Influence of organic fertilizer rates on nutrients and heavy metals dynamics in degraded soils

Limited empirical understanding of how commercialized organic fertilizers interact with degraded soils on nutrients and heavy metals dynamics constrain sustainable soil fertility management. This study investigates how different application rates of Guanoboost™ organic fertilizer influence nutrients and heavy metals dynamics in degraded soils. The study employed a controlled laboratory incubation experiment arranged in a 5 × 3 × 4 factorial design with three replications. The first factor comprised five organic fertilizer application rates (R): R0 (control), R1 (200 kg ha− 1), R2 (400 kg ha− 1), R3 (600 kg ha− 1), and R4 (800 kg ha− 1). The second factor represented three soil degradation levels (S): slightly degraded (S1), moderately degraded (S2), and severely degraded (S3) [31]. The third factor included four incubation sampling periods (T), where soils were analyzed at 21, 42, 63, and 84 days after incubation. Macronutrient composition varied significantly among soils, with S2 showing the highest levels of N (7.52 mg kg− 1), P (1.54 mg kg− 1), K (8.62 mg kg− 1), Ca (28.37 mg kg− 1), and Na (2.81 mg kg− 1), while S1 exhibited the lowest, reflecting the nutrient-leaching effect of acidity. Nutrients followed a temporal pattern, peaking around day 63, likely due to enhanced microbial mineralization. Among micronutrients, Fe and Zn were highest in S1, Mn in S3, and Cu in S2, confirming the strong influence of soil type on micronutrient solubility. Moderate fertilizer rates (600 kg ha− 1) optimized N and Fe accumulation. Heavy metal analysis showed that Al and Cr concentrations were highest in S1 (460.87 mg kg− 1 and 700.33 mg kg− 1, respectively), decreasing with higher soil pH. Overall, soil degradation level and incubation time were the dominant factors governing nutrient availability and metal behavior, emphasizing the importance of soil-specific management strategies for effective organic amendment utilization. Moderate Guanoboost™ rate (600 kg ha-1) maximized N and Fe accumulation. Mid-incubation (day 63) marked peak nutrient mineralization activity. Soil degradation and incubation time governed nutrient and metal dynamics. Higher soil pH reduced Al and Cr toxicity, enhancing soil rehabilitation.