b, the pH values of each treatment gradually increased and reached the peak along with the composting process in 1–5 days, then gradually decreased and finally stabilized. At the initial stage of composting, with the rise in temperature, the materials with high nitrogen content were mineralized dramatically. Additionally, a large amount of NH₄⁺-N was produced in this mineralization reaction, which promoted the increase in the pH value of the reactor. In the high-temperature stage, the pH value of the reactor decreased gradually due to the release of a large amount of NH₃. In the thermophilic stages, ammonification began to weaken, and the accumulation of H⁺ from nitrification and organic acids from the degradation of organic matter further reduced the pH value to a constant level in the ripening stage until the end of the test [40]. Figure 2b shows that the amendment of Mg₂⁺/P salts can significantly reduce pH (p < 0.05), while the increase of biochar can increase pH, which was consistent with the effect of Wang et al.’s (2018) study on biochar, zeolite, and wood vinegar co-composting [19]. The pH values at the end of each test treatment were all within 7.5~8.5, presenting weak alkalinity, which met the standards of composting ripening in China.

As shown in Figure 2c, the OM content in all treatments decreased gradually with time. After composting, the OM content of B₀, B₅, B₁₀, B₁₅, B₂₀, and CK treatments were 475.24, 508.65, 514.98, 518.43, 522.87 g/kg, and 268.43 g/kg, respectively. Compared with the initial stage, it decreased by 23.12%,18.18%, 17.02%, 16.85%, 16.54%, and 25.47%, respectively. The results showed that the degradation rate of OM in CK was significantly higher than that in other treatments (p < 0.05). Moreover, the degradation rate of OM decreased with biochar supplementation. The main reason for this was that it was difficult to degrade biochar when composting, and the amendment of biochar increased the content of OM in each treatment [50]. Overall, the degradation rate of OM for 0–16 days was significantly higher than that for 16–42 days. This was mainly because the microbial metabolism at the early stage of composting increased with the increase in temperature, and the decomposed OM was simple in structure and easy to degrade. In the later stage of composting, the microbial activity decreased with the decrease in temperature, and the OM that was difficult to decompose, such as lignin, became the main carbon source, so the degradation rate decreased gradually [51].

GI lower than 50% means high phytotoxicity occurs, between 50% and 80% moderated phytotoxicity and higher than 80% means the no- phytotoxic occurrence, while the products with a GI higher than 100% have phytonutrient or phytostimulant effects [52]. According to Figure 2d, the initial GI of each treatment was lower than 50%, indicating that the composting material had high plant toxicity, which also meant that there was a certain risk of using cyanobacteria directly as fertilizer without treatment. From 0 to 5 days at the beginning of composting, the GI of each treatment had little change compared with the initial stage, which was mainly attributed to the inhibition of seed germination by the production of volatile fatty acids and high concentration of NH₄⁺ in the early stage of composting [53]. Thereafter, as the composting continued, these substances were continuously degraded or transformed, and the GI in all treatments gradually increased. When GI reached over 80%, compost was considered mature and no longer phytotoxic [38]. After composting, GI in all the treatments exceeded this limit, which indicated that each treatment reached complete decomposing. Among them, the amendment of Mg²⁺/PO₄³⁻ salt (B₀) and biochar (B₅-B₂₀) helped to increase GI, mainly due to the biochar adsorption of ammonia and the formation of struvite (MgNH₄PO₄·6H₂O). This was similar to the results of Jiang et al.’s [40] study on struvite crystallization and Li et al.’s (2015) study on composting pig manure with biochar [54].

