Introduction
Amino acid fertilizers are recognized as effective bio-stimulants that contain both free and bound amino acids, playing vital roles in enhancing plant growth, development, and stress resilience (Rouphael and Colla, 2020). These compounds serve as fundamental components of proteins, supplying essential nitrogen and carbon, while also boosting metabolic processes and improving crop quality (Garcia and Perez, 2021). Common sources of amino acid fertilizers include plant, microbial, and animal proteins, which undergo hydrolysis to release peptides and amino acids that plants can readily absorb (Chen et al., 2019). Tomato, a major greenhouse crop, has shown significant improvements in chlorophyll content, vegetative growth, yield, and quality when treated with amino acid fertilizers (Gasana and Kim, 2020).  Specific amino acids such as lysine, arginine, and proline are known to help plants withstand environmental stresses like drought, salinity, and temperature variations (Garcia and Perez, 2021). A sustainable and innovative source for these fertilizers is fish processing waste, particularly from common carp, which is rich in proteins. Repurposing these by-products not only reduces environmental impact but also creates valuable agricultural inputs. Previous research confirms that fish waste-derived amino acid fertilizers can enhance plant growth, increase fruit yield, and improve biochemical traits such as protein and chlorophyll levels (Chen et al., 2019). Given the rising demand for sustainable agriculture, this study explores the potential of amino acid fertilizers made from carp waste to support eco-friendly tomato production (Rouphael and Colla, 2020).
Methodology
The research was carried out at the Caspian Sea Ecology Research Center in Sari, Iran. Common carp waste including skin, viscera, heads, and fins was collected from local markets, frozen, and processed via enzymatic hydrolysis using Alcalase and Flavourzyme (Safari et al., 2016; Safari et al., 2018; Safari et al., 2020). The process included pasteurization, enzyme treatment under controlled pH and temperature, and heat inactivation. The resulting hydrolysate was then centrifuged, filtered, and stored for further analysis.
Chemical and physicochemical properties such as total protein, soluble protein, peptide size, degree of hydrolysis, ash, fat, moisture, nitrogen, phosphorus, potassium, pH, salinity, and amino acid composition were evaluated using standard methods (Hesse, 1971; Adler-Nissen and Olsen, 1979; James, 1995; Moore, 2004; AOAC, 2005; ISIRI, 2008a, b, c; APHA, 2017).
 Microbiological safety was verified through tests for aerobic bacteria, yeast, mold, coliforms, E. coli, and Salmonella (Sallam, 2007; ISIRI 8923-1, 2007; ISO 21527-1, 2008; Hernández et al., 2009; ISIRI 11166, 2010; ISIRI 10889-3, 2013; ISO 4833-1, 2013; ISIRI 13321-2, 2014). A greenhouse trial using a completely randomized design tested five treatments: control, enzyme hydrolysate, enzyme hydrolysate enriched with phosphorus and potassium, a domestic commercial fertilizer, and an imported commercial fertilizer. Tomato seedlings in pots received weekly foliar applications, and parameters related to growth and quality such as plant height, leaf count, leaf area, fruit yield, chlorophyll, TSS, lycopene, and vitamin C were measured (Abbasi et al., 2003; Boras et al., 2011). Data were analyzed using ANOVA and Duncan’s test.
Results
The hydrolysis process significantly increased total protein content from 15.17% in raw waste to 76.61% in the final product. Fat and moisture decreased, while ash and organic nitrogen levels rose. The hydrolysate had a hydrolysis degree of 21.11%, soluble protein of 26.87 mg/mL, and contained 13.87% total amino acids and 7.67% free amino acids, including essential types like lysine and leucine. It exhibited desirable physical traits, was water-soluble, and met safety standards with no pathogenic contamination. Storage studies confirmed the product’s stability over three months. In the greenhouse, the enriched hydrolysate treatment led to the best results in plant height, leaf number, leaf area, fruit count, and fruit weight. It also outperformed other treatments in chlorophyll levels, TSS, lycopene, and vitamin C content.
Discussion and conclusion
Enzymatic hydrolysis successfully transformed carp waste into a plant-friendly fertilizer rich in bioavailable peptides and amino acids (He et al., 2013; Safari et al., 2016; Safari et al., 2018; Safari et al., 2020). The resulting product was safe, stable, and effective as a foliar spray (FAO, 2010; López-Caballero et al., 2010; Ortizo et al., 2020). Key amino acids supported chlorophyll formation and nitrogen metabolism, leading to better growth and fruit quality (Colla et al., 2015; Rouphael et al., 2017; Zhang et al., 2018). The enriched hydrolysate treatment demonstrated synergistic benefits, enhancing both yield and nutritional quality of tomatoes (Colla et al., 2014; Bulgari et al., 2015; Guo et al., 2018; Zhang et al., 2018). This method offers a sustainable way to upcycle fish waste into a high-value agricultural input (Kumar and Singh, 2019; Martinez and Smith, 2020; Ciepiela and Kamińska, 2021).
Further optimization and testing on other crops could broaden the application of this biofertilizer (Zhou et al., 2019; He et al., 2020). In summary, common carp waste-derived amino acid fertilizer presents an eco-friendly and efficient alternative to synthetic fertilizers in greenhouse tomato cultivation.
Conflict of interest
The authors declare no conflict of interest.
Acknowledgment
The authors extend their gratitude to the Caspian Sea Ecology Research Center for providing the necessary facilities and support.