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Article

A Full Chain of Applying Struvite Recovered from Biogas Slurry to Promote Vegetable Growth

by
Yunhan Li
1,2,3,
Wei Wang
1,2,3,4,
Linhe Sun
1,2,3,
Jian Cui
1,2,3,
Xiaojing Liu
1,2,3,
Jixiang Liu
1,2,3,
Yajun Chang
1,2,3,* and
Dongrui Yao
1,2,3
1
Institute of Botany, Jiangsu Province and Chinese Academy of Sciences, Nanjing 210014, China
2
Jiangsu Key Laboratory for Conservation and Utilization of Plant Resources, Nanjing 210014, China
3
Jiangsu Engineering Research Center of Aquatic Plant Resources and Water Environment Remediation, Nanjing 210014, China
4
College of Ecology and Environment, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(13), 1352; https://doi.org/10.3390/agriculture15131352
Submission received: 10 May 2025 / Revised: 15 June 2025 / Accepted: 18 June 2025 / Published: 25 June 2025
(This article belongs to the Section Agricultural Soils)
Browse Figures
Versions Notes

Abstract

The expansion of the livestock industry has led to an increase in biogas slurry discharge, which contains high levels of nitrogen (N) and phosphorous (P). Struvite precipitation is an effective method for the recovery of N and P from biogas slurry, and the recovered N and P can be applied as a slow-release fertilizer in agricultural production. To form an industrial chain for struvite recovery and application in agriculture, we investigated the factors affecting struvite recovery from biogas slurry generated on a pig farm and evaluated its efficacy as a fertilizer. The N and P recovery efficiency was higher when magnesium chloride (MgCl2) was used as a magnesium (Mg) source compared with magnesium oxide (MgO), and the optimal reaction conditions were pH 10, a reaction time of 20 min, a stirring rate of 200 rpm, and a Mg/P/N ratio of 1.2:1.0:1.0, which achieved N and P recovery rates of 81.83% and 99.67%, respectively. To further investigate the commercial utility of using struvite recovered from biogas slurry as a fertilizer, the growth and content of nutrients in two common vegetables in China were measured. The vegetable quality-related parameters of bock choy (Brassica chinensis) improved as the proportion of struvite in the fertilizer increased. Fresh weight, soluble sugar, and soluble protein increased by 194.47%, 46.13%, and 82.42%, respectively. The quality-related parameters of water celery (Oenanthe javanica (Blume) DC.) increased with an increasing proportion of struvite (27.90 mg·g−1 soluble sugar and 42.20 mg·g−1 soluble protein). The application of struvite precipitated from biogas slurry in plant cultivation shows great potential and lays a solid foundation for the resourceful recovery and utilization of biogas slurry.

Graphical Abstract

1. Introduction

Occupying a critical position in the national economy of China [1], the highly developed livestock industry has led to large quantities of discharge of biogas slurry [2]. China produces approximately 3.8 billion tons of livestock and poultry manure annually, which is equivalent to about 50% of the total nutrient input from chemical fertilizers [3]. However, the utilization rate of livestock and poultry manure and biogas slurry remains only around 76% [4]. The long-term storage of biogas slurry can increase the difficulty of treating it and result in nutrition loss and environmental contamination [5,6,7]. Chemical treatments for the recovery of nutrition from biogas slurry, along with an integrated model of vegetable cultivation, are widely used to enhance resource utilization [8]. Due to their rapid effectiveness, high recovery rates, and ease of operation, chemical treatments are well-suited for treating digestate biogas slurry [9,10]. The use of struvite precipitation has received increased attention for its technical advantages and environmentally friendly properties [11]. The recovered struvite can be used as a slow-release fertilizer for the cultivation of vegetables, which can help generate additional revenue for enterprises and improve the sustainability of biogas slurry treatment [12].
Struvite, commonly known as ammonium magnesium phosphate (MgNH4PO4), is a white to yellow crystal that contains essential elements for plant growth, including N, P, and Mg [13]. N and P can be simultaneously recovered via the addition of an external Mg source in a favorable ratio, as shown in Equation (1); the different compound forms of Mg affect the removal efficiency of struvite for N and P from wastewater [14,15,16]. Struvite precipitation is commonly used for the treatment of landfill leachate because it can efficiently remove ammonium (TAN) and total phosphorous (TP) to form precipitates that can be easily separated from the liquid [17,18]. Having similar characteristics of high TAN and TP concentration to landfill leachate, struvite precipitation can be applied to biogas slurry to reduce TAN and TP content [15,19].
HnPO43−n + Mg2+ + NH4+ + 6H2O → MgNH4PO4 · 6H2O↓ + nH+ (n = 1, 2)
Subsequently, due to its content of essential elements for plant growth and its water-insoluble nature, struvite has attracted attention for its potential as a slow-release fertilizer [20,21,22]. Le Corre et al. (2009) studied the mechanism of struvite crystallization and its potential application in agriculture because of its low solubility. Struvite can be released in environments, and the release time without plants was at least 105 days in water and 60 days in soil [20,23,24]. The solubility of struvite significantly increases when different organic acids are used to simulate root exudates, with N and P leaching rates reaching nearly 100% [25]. Additionally, isotopic labeling techniques verified that the N and P in struvite could be utilized by plants [26,27]. Previous studies have shown that struvite has a promotive effect on plant growth [28]. Struvite provides long-lasting effects comparable to quick-acting fertilizers; the use of struvite could thus provide farmers with a way to enhance vegetable growth while reducing the frequency of fertilizer application [22].
Previous studies have successfully applied struvite recovered from dairy industry wastewater, food processing wastewater, and municipal wastewater treatment plants to enhance the cultivation of vegetables [29,30,31]. However, most previous studies of biogas slurry have either focused solely on improving the efficiency of nutrient recovery from wastewater through struvite precipitation or examined the effects of commercialized struvite on plant growth. However, these research endeavors remain isolated from each other, failing to form a full research chain [32,33]. Here, we evaluated the optimal parameters for N and P recovery from biogas slurry and characterized the products of struvite precipitation under optimal reaction conditions. We also conducted fertility experiments of the recovered struvite to evaluate its efficiency as fertilizer.