3.2. Nitrogen Loss and Fixation

3.2.1. The Emission of NH₃

NH₃ emission is the main vehicle for nitrogen loss during composting [55]. It not only reduces the fertilizer of compost products but also causes environmental pollution problems [56]. As shown in Figure 3a, the NH₃ emission trends for each treatment were almost the same, with an upward trend and peaking. Large amounts of NH₃ emissions in each treatment were mainly concentrated within 0–20 d of composting, and almost no NH₃ emission was detected after that, which was consistent with Chan’s research results [17]. However, the emission rate of treatments B₀-B₂₀ was lower than CK. The maximum daily emission of NH₃ in CK appeared on day 5, reaching 324.68 g/d. The peak values of B₀, B₅, B₁₀, B₁₅, and B₂₀ treatments were 103.02, 74.25, 71.50, 59.00 g/d, and 60.75 g/d on days 9, 9, 8, 7, and 8 d, respectively. Compared with CK, it decreased by 68.27%, 77.13%, 77.98%, 81.83%, and 81.29%. This indicated that the amendment of Mg²⁺, PO₄³⁻, and biochar could effectively delay ammonia emissions and reduce its emission rate, reducing cumulative emissions, while the higher the rate of biochar application, the lower the ammonia emission rate. Fang et al. (1999) and Liang et al. (2018) found that about 40~80% of nitrogen is lost from NH₃ emissions at the high-temperature stage [57,58]. The factors affecting the volatile rate of NH₃ during composting mainly include temperature, pH, water content, particle size, mixing flip frequency, ventilation rate, and nutrients [39,59].

As shown in Figure 3b, the cumulative NH₃ emissions tended to stabilize after 20 d. Throughout the fermentation process, the CK treatment discharged NH₃ 1832.61 g, which was the highest, compared with the B₀, B₅, B₁₀, B₁₅, and B₂₀ treatments of 990.53, 668.25, 603.00, 628.13 g, and 660.75 g. Among them, B10 has the lowest cumulative NH₃ emission and decreased by 45.95%, 63.54%, 67.10%, 65.72%, and 63.94% compared with the CK, respectively. This may be due to proper amounts of biochar amendment to improve the reactor porosity and increase the reactor oxygen concentration, creating a good living environment for nitrification bacteria, which helped convert NH₄⁺ into nitrates, thus reducing the loss of nitrogen [50]. At the same time, biochar provided a good protective space and essential nutrient elements for nitrification bacteria in the thermophilic stage, improved the reactor environment, and ensured compost quality [51]. Therefore, the amendment of biochar reduces NH₃ production and emission, and effectively reduces nitrogen loss.

3.2.2. Changes in the Nitrogen Forms and TN Content

The changes in NH₄⁺−N content in all the treatments presented a similar trend, rising first and slowly reducing to stabilize (Figure 4a). When the heap was saturated, the NH₄⁺−N was continuously transformed to the NH₃ direction at high temperature and high pH values, a large number of NH₃ escaped from the heap, and the NH₄⁺−N content began to decrease. With the continuous decrease in temperature, the activities of nitrifying bacteria and other thermophilic bacteria gradually became vigorous. Through a series of biochemical reactions, such as nitrification, NH₄⁺−N was transformed into NO₃⁻−N or organic nitrogen, and the content of NH₄⁺−N tended to gradually become stable [30]. After the beginning of the test (0–5 d), with the temperature rise, the ammonifier gradually became the dominant population, and OM, under the action of ammonifier continuous decomposing, produced a large number of NH₄⁺−N and increased the NH₄⁺−N content to the peak [14]. After composting, NH₄⁺−N content in CK was 3.14 g/kg, which decreased by 40.19% compared with the initial content. However, the contents in B₀, B₅, B₁₀, B₁₅, and B₂₀ treatments were 4.43, 4.89, 5.35, 5.49 g/kg, and 5.12 g/kg, which increased by 23.40%, 56.73%, 58.28%, 52.08%, and 54.22%, respectively. The results showed that the amendment had a good effect on nitrogen fixation in different degrees. The increase in NH₄⁺−N content in all treatments was significantly higher than the CK. This was attributed to the nitrogen-fixing effect of struvite crystals and the good adsorption capacity of biochar, which fixed a large amount of NH₄⁺−N. Additionally, the strong adsorption capacity and good pore structure of biochar could adsorb NH₄⁺−N, thus reducing the conversion rate of NH₄⁺−N to NH₃ [29].

