Long-term application of manure increased soil amino acid pool under maize-maize-soybean rotation system

Purpose Manure application affects soil chemical and biochemical properties and organic nitrogen (ON). In this study, we aimed to study the long-term application of manure on soil amino acid pools and the underlying mechanisms that remain unquantified. Methods We studied the effects of 41-year manure application on soil ON fractions and amino acid pools during various crop seasons under maize-maize-soybean rotation. This study includes four treatments: no fertilization, inorganic nitrogen + phosphorus + potassium fertiliser (NPK), NPK + 13.5 t ha −1 manure, and NPK + 27 t ha −1 manure (M2NPK). Results The highest values for soil organic carbon (SOC), total nitrogen, pH, urease, and protease activity were observed in both the maize and soybean seasons under M2NPK. The contents of acidolysable ammonium nitrogen, acidolysable amino acid nitrogen (AAN), and ON in soil peaked under M2NPK in both seasons. Long-term high manure additions increased total soil amino acid content by 183% and 174% in 2019 and 2020, respectively. Acidic, alkaline, and neutral amino acid contents peaked during both seasons under M2NPK. Glycine, alanine, proline, lysine, serine, and valine were the predominant amino acid species in the soil, regardless of fertilization. M2NPK treatment led to increases of 88–151%, 79–129%, 70–114%, 87–97%, and 68–72% in the contents of Tyr, Leu, Ile, Glu, and Asp, respectively, compared to NPK. The structural equation modelling (SEM) indicated that SOC, pH, urease, and protease activity were the pathways that manure application alter the AAN pool by collectively accounting for 97% of the AAN variability. Further, our findings from SEM had a direct positive effect of AN on AAN under exogenous N input with a path coefficient of 0.35. Conclusion The addition of manure can improve soil properties, increase the amino acid pool, and enhance the amino acid pool availability under long-term crop rotation systems. In conclusion, the application of manure represents a practical approach to enhance soil nitrogen fertility and promote sustainable agricultural development.