2. Materials and Methods

2.1. Materials Collection

Biogas slurry was collected from Runqi Agricultural Technology Co., Ltd. in Liuhe, Nanjing, China. The chemical composition and concentrations of different components of the biogas slurry are shown in Table 1.
The concentrations of TP and magnesium ions (Mg2+) were significantly lower in the biogas slurry than the concentration of TAN. Sodium dihydrogen phosphate dodecahydrate (Na2HPO4·12H2O) was used as an additional P source. MgCl2 and MgO were used as Mg sources. Sodium hydroxide and hydrochloric acid were used to adjust the pH of the biogas slurry.
After placing 0.45 L of biogas slurry into beakers, the initial concentrations of TP (TP1), TAN (TAN1), and Mg2+ in the biogas slurry were measured (Figure 1). Subsequently, the amount of Na2HPO4·12H2O added was calculated based on Equation (2); the TP concentration after Na2HPO4·12H2O addition was denoted as TP2. The pH of the solution was adjusted; the stirring speed of the stirrer was then set, and the beaker was placed on the stirrer. The Mg source added was determined using Equations (3) and (4). Once the reaction was completed, the beaker was allowed to stand for 30 min. Finally, the supernatant was collected to facilitate the detection of concentrations of TP (TP3-MgCl2 and TP3-MgO) and TAN (TAN3-MgCl2 and TAN3-MgO). The precipitate was filtered and then placed on absorbent paper to air dry naturally.
Weight of Na2HPO4·12H2O (mg) = (TAN1/18) × 358.14 × 0.45
Weight of MgCl2·6H2O (mg) = (TAN1/18) × 203.3 × 0.45
Weight of MgO (mg) = (TAN1/18) × 40.3 × 0.45

2.2. Crystallization Parameters

Reaction parameters including the pH, reaction time, stirring speed, and molar ratio were tested, and the specific values tested are shown in Table 2 [31]. The initial experiment was conducted with a reaction time of 30 min, a stirring rate of 100 rpm, and a P/Mg/N ratio of 1:1:1 (Figure 1).

2.3. Characterization of Precipitations

Characterization of the precipitations was carried out using X-ray diffraction (XRD) (Ultima IV, Rigaku, Denver, CO, USA) and scanning electron microscopy (SEM) (Quanta 200, FEI, Lausanne, Switzerland), as shown in Figure 1.
For XRD analysis, the collected precipitates were ground into a fine powder by passing them through a 320-mesh sieve and then mounted onto a sample holder. The surface was leveled and compacted using a glass slide to ensure a flat, uniform surface. The scanning range was set from 10° to 80° (2θ), with a step size of 0.02° per step and a scanning rate of 5° per min. A Cu target was used as the X-ray source, with an operating voltage of 40 kV and a current of 40 mA.
For SEM, a small amount of the powdered sample was collected using a sampling spatula and evenly distributed onto a specimen stub coated with conductive adhesive. The sample was gently pressed to enhance adhesion, and any loose particles were removed using a rubber suction bulb. The prepared stub was then gold-coated using an ion sputter coater and subsequently transferred to an environmental scanning electron microscope for imaging.