As shown in Figure 4b, during days 0 to 5 of the composting stage, NO₃⁻−N content in all treatments showed a slow increase, mainly because the high temperature and high NH₄⁺-N content inhibited the activities of the nitrifier [60]. Within 10 to 16 days, with the decrease in temperature and the release of a large amount of NH₃, the content of NO₃^--N in all treatments showed a sharp increase. This was mainly due to the enhanced nitrifier activity that transforms the reactor-free NH₄⁺−N into NO₃⁻−N, so its content increases. Then, the NO₃⁻−N content stabilized, and after composting, the B₀, B₅, B₁₀, B₁₅, B₂₀, and CK treatments were 556, 583, 595, 606, 577 mg/kg, and 600 mg/kg and increased 247.50%, 257.67%, 271.88%, 274.07%, 277.07%, 277.12%, and 268.10% relative to the initial content. The increase in NO₃⁻−N content of B₁₀, B₁₅, and B₂₀ was higher than CK and increased with a greater rate of biochar application. The results indicated that the amendment of a certain rate of biochar application was beneficial to the fixation of NO₃⁻−N in the composting process.

As shown in Figure 4c, the TN content in each treatment showed a slightly decreasing trend during days 0–5 at the beginning of composting, which was mainly due to the emission wherein NH₃ began to be released with the decomposition of organic matter, resulting in different degrees of nitrogen loss in each treatment. During days 5 to 15 of composting, although NH₃ emission was significant, the TN content still gradually increased, which was consistent with the research results of Luo et al. and Nasini et al. [61,62]. This was mainly due to the degradation rate of carbon-containing organic matter being higher than the emission rate of NH₃. As the organic compounds degraded, the heap mass was constantly lost, increasing the TN concentration. After composting, the TN content in CK was 21.53 g/kg, up 26.87% compared to the initial content. Meanwhile, in B₀, B₅, B₁₀, B₁₅, and B₂₀ treatments, it was 26.02, 29.74, 32.69, 31.87 g/kg, and 30.55 g/kg, up 54.42%, 76.39%, 92.18%, 90.04%, and 80.45%, compared to the initial content, respectively. Comparing the increased range of TN content between treatments showed that the B₁₀ treatment was the best for reducing nitrogen loss. It can be seen in Figure 4a–c that the main forms of nitrogen-containing substances in the composting process are organic nitrogen (51.68 to 81.6%) and inorganic nitrogen in the form of ammonia nitrogen (16.37 to 47.07%).

3.2.3. Nitrogen Loss and Fixation Rates

According to the principle of material conservation and nitrogen balance, the nitrogen loss rate and fixed rate in different treatments of cyanobacteria compost are calculated in

Table 2. Efficiency of nitrogen-loss and inhibiting nitrogen-loss during composting of cyanobacteria in different treatments.

Treatments	RAN-−loss (%)	RN−loss (%)	RAN−loss/RN−loss (%)	RNF (%)
CK	12.92	14.46	89.35	—
B0	6.98	8.48	82.31	41.35
B5	4.71	6.23	75.60	56.91
B10	4.25	5.56	76.44	61.54
B15	4.42	5.89	75.04	59.26
B20	4.65	6.11	76.10	57.74

Nitrogen loss of all treatments was compared with the initial TN.

. We can see that NH₃ emission was the main vehicle of nitrogen loss, accounting for 75.04 to 89.35%, which was consistent with previous studies [16,57,58]. This showed that inhibiting NH₃ release during composting was an important means of controlling nitrogen loss in compost. With the amendment of Mg/P salts and biochar (B), nitrogen loss decreased by 41.35 to 61.54%. This comparison shows that the amendment of 10% biochar had the best effect on reducing reactor nitrogen loss.

3.3. Evaluation of Safety and Fertilizer Efficiency of Compost Products

3.3.1. Degradation Effect of Microcystins

The most common and potently toxic microcystins are the microcystin variant with arginine (MC-RR) and the microcystin variant with leucine (MC-LR). Microcystins are harmful to the liver of humans and animals, but they can be degraded by acids, permanganate, H₂O₂, ozone, ultrasonic irradiation, ultraviolet light, and microorganisms [63,64]. This experiment focused on the content and degradation rate of MC-RR and MC-LR in samples before and after composting.

As shown in

Table 3. Changes in microcystin content during composting of cyanobacteria in different treatments.

Treatments	MC-RR (μg·kg−1)			MC-LR (μg·kg−1)
	0 d	42 d	Degradation Rate (%)	0 d	42 d	Degradation Rate (%)
CK	330.65	5.46	98.35	235.56	6.14	97.39
B0	325.46	ND	100	226.74	4.39	98.06
B5	327.64	4.65	98.58	235.18	4.97	97.89
B10	330.38	ND	100	231.64	4.05	98.25
B15	342.54	3.85	98.88	238.27	4.53	98.10
B20	336.95	5.83	98.27	244.45	5.24	97.85

ND means not detected.

