Nutrient‐Specific Effects of N, P, and K on Cassava Productivity Under 5‐Year Manure‐Based System in Loamy Sand Soil: Role of Soil N Availability

Long‐term integrated fertilization can improve crop productivity and soil fertility in tropical sandy soils; however, nutrient‐specific responses of cassava productivity and soil N dynamics remain poorly understood. This study evaluated the effects of nitrogen (N), phosphorus (P), and potassium (K) fertilization under a 5‐year chicken manure (CM)‐based system on cassava yield, nutrient uptake, nutrient use efficiency (NUE), soil properties, and N mineralization. Three independent split‐plot field trials (CM–N, CM–P, and CM–K) were established in 2018, with cassava yield and soil parameters assessed during the fifth cropping cycle (2022/2023 growing season). In each trial, the focal nutrient was excluded from the blanket fertilization and applied at three treatment rates, while the other two nutrients were uniformly supplied. Integrated application of CM and chemical fertilizers significantly increased cassava yield and plant nutrient uptake, while responses of NUE indices varied among the independently conducted N, P, and K trials. CM combined with the highest N and P rates increased fresh tuber yield by 40.4% and 40.8% in the CM–N and CM–P trials, respectively; in the CM–K trial, CM with or without K increased yield by 59.1%–81.0%. CM also increased soil organic carbon (SOC) and nutrient availability, although responses in water‐stable aggregates (WSAs) were inconsistent. Under laboratory incubation conditions, N mineralization showed limited responses to P fertilization in the CM–P trial. In the CM–N and CM–K trials, CM combined with the respective mineral fertilizers was associated with changes in the potentially mineralizable N pool and mineral N release kinetics. Importantly, increases in the potentially mineralizable N pool were not necessarily accompanied by increases in the mineralization rate constant, indicating treatment‐dependent mineralization kinetics. Overall, the findings demonstrate nutrient‐specific responses to integrated CM and mineral fertilization and indicate that mineral nutrient requirements should be considered individually when developing CM‐based fertilizer management for cassava on low‐fertility sandy soils.