Objectives Sandy fluvo-aquic soil is characterized by prominent constraints, including poor soil structure, intense mineralization, and low water- and nutrient-retention capacity. This study investigated the effects of biochar, manure, and their combined application on soil nitrogen and phosphorus fractions and their transformation at different soil depths, providing a theoretical and technical basis for alleviating soil constraints and improving soil fertility in sandy fluvo-aquic soil.

Methods A wheat–maize rotation experiment was established in October 2021 in Langgongmiao Town, Xinxiang County, Henan Province, China. Seven treatments were implemented during the winter wheat season: conventional chemical fertilizer (CK); one-time biochar application at 15 t/hm²(T1); annual biochar application at 12 t/hm²(T2); replacement of 20% (J1) or 40% (J2) of chemical fertilizer N with manure; and annual application of 12 t/hm² biochar combined with J1 or J2 (TJ1 and TJ2, respectively). Soil samples were collected during the winter wheat harvest in June 2024 from three depths: 0–20 cm (topsoil), 20–40 cm (middle soil), and 40–60 cm (deep soil). The contents of inorganic N and acid-hydrolyzable organic N fractions (acid-hydrolyzable ammonium N, amino acid N, amino sugar N, unknown N, and non-acid-hydrolyzable N), labile P (H₂O-P, NaHCO₃-Pi, NaHCO₃-Po), moderately labile P (NaOH-Pi, NaOH-Po), and stable P (HCl-P, Residual-P), as well as the activities of extracellular enzymes involved in N and P transformation, were determined.

Results The contents of acid-hydrolyzable organic N fractions under T1 did not differ significantly from those under CK in any soil layer. Compared with CK, T2 significantly increased β-acetylglucosaminidase activity in the topsoil, increased total N contents in the topsoil and middle soil by 33.3%–52.0%, increased acid-hydrolyzable amino acid N content in the topsoil, and promoted the accumulation of stable P in the topsoil. Compared with CK, J1 and J2 significantly increased β-acetylglucosaminidase and phosphatase activities in the topsoil, as well as total N and available N and P contents across the 0–60 cm soil profile. They also significantly increased acid-hydrolyzable amino acid N content in the topsoil and acid-hydrolyzable ammonium N content in the middle soil. Labile P content increased by 31.1%–89.9% in the topsoil and by 29.6%–117.7% in the middle soil, while moderately labile P content in the middle soil increased by 51.0%–114.2%. Compared with T2, J1, and J2, TJ1 and TJ2 further increased the activities of enzymes involved in N and P transformation across the soil profile, particularly increasing inorganic N and labile P contents in the topsoil. They also increased the proportion of acid-hydrolyzable unknown N in the topsoil and reduced the accumulation of inorganic N in the deep soil. In addition, TJ1 significantly increased labile and moderately labile P contents in the middle soil.

Conclusions Manure application significantly increased total N content in the 0–40 cm soil layer and increased the proportions of the more labile acid-hydrolyzable ammonium N and amino acid N fractions, thereby enhancing the activities of extracellular enzymes involved in N and P transformation and promoting P activation in the topsoil and middle soil of sandy fluvo-aquic soil. On this basis, annual application of biochar at 12 t/hm² further promoted the transformation of N into the acid-hydrolyzable unknown N fraction and enhanced the retention of readily decomposable organic N and labile P pools. This combination increased labile P content in the topsoil and stabilized P pools in the middle and deep soil layers, thereby reducing the potential leaching losses of N and P. Therefore, replacing 20% of conventional chemical fertilizer N with manure, combined with annual biochar application, is a feasible strategy for increasing the capacity and supply intensity of soil N and P pools in sandy fluvo-aquic soil.