, the degradation rate of MC-RR and MC-LR in the test process could reach 97.39 to 100%, which indicates that the high-temperature aerobic composting process was conducive to the degradation of MC. After composting, the degradation trend of MC content in each test treatment was consistent with the research results of Tang et al. (2021) and Han et al. (2019). In general, B₁₀ treatment had the best comprehensive degradation effect on MC and effectively reduced the safety risk of cyanobacteria composting [41,65].

3.3.2. Effectiveness Evaluation of the Fertilizer

The National Standard for Organic Fertilizer of China (NY 525-2012) stipulates the technical indicators of organic fertilizer, in which the content of organic matter (based on drying) is ≧45%, and the amount of total nutrient (N + P₂O₅ + K₂O) content (TNC) (based on drying) is ≧5%. The fertilizer effectiveness of composting products is shown in

Table 4. Fertilizer efficiency of cyanobacteria composting products in different treatments.

Treatments	OM (%)	TN (%)	TP (%)	TK (%)	TNC (%)
CK	46.28	2.15	1.39	1.00	6.53
B0	47.52	2.60	2.08	1.89	9.64
B5	50.87	2.97	2.47	2.13	11.19
B10	51.50	3.27	2.69	2.34	12.23
B15	51.84	3.19	2.55	2.25	11.74
B20	52.29	3.06	2.29	1.95	10.65

. After composting, the OM content of each treatment was 46.28 to 52.29%, including the highest OM content of B₂₀ (52.29%), mainly because the increase in biochar improved the carbon content and thus improved the total organic matter content. The TNC for all composting experiments ranged from 6.53% to 12.23%, which met the requirements of the NY 525-2012 standard. The TNC of B₁₀ treatment reached the highest at 12.23%. Overall, the combined amendment of 10% biochar and Mg(OH)₂/KH₂PO₄ can achieve better comprehensive benefits in cyanobacteria composting.

3.4. Effects of Different Fertilization Treatments on Soil and Runoff of a Maize Field

3.4.1. Effects of Different Fertilization Treatments on Soil Nutrient Fixation

Three different fertilization treatments were applied to maize field experiments in Zipeng, as shown in

Table 5. Effects on nutrients in the soil of maize fields in different fertilization treatments.

Period	Treatments	SOM (g·kg−1)	TN (g·kg−1)	TP (g·kg−1)	TK (g·kg−1)
Pre-planting	PCK	24.75 ± 0.23 c	1.45 ± 0.03	0.68 ± 0.05	17.54 ± 0.05
	C0	24.22 ± 0.16 d	1.31 ± 0.02	0.37 ± 0.02	17.17 ± 0.08
Late harvest	CF	25.26 ± 0.20 b	1.60 ± 0.02	0.75 ± 0.03	18.21 ± 0.07
	OF	27.09 ± 0.12 a	1.72 ± 0.05	0.76 ± 0.02	20.89 ± 0.11

Different superscript letters after data in the same column indicate significant differences between treatments (p < 0.05). Data are shown as mean ± SD, n = 3 in all cases.

. The compost products of the B₁₀ treatment were the same as the organic fertilizer. The relevant indexes of soil nutrients in the pre-planting period (PCK) were equivalent to the background values of soil nutrients in the maize field. After planting the maize, the soil nutrient content in the two treatments of CF and OF increased to varying degrees, except for the C0 treatment, which decreased slightly compared with the PCK. Under different fertilization measures, the OF treatment had the highest fixation rate of TN, TP, TK, and SOM, which was 18.62%, 11.76%, 19.10%, and 9.45%, respectively. The C0 treatment had the lowest fixation rate of TN, TP, TK, and SOM, which was −9.66%, −45.59%, −2.11%, and −2.14%, respectively. This was mainly due to the output of soil nutrients to crops under C0 treatment, resulting in a decrease in their content. At the same time, CF and OF treatments were supplemented by external fertilization. Compared with CF treatment, the SOM, TN, TP, and TK contents of OF treatment increased by 7.24%, 7.5%, 1.33%, and 14.72%, respectively. Among them, the SOM, TN, and TK contents were much more affected by fertilization treatments than TP. The soil nutrient contents of CF and OF treatments were significantly different from those of C0 treatments (p < 0.05). The results show that the effect of organic fertilizer on soil nutrient improvement was better than that of compound chemical fertilizer, mainly due to the affinity between organic fertilizer and soil, and the nutrients were easily stored in the soil and released slowly. Additionally, compound chemical fertilizer will weaken the soil hardening and water and fertilizer holding capacity, resulting in nutrient volatilization and loss [66].

