Objectives Sandy fluvo-aquic soil has prominent obstacles such as poor structure, intense mineralization, and weak water and fertilizer retention. Exploring the effects of biochar, manure and their combined application on soil nitrogen and phosphorus components and transformation at different depths can provide theoretical and technical support for its obstacle reduction and soil fertility improvement.

Methods In October 2021, a wheat-maize rotation experiment was set up in Langgongmiao Town, Xinxiang County, Henan Province. The winter wheat season was set up seven treatments: conventional chemical fertilizer (CK), and on this basis, one-time biochar application (T1, 15 t/hm²), annual biochar (T2, 12 t/hm²), manure instead of chemical fertilizer nitrogen 20% (J1), manure instead of chemical fertilizer nitrogen 40% (J2), J1 combined with biochar (TJ1, 12 t/hm²) and J2 combined with biochar (TJ2, 12 t/hm²). Soil samples were collected at 0–20 cm (topsoil), 20–40 cm (middle soil) and 40–60 cm (deep soil) during the winter wheat harvest period in June 2024. The contents of soil inorganic nitrogen and acid-hydrolyzed organic nitrogen components (acid-hydrolyzed ammonium nitrogen, acid-hydrolyzed amino acid nitrogen, acid-hydrolyzed amino sugar nitrogen, acid-hydrolyzed unknown nitrogen, non-acid-hydrolyzed nitrogen), labile phosphorus (H₂O-P, NaHCO₃-Pi, NaHCO₃-Po), moderately labile phosphorus (NaOH-Pi, NaOH-Po), and stable phosphorus (HCl-P, Residual-P), as well as extracellular enzyme activities related to nitrogen and phosphorus transformations were analyzed.

Results The content of acid-hydrolyzable organic nitrogen fractions in each soil layer under T1 showed no significant difference compared with CK. In contrast, T2 increased the activity of β-acetyl-glucosaminidase in the topsoil, significantly increased the total nitrogen content in the topsoil and middle soil by 33.3%−52.0%, enhanced acid-hydrolyzable amino acid nitrogen in the topsoil, and promoted the accumulation of stable phosphorus content in the topsoil. Compared with CK, J1 and J2 treatment significantly increased β-acetyl-glucosaminidase and phosphatase activites in the topsoil, elevated total N across the 0–60 cm soil layer, and increased available N and P contents. J1 and J2 treatments also significantly increased acid-hydrolyzable amino acid nitrogen in the topsoil, and acid-hydrolyzable ammonium nitrogen in the middle soil layer, and raised labile phosphorus content by 31.1%−89.9% in the topsoil and by 29.6%−117.7% in the middle soil, as well as moderately labile phosphorus content by 51.0%−114.2% in the middle soil. Compared with T2, J1 or J2 treatment, TJ1 and TJ2 treatment further increased the activities of N and P transforming enzymes across all soil layers, particularly significantly increased the content of inorganic N and labile P in topsoil, increased the proportion of acid-hydrolyzed unknown nitrogen in the topsoil, and reduced the accumulation of inorganic nitrogen in the deep soil. TJ1 also significantly increased the content of labile and moderately labile phosphorus in the middle soil.

Conclusions The application of manure significantly increased the total N content in the 0–40 cm soil layer, raised the proportions of highly active acid-hydrolyzable ammonium N and acid-hydrolyzable amino acid N fractions, thereby enhancing the activities of extracellular enzymes involved in N and P transformation, and promoting the activation of phosphorus in the surface and middle layers of sandy fluvo-aquic soil. On this basis, the annual combined application of biochar at 12 t/hm² further facilitates the transformation of N to acid-hydrolyzable unknown form, while strengthening the retention of easily decomposable organic nitrogen and active phosphorus pools. This presents the effects of increasing the active phosphorus content in the surface layer and stabilizing the phosphorus pools in the middle and lower soil layers, effectively reducing the leaching loss of nitrogen and phosphorus nutrients. Therefore, in sandy fluvo-aquic soil regions, replacing 20%−40% of the conventional chemical fertilizer nitrogen application rate with manure, combined with the application of biochar, is a feasible measure to improve the capacity and supply intensity of nitrogen and phosphorus pools in sandy fluvo-aquic soil.