2.4. Precipitations on Plant Growth

We conducted experiments using two commonly cultivated vegetables in southern China: water celery and bock choy (Figure 1). Water celery is known for its distinctive flavor and is rich in proteins, vitamins, and other nutrients; it is thus widely known for its health-promoting properties [34]. Further, water celery has been used in livestock wastewater treatment and has been proven to be edible [35,36]. Bok choy is rich in vitamins and minerals and is a common vegetable in Chinese cuisine [37]. Potting experiments with bock choy and water celery were conducted to evaluate the fertility of struvite recovered from the biogas slurry. A commercial slow-release fertilizer (1 g containing 0.15 g N, 0.15 g P, and 0.15 g potassium) was used for comparative analysis. This study focuses on the recovery and resource utilization of nitrogen and phosphorus from biogas slurry, so the dosage of fertilizer was based on N content (0.15 g for 1 kg soil) and Equation (5). Due to the 1:1 molar ratio of N/P in struvite, applying it based on N requirements will inevitably result in higher levels of P compared to commercial slow-release fertilizers. Moreover, due to the phosphate residual effect, even when applied in excess, the nutrient use efficiency by plants remains limited [38]. The experiments comprised seven groups, and each group contained three biological replicates, with struvite accounting for 0%, 20%, 40%, 60%, 80%, and 100% of the fertilizer dosage, in addition to a group without fertilizer as a control (Table 3). All pots were subjected to 25 °C and 7000 lux illumination. For bock choy, 6 seeds were sown in pots containing 500 g of soil. After germination, one seedling with uniform growth was selected, and the others were removed. For water celery cultivation, three seedlings with a fresh weight of 10 g and a height of 15 cm were planted in pots containing 1 kg of soil. As an aquatic vegetable, maintaining sufficient soil moisture is essential; therefore, we watered the soil every three days, ensuring the water level remained 1 cm above the soil surface.
Dosage of precipitations (g) = 0.15/14 × 137.3 × soil weight × %mass in fertilizer
The potting experiment lasted 45 days for bock choy and 90 days for water celery. The duration of the experiments was shorter than the nutrient release period of struvite in soil reported by Min et al. (2019) (63 days), and that in water reported by Latifian et al. (2012) (105 days) [20,24]. After the experiments, photographs were taken to record the growth condition, and root length, chlorophyll content, soluble protein, and soluble sugar were recorded [39,40]. As edible vegetables, the appearance, texture, flavor, and nutritional value of water celery and bock choy are key criteria for assessing their quality and commercial value [41]. Fresh weight is one of the important indicators of vegetable yield. Since both water celery and baby bock choy are harvested and consumed in their fresh form, fresh weight more accurately reflects the actual yield in line with industrial production needs [42].

2.5. Analytical Methods

TAN was measured by an ammonia photometer (HI96715, HANNA, Buhl, MN, USA). TP was analyzed using the ammonium molybdate spectrophotometric method (GB 11893–89 [43]). pH was measured with a pH meter (PHS–25, INESA, Shanghai, China). The N and P recovery rates were calculated using Equations (6) and (7) [15]. MDI Jade 6 (Stat-Ease, Minneapolis, MN, USA) was used to analyze the XRD data.
TAN recovery rates (%) = (TAN1 − TAN3)/TAN1 × 100%
TP recovery rates (%) = (TP2 − TP3)/TP2 × 100%
Parameters, including fresh weight, plant height, and root length, were assessed to test the effect of struvite on plant growth. Leaves from the same part of the plants were used to determine the chlorophyll content according to the ethanol method, and the concentration of chlorophyll a (Chla) and chlorophyll b (Chlb) was measured according to Equations (8)–(10) [44]. Soluble sugar was assessed using the sulfate–anthrone method. Soluble protein was used to measure the nutrition quality [45]. Vitamin C (VitC) was assessed by the method of 1,10-phenanthroline spectrophotometry [46].
Chla = 13.95A665 − 6.88A649
Chlb = 24.96A649 − 7.32A665
Total chlorophyll = Chla + Chlb
Data were analyzed using one-way analysis of variance in SPSS 25.0 software (SPSS Inc., Chicago, IL, USA). Linear fitting and Duncan’s test were performed to assess the significance of differences between groups to examine changes in N and P removal rates and the quality of potted experimental plants. Paired-sample t-tests were used to analyze the differences in N and P recovery under MgO and MgCl2 addition under optimal conditions. Graphs were generated using Origin 2021 (OriginLab Corporation, Northampton, MA, USA). Photo editing was performed using Adobe Photoshop CC (Adobe, San Jose, CA, USA) to enhance visual presentation without altering the integrity of the data.