3.4.2. Effect of Different Fertilization Treatment on the Total Nutrient Loss of Runoff in a Maize Field

According to

Table 6. Effects on the total nutrient loss of runoff from maize field in different fertilization treatments.

Treatments	TN (kg·hm−2)	NH4+−N (kg·hm−2)	NO3−-−N (kg·hm−2)	TP (kg·hm−2)	TK (kg·hm−2)
C0	5.17 ± 0.32 c	0.62 ± 0.02	2.15 ± 0.15	2.54 ± 0.03	1.83 ± 0.02
CF	12.63 ± 0.15 a	1.18 ± 0.02	6.71 ± 0.07	13.75 ± 0.01	20.49 ± 0.05
OF	8.42 ± 0.07 b	0.87 ± 0.03	4.05 ± 0.05	14.01 ± 0.02	15.83 ± 0.11

Different superscript letters after data in the same column indicate significant differences between treatments (p < 0.05). Data are shown as mean ± SD, n = 3 in all cases.

, the total nutrient loss of CF and OF treatment were significantly higher than that of C0 by 144.29% and 62.86%, respectively. The TN loss was recorded as the highest in CF, and it was 144.29% and 49.82% higher than C0 and OF, whereas the TP loss was recorded as the highest in OF, and it was 1.89% and 4.52% higher than CF and C0, respectively. The TK loss was recorded as the highest in CF. That of CF and OF treatments compared with C0 increased by 1019.67% and 765.03%, respectively. This suggested that one of the most important sources of nutrient loss in the maize field was fertilization. At the same time, applying organic fertilizer could reduce TN and TK loss to a certain extent but would increase the TP loss. This is because the N and K in the chemical fertilizer were soluble and easily lost due to the influence of surface runoff, while the application of organic fertilizer increased the formation of soil aggregates and improved the effectiveness of soil, water, and fertilizer conservation [67]. However, the P as the form of soluble phosphate exists in chemical fertilizers, which easily reacts with cations (e.g., Fe³⁺, Al³⁺, and Ca²⁺) in the soil to form phosphate precipitation and become fixed. Wang et al. (2001) reported that the main form of P in runoff water is granular P [68]. At the same time, the P in organic fertilizers is mostly granular P, which is not easily fixed by soil and is relatively easily lost with runoff [69]. Previous studies showed that biochar amendment and organic manure fertilizer improved soils’ water-holding capacity [56,58]. Additionally, the increased soil water-holding capacity could decrease the amount of runoff water, which is beneficial for reducing N and P losses by runoff. Therefore, the comprehensive application of 10% biochar combined with the amendment of organic fertilizer in agriculture was an effective pattern for purifying water bodies and increasing nutrient retention in soils. This was consistent with the findings of Zhang et al. [67] in which soil alkaline hydrolyzable nitrogen and available potassium contents were significantly increased by biochar amendment, but there was no significant impact on the soil’s available P.

In terms of N loss in different forms, NH₄⁺-N loss of C0, CF, and OF treatments accounted for 11.99%, 9.34%, and 10.33% of TN loss, respectively. The NO₃⁻-N loss accounted for 41.59%, 53.13%, and 48.10%, respectively. The results indicated that the NO₃⁻-N loss accounted for the main portion of N loss in the runoff of the maize field, which was much higher than that of NH₄⁺-N. This was consistent with the previous research results of Sneh Goyal et al. [70]. On the one hand, the microorganisms in the soil convert NH₄⁺-N to NO₃⁻-N, which increases the NO₃⁻-N content in runoff. On the other hand, because the charge of NO₃⁻-N is consistent with soil particles, they easily repulsed each other and were easily washed away by surface runoff. Adding co-compost during maize cultivation could help reduce levels of soil erosion and nutrient runoff, resulting in a lower need for chemical fertilizer and water irrigation through improved water and nutrient retention, with increased maize productivity as a result [71].

