Integrated Application of Biochar and Polyacrylamide with Conservation Tillage Promotes Fertilizer-N Recovery in the Short Term

Although conservation tillage (CT), polyacrylamide (PAM), and biochar (BC) are each known to enhance soil organic carbon (SOC), their combined short-term effects on SOC sequestration and fertilizer-nitrogen (N) recovery remain unclear. A 15N-tracing study involving two tillage regimes (rotary tillage (RT) and CT) and four amendments (no amendment (CK), PAM, BC, and PAM + BC (PMC)) was conducted across two seasons from a wheat–maize rotation in an infertile soil. Compared with RT, the crop-recovered fertilizer-N under CT was significantly increased by 15.2%. Relative to CK, the SOC stock, non-labile SOC (NLOC) stock, and total fertilizer-N recovery were increased by 15.9–28.7%, 19.5–48.5%, and 10.10–57.37 kg N ha−1, respectively, in BC amendment under both RT and CT. Soil aggregate stability was significantly improved in PAM amendment, particularly under CT and in the maize season, thereby reducing SOC lability. The structural equation model indicated that the short-term promotion of fertilizer-N recovery was driven by the increase in total SOC and NLOC. The adoption of PMC under CT exhibited the lowest SOC lability and the highest total fertilizer-N recovery at the end of the trial. These findings demonstrated the short-term effectiveness of integrating CT, PAM, and BC to enhance fertilizer-N recovery by promoting crop N utilization and increasing the quantity and quality of SOC.