3. Results and Discussion

3.1. Optimization of Crystallization Parameters for Recovery of N and P

3.1.1. pH

pH affects chemical reaction equilibria and solution supersaturation [47]. As shown in Figure 2a,b, pH had no significant effect on the TP recovery (94.76% to 99.17%) under MgCl2 addition; TAN recovery peaked at 74.09% at pH 10. The effect of pH was also small for MgO, with TP recovery ranging from 68.07% to 70.07%, and TAN recovery peaked at 31.78% at pH 11.0. However, due to the volatilization of TAN above pH 10.5 [48], the optimal pH values were determined as 10.0 for MgCl2 and 10.5 for MgO in this study.

3.1.2. Reaction Time

An excessively long reaction time can increase processing costs; however, the reaction might be incomplete when the reaction time is excessively short. The reaction time thus needs to be optimized [49]. Between 10 and 60 min, TP recovery was stable, averaging 98.87% for MgCl2 and 65.69% to 70.13% for MgO (Figure 2c,d). TAN recovery increased significantly from 10 to 20 min but plateaued afterward. Thus, 20 min was optimal for both MgCl2 and MgO treatments.

3.1.3. Stirring Rate

Given the large volume of biogas slurry discharged from pig farms, it must be promptly treated. An appropriate stirring rate increases the probability of molecular collisions, promotes reactions, and accelerates the reaction rate [50]. As shown in Figure 2e,f, stirring had no effect on TP recovery, which remained stable at approximately 99.49% for MgCl2 and 67.84% for MgO. Although stirring had a major effect on TAN recovery, the magnitude of stir speed has little impact on the TAN recovery rate for both Mg sources. The optimal stirring rate was determined to be 200 rpm.

3.1.4. Molar Ratio

The formation of struvite depends on a specific molar ratio of N, P, and Mg, and a supersaturated solution favors the progression of the reaction [23]. A suitable molar ratio can both promote the reaction and avoid excessive cost. When the concentration of a solution exceeds its saturation point, crystallization can occur rapidly without altering the reaction conditions. P/N had little effect on the TAN recovery rates (Figure 2g). However, MgCl2 concentrations significantly influenced N and P recovery rates; moreover the highest recovery rates were observed at a P/Mg/N ratio of 1.0:1.2:1.0, achieving a TAN recovery rate of 81.83% (Figure 2i). However, as a sparingly soluble solid, MgO is less reactive with N and P in solution. In Figure 2h,j, the increase in particulate matter in the solution reduces the TP recovery rate and the purity of struvite [51]. Therefore, the optimal molar ratio for MgCl2 treatment was determined to be 1.0:1.2:1.0 (P/Mg/N); for MgO treatment, it was 1.0:1.0:1.0.
Under optimal conditions, the TP and TAN recovery rates of batches with MgCl2 added were 99.67% and 81.83%, respectively, reducing TP from 7322.04 to 24.30 mg·L−1 and TAN from 1387.33 to 252.67 mg·L−1. The TP and TAN recovery rates of batches with MgO addition were 69.19% and 22.32%, respectively, which reduced TP from 6206.67 to 1912.15 mg·L−1 and TAN by 351.33 mg·L−1.

3.2. Comparison of Two Mg Sources

3.2.1. Recovery Rate

Under optimal recovery conditions, MgCl2 addition achieved a P and N recovery rate of 99.67%, which was significantly higher than the P recovery rate of MgO (69.19%) under optimal conditions (Figure 3). The difference in N recovery efficiency between the two was 59.51%. Thus, the observations suggest that recovery rates are higher under MgCl2 addition than under MgO addition, and P recovery rates are significantly higher than N recovery rates, which stems from side reactions.

3.2.2. Characterization of Struvite

To verify whether the recovered precipitation contained struvite and determine its content and the main by-products, XRD and SEM analyses were performed on the recovered precipitation. The XRD analysis confirms the presence of struvite in precipitations (Figure 4a). The purity of the precipitation produced using MgCl2 as Mg source was 79.00%. No product of co-precipitation of N and P with other heavy metals was detected. Although overdosing Mg can lead to the formation of Mg3(PO4)2, only MgHPO4 was detected in the precipitation, which comprised 6.10% of the precipitation. The purity of the precipitation produced using MgO as the Mg source was 43.60%, with 54.10% of the precipitation being MgO. Thus, the purity of the struvite crystals was higher in the precipitation produced using MgCl2 compared with that produced using MgO. Figure 4b shows a standard sample of 98% purity struvite, which had a long columnar structure with a length of approximately 50 μm. As shown in Figure 4c, struvite using MgCl2 as the Mg source had a sheet-like morphology with relatively large particles that were up to approximately 50 μm in length. As shown in Figure 4d, struvite using MgO had a long columnar shape and was attached to the surface of MgO; the particles were also smaller in size. These results indicate that the precipitation produced using MgCl2 was better than that using MgO based on the purity of struvite crystals and crystal size. Therefore, the precipitations produced using MgCl2 were collected to investigate their effects on plant growth.