3.5. Effects of Different Fertilization Treatments on the Growth and Quality of Maize

3.5.1. Effects on the Maize Growth Index

As can be seen from

Table 7. Effects of maize growth indicators in different fertilization treatments.

Treatments	Plant Height (cm)	Quality per Plant (g)	Leaf Area (cm2)	Ear Length (cm)	Ear Diameter (cm)	Kernel Numbers per Ear	100-Grains Weight (g)
CK	138.6 ± 7.0 a	603.5 ± 15.7 a	512.6 ± 44.1 a	13.1 ± 0.8 a	5.6 ± 0.3 a	302.9 ± 17.8 a	25.9 ± 0.7 a
CF	154.3 ± 4.5 b	763.2 ± 16.5 c	532.3 ± 54.3 b	15.3 ± 1.3 b	6.3 ± 0.1 b	336.5 ± 6.1 b	27.6 ± 0.7 b
OF	153.8 ± 4.4 b	745.4 ± 12.8 b	535.1 ± 53.0 b	15.5 ± 0.6 b	6.2 ± 0.4 b	342.4 ± 4.7 c	28.1 ± 0.8c

Different superscript letters after data in the same column indicate significant differences between treatments (p < 0.05). Data are shown as mean ± SD, n = 3 in all cases.

, compared with C0 treatment, CF and OF treatment showed significant differences in plant height, weight per plant, panicle length, panicle diameter, grain number per panicle, 100-grain weight, and other growth indicators (p < 0.05), but there were no significant differences in leaf area (p > 0.05). Compared with the C0 treatment, the maize plant height of CF and OF treatments increased by 11.33% and 10.97%, respectively. It reached 154.3 cm in CF treatment, which was the best among the three treatments. Compared with CF treatment, it was reduced by 0.3% in OF treatment. Compared with the C0 treatment, the maize weight per plant of CF and OF treatments increased by 26.46% and 23.51%, respectively. Among them, the CF treatment showed the best performance, which reached 763.2 g. The weight per plant of OF treatment was 2.33% less than that of CF treatment. Compared with the C0 treatment, the maize leaf area of CF and OF treatments increased by 3.84% and 4.39%, respectively. It was 535.1 cm² in OF treatment, which was the best among the three treatments. Compared with CF treatment, the maize leaf area increased by 0.53% in OF treatment. Compared with the C0 treatment, the maize ear length of CF and OF treatments increased by 16.79% and 18.32%, respectively. It was 15.5 cm in OF treatment, which was the best among the three treatments. Compared with CF treatment, it increased by 1.31% in OF treatment. Compared with the C0 treatment, the maize ear diameter of CF and OF treatments increased by 12.5% and 10.71%, respectively. It reached 6.3 cm in CF treatment, which was the best among the three treatments. Compared with CF treatment, it was reduced by 1.59% in OF treatment. Compared with the C0 treatment, the CF and OF treatments increased the number of maize kernels per year by 11.09% and 13.04%, respectively. Among them, it was 342.4 in OF treatment, which was the best among the three treatments. Compared with CF treatment, it increased by 1.75% in OF treatment. Compared with the C0 treatment, the CF and OF treatments increased the weight of 100-grains by 6.56% and 8.49%, respectively. Among them, it was 28.1 g in OF treatment, which was the best among the three treatments. Compared with CF treatment, it increased by 1.81% in OF treatment. In general, CF and OF treatments have different improvements in growth indexes compared with CK. CF and OF treatment had advantages in different growth indicators, but there is no significant difference between the two treatment treatments. Gao et al. (2020) found that organic fertilizer could improve the growth of maize and morpho-physiological parameters when studying bio-organic fertilizer, which was mainly attributed to bio-organic fertilization causing an obvious increase in the microbial activity by enhancing acid phosphatase and dehydrogenase enzymes, bacterial count, and mycorrhizal colonization levels in maize rhizosphere compared with chemical fertilization [72].

