The initial pH varied slightly among the treatments, with values of 7.4 for CK, 7.3 for SSP5, 7.1 for SSP10, and 7.0 for SSP15, aligning with the range reported in previous studies on swine manure composting systems [16]. Due to the volatilization and decomposition of organic acids [26], the pH exhibited an upward trend as composting began, eventually settling in the range of 7.4 to 7.7 (Figure 2a). This range aligns with the optimal pH values that are conducive to plant growth, nutrient accessibility, and microbial activity [27]. Notably, SSP10 and SSP15 significantly lowered the pH in both the raw materials and the resulting compost products, likely due to the inherent acidity of superphosphate (pH = 1.5). The effect of superphosphate addition on pH reduction has also been observed by several researchers in small-scale experiments [28,29].

Electrical conductivity (EC) reflects the salinity levels of compost, and elevated EC can pose risks to seed germination and plant health when compost is utilized [30]. As shown in Figure 2b, the EC increased with the addition of superphosphate. The final EC values for the CK, SSP5, SSP10, and SSP15 treatments were 4.27, 5.70, 7.36, and 9.04 mS·cm⁻¹, respectively, exceeding the recommended standard values (4 mS·cm⁻¹) for compost applications to soil and potentially causing phytotoxicity [31]. However, EC is influenced by the properties of raw materials and external additives, and an EC value exceeding 4 mS/cm does not necessarily indicate immaturity [32]. Similar increases in EC with the addition of superphosphate have been found by various researchers under laboratory conditions [15,17,33]. However, the EC values in our study were significantly higher, likely due to the dissolution of PO₄³⁻, HPO₄²⁻, H₂PO₄⁻, and Ca²⁺ from superphosphate, facilitated by the organic acids generated during the initial and thermophilic phases of composting [15]. Nevertheless, previous reports have indicated that the rise in EC due to added Ca²⁺ may not be of significant concern [18]. This is because most Ca²⁺ is expected to either precipitate as minerals or be preferentially adsorbed by the soil when compost is applied [34].

The addition of superphosphate significantly decreased the compost C/N ratio during the composting period (Figure 2c). Owing to the absence of high-C co-composting materials such as straw, the initial C/N ratio was relatively low compared to previous studies; however, the final C/N ratios of the compost products, ranging from 12.0 for SSP15 to 16.0 for CK, were consistent with the values reported in those studies [24,33]. Traditionally, the C/N ratio has served as a key indicator for assessing the stability of the composting process and the maturity of the final product, as it reflects the balance between nitrogen retention and the conversion of carbon into CO₂ and humus [35]. A final C/N ratio <21 is accepted as the maturity standard for compost products [36], and a C/N ratio of 15 or even lower is deemed preferable for agricultural use [37]. Given these standards, all the treatments in our study met the accepted C/N ratio standards, with SSP15 emerging as the best for soil application.

The germination index (GI) is a critical parameter for evaluating compost phytotoxicity and maturity, effectively reflecting characteristics such as the cumulative temperature, dissolved organic carbon (DOC), and water-extractable nitrogen (Nw) [23,38]. As shown in Figure 2d, the GI increased steadily throughout the composting process. However, the addition of substantial quantities of superphosphate adversely affects the GI of compost products, which contrasts with the observations of Wang et al. (2022) [39]. SSP15 exhibited the lowest GI at 70.4%, which was significantly lower than that of any other treatment. This inhibition is primarily attributed to the fact that a high salt concentration limits microbial activity, which is supported by the significant negative correlation observed between the EC and GI. It is worth noting that Yang et al. (2015) [18] reported a similar inhibitory influence of superphosphate on GI during kitchen waste composting. Theoretically, a GI > 50% is regarded as the maturity standard, and a GI > 80% is considered nonphytotoxic to plants [40]. Although only the CK treatment in this study achieved a GI surpassing 80%, all treatments met the maturity criterion.

The nutrient content of the final composting product is presented in Table 1, and all the treatments met the requirements of the China National Agricultural Organic Fertilizer Standard (NY 525-2021) [25]. Superphosphate has the potential to increase both TN and P contents, and all superphosphate addition treatments yielded a higher TN and P₂O₅ compared with CK, with SSP15 showing the highest TN, P₂O₅, and K₂O contents. However, there were no significant differences in the TOC content between the control and the superphosphate-amended treatments.

Superphosphate can reduce N losses during composting, thereby enhancing the TN content. This finding is consistent with several previous studies conducted at both the laboratory scale and industrial scale [41,42]. Previous research has revealed that multiple mechanisms contribute to this effect. First, a decrease in pH resulting from superphosphate addition can mitigate the volatilization of ammonium to NH₃ by suppressing ammonification [43]. Additionally, the free acids (H₂SO₄ and H₃PO₄) in superphosphate chemically stabilize NH₄⁺ by converting it into ammonium phosphate and ammonium sulfate, both of which effectively reduce NH₃ emissions by limiting its source [44]. Furthermore, the phosphate ions (PO₄³⁻, HPO₄²⁻, and H₂PO₄⁻) in superphosphate can react with ammonium and magnesium ions to form struvite crystals, which stabilize nitrogen in the compost matrix [45,46].

According to the IPCC protocol, CH₄ and N₂O emissions are converted to CO₂-equivalents using their 100-year global warming potentials of 28 and 265 times that of CO₂, respectively [11]. Considering the inadequate carbon trading market in the city where the factory is located, we estimated the environmental economic benefits of greenhouse gas emission reductions using the latest estimate of the social cost of carbon (USD 185 per ton of CO₂) by Camilleri et al. (2023) [54]. The transportation and purchase cost of superphosphate used in this experiment is USD 100 per ton. Taking into account the current market fertilizer prices, the economic benefits associated with the retention of N, P₂O₅, and K₂O in the compost are estimated to be USD 0.72, 1.00, and 0.60 per kilogram, respectively. Furthermore, by reducing N loss during the composting process and retaining N, the addition of superphosphate also decreases the demand for and production of mineral N fertilizers. The economic benefits resulting from the saved use and production of mineral N fertilizers are calculated based on the GHG emissions associated with N fertilizer production processes described by Zhang et al. (2013) [55] in combination with the carbon social cost estimated by Camilleri et al. (2023) [54].

Taking into account that the average carbon emissions price (CEP) in China stood at only USD 6 per ton in 2020 [56], significantly lower than the environmental cost of USD 185 per ton utilized in our analysis, the economic burden associated with superphosphate addition surpasses the benefits derived solely from GHG emissions reduction (Table 2). Nonetheless, when accounting for the value of nutrients conserved through composting, incorporating superphosphate in compost production enhances P content and mitigates N loss, resulting in overall increased economic gains. Moreover, the superphosphate addition enables more N recovery from compost, thereby decreasing the demand for and production of mineral N fertilizers. Consequently, this significantly reduces GHG emissions during fertilizer production and usage, leading to substantial environmental and economic benefits.