3.3. The Application of Struvite Precipitation in Agriculture

Potting experiments were performed to evaluate the growth of the terrestrial vegetable bock choy and the aquatic vegetable water celery using different ratios of commercial slow-release fertilizer and struvite derived from biogas slurry.

3.3.1. Terrestrial Vegetable Cultivation

The application of struvite precipitation significantly increased the root length and fresh weight of bock choy (Figure 5a–c); as the proportion of struvite precipitation in the fertilizer increased, its growth-promoting effect became more pronounced. When the proportion of struvite in fertilization was 80% or more, the fresh weight was higher than that of other groups, reaching 8.20 g and 8.35 g for group B5 and B6, respectively.
The effects of the application of struvite precipitation on vegetable quality-related parameters were investigated. The soluble sugar content increased as the proportion of struvite increased in the fertilizer; it was highest in group B6 (44.59 mg·g−1), followed by group B5 (39.55 mg·g−1) (Figure 5d). The soluble protein content in groups B5 and B6 was 22.55 mg·g−1 and 22.49 mg·g−1, respectively, which was significantly higher than that of the other groups and the control group (Figure 5e). Commercial fertilizers reduced the soluble protein content in bock choy (Figure 5e), whereas the addition of struvite did not have such a negative effect. In addition, application of struvite precipitation also increased the VitC content (Figure 5f).
Chlorophyll content is an important indicator of the photosynthetic capacity of plants, and a high chlorophyll content enhances photosynthesis. Since struvite contains Mg, it can affect chlorophyll synthesis [52]; the chlorophyll content in the leaves increased as the proportion of struvite in the fertilizer increased. The chlorophyll content was highest in groups B5 and B6, which was approximately 0.68 mg·g−1 (Figure 5g).

3.3.2. Aquatic Vegetable Cultivation

The stem and leaves of water celery were larger in the fertilized groups than in the CK (Figure 6a). As the proportion of struvite increased, no significant differences in weight, root length, and height between groups O3 to O6 and group O1 were observed (Figure 6b). However, the root/shoot ratio significantly increased with the content of struvite, indicating a shift in the growth strategy of water celery.
Group O6 had the highest soluble sugar and soluble protein content (30.15 mg·g−1 and 42.89 mg·g−1, respectively), and they significantly differed in Group O6 compared with the other fertilized groups (Figure 6c,d). No significant differences in the VitC content were observed among groups, and it ranged between 2108.79 nmol·g−1 and 3495.72 nmol·g−1 (Figure 6e). The chlorophyll content was highest in Group O6 (Figure 6f).