3.5.2. Effects on the Nutrient Content of Maize Grain and Straw

As shown in

Table 8. Effects on the nutrient content of maize kernels and maize stalks in different fertilization treatments.

Part	Treatments	Nitrogen Content (g·kg−1)	Phosphorus Content (g·kg−1)	Potassium Content (g·kg−1)
maize kernels	CK	8.85 ± 0.83 a	2.08 ± 0.89 a	1.54 ± 0.31 a
	CF	12.56 ± 0.69 b	3.15 ± 0.36 b	2.97 ± 0.25 b
	OF	13.18 ± 0.71 c	3.24 ± 0.31 c	3.28 ± 0.40 c
maize stalks	CK	8.53 ± 0.56 a	1.23 ± 0.04 a	9.54 ± 0.84 a
	CF	11.47 ± 0.49 b	1.31 ± 0.21 c	11.29 ± 0.92 b
	OF	14.38 ± 0.51 c	1.26 ± 0.15 b	13.36 ± 0.95 c

Different superscript letters after data in the same column indicate significant differences between treatments (p < 0.05). Data are shown as mean ± SD, n = 3 in all cases.

, the N, P, and K contents in maize kernels of CF and OF treatments were significantly improved compared with the C0 treatment. The N, P, and K contents of CF treatment compared with C0 increased by 41.92%, 51.44%, and 92.86%, respectively. Compared with CK, they increased by 48.93%, 55.77%, and 112.99% in OF treatment, respectively. According to the increase in each nutrient, the K content in maize kernels was greatly affected by fertilization. Compared with CF treatment, the contents of N, P, and K in OF treatment increased by 4.94%, 2.86%, and 10.44%, respectively, indicating that organic fertilizer was more beneficial to the absorption of nutrients in maize grains to a certain extent.

The nutrient contents of N, P, and K in maize stalks under CF and OF treatments were significantly improved compared with CK. Compared with CK, the contents of N, P, and K in CF treatment increased by 34.47%, 6.50%, and 18.34%, respectively. Compared with CK treatment, the contents of N, P, and K in OF increased by 68.58%, 2.44%, and 40.04%, respectively. The results showed that the N and K contents of corn stover were significantly affected by fertilization, but the P content was less affected by fertilization. The contents of N and K in OF treatment were significantly higher than those in CF, but there was no significant difference in P content between the two treatments. Compared with CF treatment, the contents of N and K in OF treatment were increased by 25.37% and 18.33%, respectively, while the contents of P were decreased by 3.82%, indicating that the application of organic fertilizer was more beneficial to the absorption of N and K nutrients in corn stover. When Gao et al. (2020) studied the mechanism of bio-organic fertilizer’s promotion of crop growth, they found that bio-organic fertilizer could significantly improve the activity of maize rhizosphere microorganisms and indole acetic acid [72]. Moreover, the bio-organic fertilizer enhanced the activity of maize rhizosphere acid phosphatase and dehydrogenase enzymes.

The N, P, or K agronomic coefficients of organic fertilizer were calculated based on the weight of organic fertilizer on crop N, P, or K element absorption and the promotion effect of the soil fertilizer at the same N, P, or K application level. This index combines crop absorption and soil fertilizer level, reflecting the comprehensive benefit of organic fertilizer on the soil-crop system. The N, P, and K source agronomic coefficient of cyanobacteria organic fertilizer was 1.424, 1.001, and 1.16, respectively, which shows that the N fertilizer effect of OF was better than the CF, and P /K fertilizer effect was comparable to that of CF. In this study, the P agronomic coefficients (1.001) of the bio-organic fertilizer (1.001) were similar to the P agronomic coefficients of pig manure calculated by Hao et al. (2018) after an 8-year corn-soybean rotation production in calcareous soils of Canada, including liquid pig manure, solid pig manure, and complex fermented pig manure [73]. The P agronomic coefficients of manure were 0.99, 1.08, and 0.87, respectively, with an average of 0.98.

3.5.3. Effects on the Yield and Quality of Maize

As seen in

Table 9. Effects on yield and quality of maize in different fertilization treatments.

