Reduced-rate controlled-release fertilizer combined with humic acid: responses of paddy soil properties and rice nitrogen-use performance

Background To explore the regulatory mechanisms of humic acid (HA) combined with different fertilizer types and application rates on paddy soil structure, soil nitrogen supply, and rice yield formation, a two‑year field experiment was conducted with conventional bulk fertilizer (BBF) and controlled‑release fertilizer (CRF). Methods Treatments of single fertilizer application and HA co‑application were set up to systematically investigate soil aggregate composition, dynamic changes of soil NH 4 + -N and NO 3 – -N, rice tiller population, leaf area index, dry matter accumulation, yield components, and nitrogen use efficiency (NUE). Pearson correlation and principal component analysis (PCA) were employed to elucidate factor effects and variable associations. Results The results showed that HA significantly increased the proportion of > mm macro‑aggregates via organic cementation, optimized soil aggregate structure, and improved soil water and nutrient retention capacity. HA effectively retained soil NH 4 + -N, increased available nitrogen concentration during the critical tillering and panicle initiation stages, and avoided excessive NO 3 – -N accumulation, thereby reducing the risk of nitrate leaching in paddy fields. In addition, the combined application of HA and CRF achieved higher yield and NUE under reduced nitrogen application conditions. Conclusion In conclusion, the co‑application of controlled‑release fertilizer and HA can optimize paddy soil structure, regulate soil nitrogen transformation, improve rice productivity and nitrogen use efficiency, and balance grain yield improvement and soil quality amelioration. Further research is needed on the scientific dosage adjustment of these two materials.