4. Discussion

The optimal pH for MgCl2 was slightly higher than that for MgO. This might stem from the fact that recovery of N and P from the biogas slurry is more complete under MgCl2 application, a conclusion supported by Figure 3. The precipitation reaction resulting in struvite formation releases hydrogen ions; consequently, a greater number of hydroxide ions are required to neutralize these protons and drive the reaction forward [53]. After 20 min of the reaction, no significant differences in the removal rates of TP and TAN were observed, indicating that the chemical reactions proceeded rapidly and were essentially completed within 20 min [49]. Increasing the stirring rate increases the reactant collision frequency and reaction uniformity [50]; Figure 2e,f also shows that increasing the stirring rate from 0 to 200 rpm significantly enhances the recovery efficiency of both N and P. Crystallization efficiency is dependent on the degree of supersaturation [23]. When the solution concentration exceeds its saturation threshold, crystallization can occur rapidly even without altering other reaction conditions. In this study, the optimal molar ratio for struvite recovery was P/Mg/N = 1:1.2:1, which is consistent with the optimal ratio reported by Donten et al. (2012) and Min and Park (2021), (1.1:1.2:1) [53,54]. Since Mg can form multiple low-solubility compounds with P, an increase in Mg dosage enhances P recovery efficiency [55,56]. Gadekar and Pullammanappallil (2010) investigated the effect of molar ratios on struvite precipitation using simulated wastewater and MgCl2 as the Mg source [57]. They found that the molar ratio significantly influences both the recovery efficiency and the purity of struvite in the precipitate.
Our findings indicate that both MgCl2 and MgO can achieve the recovery of N and P from biogas slurry. However, the effect of MgCl2 on recovery rates in the experiment is more pronounced than that of MgO, as shown in Figure 3. Following the addition of MgO into the solution, hydrolysis takes place at the surface of MgO, initiating the reaction around the MgO solid [58]. Some researchers have used additional acids to increase the recovery rate, but this has led to increased costs [16,59]. The reaction area likely increased and recovery rates were likely enhanced when MgCl2 was used as a Mg source [60]. Furthermore, the recovery rates of P were significantly higher than those of N, and the recovery rates varied among Mg salts. Due to side reactions, the possible magnesium phosphate species were struvite (Ksp = 10−13.26), magnesium hydrogen phosphate (MgHPO4) (Ksp = 1.02 × 10−7), and magnesium phosphate (Mg3(PO4)2) (Ksp = 10−25.2), which enhance the P recovery rates [55,56].
Due to the dissociated Mg2+, phosphate (PO42−), and sodium hydrogen phosphate (HPO4−), MgHPO4 is formed, and sodium ions (Na+) replace hydrogen ions (H+) to form magnesium sodium phosphate (MgNaPO4), which is more stable than MgHPO4 [61]. The excess solid MgO in the solution provides a large amount of active growth sites for struvite (Figure 4c). The struvite crystals attached to the surface of MgO particles lead to the formation of polyhedrals [62,63]. However, the struvite attached to the surface of MgO limits further improvement in recovery rates [64,65]. In Figure 4b, the flake-like formation of struvite crystals was observed when MgCl2 was used as the Mg source, which occurs under high pH and in highly oversaturated solutions [66,67]. The size of the struvite crystals when MgCl2 was used as a Mg source was much larger than that when MgO was used, and larger crystals have long-term effects in the soil. In addition, Yao et al. (2023) reported, using synthetic wastewater, that anionic surfactants led to the optimal P recovery at 24 min, which was longer than that of the control group, while cationic and zwitterionic surfactants had no significant effect on crystallization [68]. In this study, the P recovery reached its optimum at 20 min, indicating that the experimental results were not affected by anionic surfactants.
In the pot experiment, both the growth and quality indicators of bock choy were improved by the addition of struvite. Struvite contains N, P, and Mg, which are essential elements for plant growth. N in struvite exists in the form of TAN, which can be directly absorbed by plants and used for protein synthesis without expending large amounts of energy. This may explain the observed increase in soluble protein content following struvite application [69]. Mg is the central component of chlorophyll, and its deficiency severely impairs photosynthetic efficiency and carbohydrate formation in plants [70]. The Mg in struvite is absorbable by plants; struvite thus provides an adequate source of this crucial element. Furthermore, studies in crops such as rice and maize have shown that an appropriate level of TAN positively influences chlorophyll content and thus enhances photosynthetic efficiency [71]. Since photosynthesis is the primary pathway for carbon dioxide fixation in plants, this improvement ultimately increases the biomass productivity of bock choy [72].
The application of struvite had a positive effect on most vegetable quality indicators for water celery. Li et al. (2003) applied quick-acting fertilizer and struvite to two aquatic plants, water spinach (Ipomoea aquatica) and water convolvulus (Ipomoea aquatica, I. reptans), and struvite was found to promote the growth of these vegetables [73]. Since water celery has a well-developed root system, TAN can alter the distribution of carbohydrates in plants, prioritize the allocation of resources to the underground parts, and significantly increase the root/shoot ratios [74]. TAN can enhance root activity by promoting lignin synthesis in roots and accelerate the transport of water and nutrients [75]. In studies of sweet potatoes, TAN has been shown to increase yield by promoting the growth of young roots and increasing the number of storage roots [76]. Thus, the application of struvite may be beneficial to plant roots. Furthermore, the effects of struvite addition on the growth of terrestrial and aquatic plants differ. These effects vary because of differences in the root environment, nutrient availability, and uptake mechanisms between these two types of plants [77]. The ability of plants to alter their rhizosphere is weakened in aquatic environments; struvite dissolves better under low-pH conditions, and the nutrient absorption efficiency of struvite in water is low [78]. Commercial slow-release fertilizers typically achieve their controlled-release effect through physical coatings or microbial degradation, which makes them less vulnerable to variation in environmental conditions and plant types [79]. The effects of struvite on the growth of aquatic plants are weaker than those of commercial slow-release fertilizers.