Treatments	Yield (kg·ha−1)	Crude Fat Content (%)	Crude Protein Content (%)	Crude Starch Content (%)
CK	3487.94 ± 123.25 a	5.13 ± 0.32 a	6.05 ± 0.45 a	75.37 ± 10.52 a
CF	5836.23 ± 78.94 c	5.68 ± 0.19 b	8.27 ± 0.63 b	85.94 ± 14.85 b
OF	5624.77 ± 95.33 b	5.93 ± 0.27 c	8.96 ± 0.39 c	88.23 ± 7.97 c

Different superscript letters after data in the same column indicate significant differences between treatments (p < 0.05). Data are shown as mean ± SD, n = 3 in all cases.

, the maize yield of CF and OF treatments increased by 67.33% and 61.26%, respectively, compared with CK, showing significant differences, which indicated that the fertilization treatment had a significant effect on the maize yield’s increase. Compared with CF treatment, the yield of maize of OF decreased by 3.62%, but there was no significant difference between the two treatments (p > 0.05), indicating that the application of compound fertilizer was better than organic fertilizer in increasing maize yield to some extent. This was consistent with the findings of Gil et al.’s (2020) study, wherein the maize yields in soils amended with high-quality organic matter, except manure, were equal to or smaller than those of solely mineral fertilizer [74]. Additionally, Zhang et al. (2021) found that biochar and organic fertilizer had a long-term effect on maize yield [67].

The contents of crude fat, crude protein, and crude starch of maize kernels under CF and OF treatments were higher than those under CK and showed significant differences. Compared with CK treatment, in CF, they increased by 10.72%, 36.69%, and 14.02%, respectively. Compared with CK treatment, in OF treatment, they increased by 15.59%, 48.10%, and 17.06%, respectively. The results showed that the content of crude protein was much more affected by fertilization than that of crude fat and crude starch. Compared with CF treatment, the contents of crude fat, crude protein, and crude starch in OF treatment increased by 4.40%, 8.34%, and 2.66%, respectively, indicating that applying organic fertilizer was more beneficial to the improvement of maize quality to a certain extent. When Gao et al. [72] studied bio-organic fertilizer, they also found that it could improve the contents of starch, protein, and amino acid in maize kernels. Additionally, they found that the mechanism of their results was the enhancement of α-amylase and gibberellins activities, as well as decreased abscisic acid levels in the maize kernels compared to the chemical fertilizers.

4. Conclusions

The combined amendment of 10% biochar and 15% Mg(OH)₂/KH₂PO₄ solved the problem of nitrogen loss and the risk of microcystin pollution in the co-composting of cyanobacteria. The nitrogen fixation rate was 61.54%. The degradation rates of Mc-rr and Mc-lr were 100% and 98.25%, respectively, and the content of N + P₂O₅ + K₂O was 12.23%. In addition, the results of field experiments showed that applying this organic fertilizer could improve soil nutrients and maize quality. The increase in the rate of soil TN fixation was as follows: OF (18.62%) > CF (10.34%) > C0 (−9.66%). Compared with CF treatment, soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), and total potassium (TK) contents in OF treatment increased by 7.24%, 7.5%, 1.33%, and 14.72%, respectively. Compared with the application of CF, the loss of TN and TK in farmland runoff could be reduced by 33.33% and 22.74%, respectively. Moreover, OF treatment was more conducive to the absorption of nutrients by maize kernels and stalks. The TN and TK of maize stalks increased by 25.37% and 18.33% compared with CF treatment, and the TN, TP, and TK contents in maize kernels increased by 4.94%, 2.86%, and 10.44% compared with CF, respectively. Although the maize yield in the OF treatment was slightly lower than the CF (with a 3.63% reduction), OF treatment improved maize quality, increasing the crude fat, crude protein, and crude starch content by 4.40%, 8.34%, and 2.66% compared with the CF treatment, respectively. The N, P, and K source agronomic coefficient of cyanobacteria organic fertilizer was 1.424, 1.001, and 1.16, respectively, which shows that the N/K fertilizer effect of OF was better than CF, and P fertilizer effect was comparable to that of CF. Thus, these research results have improved the information and data on the value of cyanobacteria organic waste and the circular economy solutions of organic fertilizers suitable for the production of agroecosystems. Therefore, cyanobacterial co-composting has the potential to replace conventional chemical fertilizers, providing better ideas for the engineering application of cyanobacterial composting.