Reduced Nitrogen Fertilizer Combined with Organic Fertilizer Affects Growth and Soil Physicochemical Properties of Sapindus delavayi (Franch.) Radlk.

Sapindus delavayi (Franch.) Radlk. is a non-wood tree species of considerable ornamental, ecological, and medicinal value, and its fruits are rich in saponins, with notable cleansing and skin-care properties. However, this tree species is currently facing the dilemma of low fruit yield and unstable fruiting. Fertilization is an effective measure to improve this cultivation situation. Considering the harm of nitrogen fertilizer in soil, we designed four schemes to replace nitrogen fertilizer with organic fertilizers in this study, aiming to obtain the optimal fertilization combination for the growth of Sapindus delavayi (Franch.) Radlk. Three-year-old seedlings were subjected to applications of nitrogen fertilizer, organic fertilizer, and their combinations to evaluate their effects on plant physiological responses and soil physicochemical properties. Results revealed that the sole application of organic fertilizer significantly promoted the elongation of new shoots. Compared with the unfertilized control, the combined treatment of 30% organic fertilizer and 70% nitrogen fertilizer significantly increased soil nitrate nitrogen content by 161.54%, while ammonium nitrogen content decreased by 24.30%. Principal component analysis indicated that glutamine synthase (GS), glutamate synthase (GOGAT), and nitrate reductase (NR) were the major enzymes influencing leaf physiological responses. Therefore, we concluded that fertilization has affected the rate of nitrogen assimilation and altered the process of amino acid synthesis, thereby influencing the metabolism and maintenance of cellular function in Sapindus delavayi (Franch.) Radlk. Structural equation modeling further indicated that fertilization primarily influenced total carbon content in plant leaves by affecting soil organic matter and alkali-hydrolyzable nitrogen. These findings elucidate the regulatory relationship between plant physiological processes and soil properties under co-application of nitrogen and organic fertilizer, providing a scientific reference for field fertilization management of this tree species.