5. Conclusions

We developed a complete industrial chain in which struvite was used to recover N and P from biogas slurry for its subsequent application to potted plants. The N and P recovery efficiency was significantly higher when MgCl2 was used as the Mg source compared with MgO; the optimal reaction conditions were pH 10, a reaction time of 20 min, a stirring rate of 200 rpm, and a Mg/P/N molar ratio of 1.2:1.0:1.0, which achieved N and P recovery rates of 99.67% and 81.83%, respectively. SEM and XRD confirmed the presence of struvite in the precipitate, which had a purity of 79.00%. We also compared the effects of struvite and commercial slow-release fertilizers on vegetable growth for the first time. The addition of struvite was positively correlated with the growth and quality indicators of bock choy. The fresh weight of the full struvite group was 2.90 times that of the commercial slow-release fertilizer group; the soluble sugar content was 44.58 mg·g−1 and the soluble protein content was 22.55 mg·g−1. The growth strategy of water celery was altered by struvite, as evidenced by a significant increase in the root/shoot ratio. The quality parameters of water celery were promoted by struvite, and the soluble sugar content and soluble protein content were 1.20 times and 1.09 times higher in the full struvite group than in the commercial slow-release fertilizer group, respectively; the VitC content was 2996.29 nmol·g−1 in the full struvite group. The application of struvite recovered from biogas slurry to vegetables had a significant positive effect on various quality indicators of water celery and nearly all parameters of bock choy. Overall, we established a complete industrial chain for the utilization of struvite recovered from biogas slurry to enhance vegetable growth. Our findings provide crucial insights that will help optimize the recovery of nutrients from wastewater using struvite and the use of this material in real-world agricultural scenarios to promote crop growth.

Author Contributions

Material: Y.L. and W.W.; methodology: Y.C. and W.W.; formal analysis: Y.L.; software: J.C. and X.L.; visualization: Y.L. and J.L.; writing—original draft preparation: Y.L.; writing—review and editing: L.S. and Y.C.; funding acquisition: Y.C. and D.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Jiangsu Agriculture Science and Technology Innovation Fund (CX(21)2012).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to containing unpublished patent information.

Acknowledgments

We would like to express our gratitude to Nanjing Forestry University Advanced Analysis and Testing Center for providing XRD and SEM testing services, and to Sanshubio Co., Ltd. in Shanghai, China, for providing vegetable quality-related parameter testing services. We are grateful to all lab members for their suggestions, support, and encouragement.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 15 01352 g001
Figure 1. Experimental schematic diagram.
Figure 1. Experimental schematic diagram.
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Figure 2. The concentrations of TP and TAN after the recovery and recovery rates of TP and TAN using two Mg sources. Under different pH conditions, (a) MgCl2, and (b) MgO. At different reaction times, (c) MgCl2, and (d) MgO. At different stirring speeds, (e) MgCl2, and (f) MgO. At different Mg/N ratios, (g) MgCl2, and (h) MgO. At different P/N ratios, (i) MgCl2, and (j) MgO. Different lowercases indicate significant differences at p < 0.05 according to Duncan’s multiple range test.
Figure 2. The concentrations of TP and TAN after the recovery and recovery rates of TP and TAN using two Mg sources. Under different pH conditions, (a) MgCl2, and (b) MgO. At different reaction times, (c) MgCl2, and (d) MgO. At different stirring speeds, (e) MgCl2, and (f) MgO. At different Mg/N ratios, (g) MgCl2, and (h) MgO. At different P/N ratios, (i) MgCl2, and (j) MgO. Different lowercases indicate significant differences at p < 0.05 according to Duncan’s multiple range test.
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Figure 3. Comparison of N and P recovery rates under optimum conditions using different Mg sources. Error bars represent standard deviation. ** and *** indicate significance at p < 0.01 and p < 0.001, respectively.
Figure 3. Comparison of N and P recovery rates under optimum conditions using different Mg sources. Error bars represent standard deviation. ** and *** indicate significance at p < 0.01 and p < 0.001, respectively.
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Figure 4. SEM and XRD images of produced precipitations. (a) XRD analysis of precipitations and struvite reagent. (b) Precipitation of struvite reagent. (c) Precipitation of MgCl2. (d) Precipitation of MgO.
Figure 4. SEM and XRD images of produced precipitations. (a) XRD analysis of precipitations and struvite reagent. (b) Precipitation of struvite reagent. (c) Precipitation of MgCl2. (d) Precipitation of MgO.
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Figure 5. Photographs and quality-related parameters of bock choy. (a) Photographs after cultivation for 1.5 months. CK: control group, cultivation without fertilizer. Struvite precipitations composing 0% (B1); 20% (B2); 40% (B3); 60% (B4); 80% (B5); and 100% (B6) of the fertilizer dosage. (b,c): Fresh weight and root length. Quality parameters including (d) soluble sugar, (e) soluble protein, (f) VitC, and (g) chlorophyll concentration. Different lowercases indicate significant differences at p < 0.05 according to Duncan’s multiple range test.
Figure 5. Photographs and quality-related parameters of bock choy. (a) Photographs after cultivation for 1.5 months. CK: control group, cultivation without fertilizer. Struvite precipitations composing 0% (B1); 20% (B2); 40% (B3); 60% (B4); 80% (B5); and 100% (B6) of the fertilizer dosage. (b,c): Fresh weight and root length. Quality parameters including (d) soluble sugar, (e) soluble protein, (f) VitC, and (g) chlorophyll concentration. Different lowercases indicate significant differences at p < 0.05 according to Duncan’s multiple range test.
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Figure 6. Photographs and quality-related parameters of water celery. (a) Photographs after cultivation for 1.5 months. CK: control group cultivated without fertilizer. Struvite precipitations composing 0% (O1); 20% (O2); 40% (O3); 60% (O4); 80% (O5); and 100% (O6) of the fertilizer dosage. (b) Fresh weight (yellow), height (red), root length (blue), and root/shoot ratios (green). Quality parameters including (c) soluble sugar (yellow), (d) soluble protein (red), (e) VitC (blue), and (f) chlorophyll concentration (yellow for Chla and red for Chlb). Different lowercases indicate significant differences at p < 0.05 according to Duncan’s multiple range test.
Figure 6. Photographs and quality-related parameters of water celery. (a) Photographs after cultivation for 1.5 months. CK: control group cultivated without fertilizer. Struvite precipitations composing 0% (O1); 20% (O2); 40% (O3); 60% (O4); 80% (O5); and 100% (O6) of the fertilizer dosage. (b) Fresh weight (yellow), height (red), root length (blue), and root/shoot ratios (green). Quality parameters including (c) soluble sugar (yellow), (d) soluble protein (red), (e) VitC (blue), and (f) chlorophyll concentration (yellow for Chla and red for Chlb). Different lowercases indicate significant differences at p < 0.05 according to Duncan’s multiple range test.
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Table 1. Parameters of biogas slurry used in this study.
Table 1. Parameters of biogas slurry used in this study.
ParameterValuesUnits
Mg2+11.08 mg·L−1
TP152.04 mg·L−1
TAN1338.22 mg·L−1
pH8.0–8.2–
Table 2. The values for the different parameters tested.
Table 2. The values for the different parameters tested.
ParameterGroupsUnit
pH value9.0, 9.5, 10.0, 10.5, 11.0, 11.5 (treatment of MgCl2)
7.0, 8.0, 9.0, 10.0, 11.0, 12.0 (treatment of MgO)		–
Reaction time10, 20, 30, 60min
Stirring rate0, 100, 200, 300, 400, 500, 600 (treatment of MgCl2)
0, 100, 200, 300, 400 (treatment of MgO)		rpm
P/N0.8, 0.9, 1.0, 1.1, 1.2, 1.3–
Mg/N0.8, 0.9, 1.0, 1.1, 1.2, 1.3–
Table 3. The potting experiment groups with different fertilizer ratios.
Table 3. The potting experiment groups with different fertilizer ratios.
VegetableFertilizer Ratio
Struvite-0%20%40%60%80%100%
Commercial fertilizer-100%80%60%40%20%0%
Water celery CKO1O2O3O4O5O6
Bock choy CKB1B2B3B4B5B6
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Li, Y.; Wang, W.; Sun, L.; Cui, J.; Liu, X.; Liu, J.; Chang, Y.; Yao, D. A Full Chain of Applying Struvite Recovered from Biogas Slurry to Promote Vegetable Growth. Agriculture 2025, 15, 1352. https://doi.org/10.3390/agriculture15131352

AMA Style

Li Y, Wang W, Sun L, Cui J, Liu X, Liu J, Chang Y, Yao D. A Full Chain of Applying Struvite Recovered from Biogas Slurry to Promote Vegetable Growth. Agriculture. 2025; 15(13):1352. https://doi.org/10.3390/agriculture15131352

Chicago/Turabian Style

Li, Yunhan, Wei Wang, Linhe Sun, Jian Cui, Xiaojing Liu, Jixiang Liu, Yajun Chang, and Dongrui Yao. 2025. "A Full Chain of Applying Struvite Recovered from Biogas Slurry to Promote Vegetable Growth" Agriculture 15, no. 13: 1352. https://doi.org/10.3390/agriculture15131352

APA Style

Li, Y., Wang, W., Sun, L., Cui, J., Liu, X., Liu, J., Chang, Y., & Yao, D. (2025). A Full Chain of Applying Struvite Recovered from Biogas Slurry to Promote Vegetable Growth. Agriculture, 15(13), 1352. https://doi.org/10.3390/agriculture15131352

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Article metric data becomes available approximately 24 hours after publication online.
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