Introduction Phosphorus is an essential nutrient for plant growth, but its bioavailability is low in calcareous soils due to precipitation with calcium. Cow bone char (containing hydroxyapatite) can act as a slow-release phosphorus source. The particle size of bone char significantly influences phosphorus release kinetics; reducing the particle size to the nano-scale increases the specific surface area and accelerates phosphorus release. Despite studies on macro-sized bone char, limited information is available regarding the effect of nano-sized bone char on the chemical forms of phosphorus and its release kinetics in calcareous soil. Therefore, this study was conducted to compare the effects of macro- and nano-sized cow bone char on phosphorus availability in a calcareous soil.

Materials and Methods This experiment was conducted as a completely randomized design with three replicates to evaluate the effects of cow bone char and nano cow bone char at rates of 200 and 400 mg P kg⁻¹ soil compared to a control treatment. To prepare bone char, raw cow bones were cleaned several times and then dried at 110°C for 24 hours. The obtained bone char was ground to nano-scale using a mechanical ball mill. The treatments were incubated for two months at 25 ± 2°C, and soil moisture was maintained at 70% of field capacity (FC); then the samples were air-dried. Mineral phosphorus fractions were measured using the Jiang and Guo method. Phosphorus release kinetics were determined by adding 20 mL of 0.5 M NaHCO₃ to 1 g of each treatment, shaking for 1 to 72 hours (seven time points), and then centrifuging. Phosphorus concentration in the extracts was determined using a colorimetric method. Subsequently, the data obtained from the phosphorus release kinetics experiment were fitted to pseudo-first-order, pseudo-second-order, and Elovich equations. Analysis of variance (ANOVA) was performed using SPSS 26 software to evaluate the effects of different phosphorus fertilizer levels on various P forms and release kinetics. Mean comparisons were conducted using Duncan's test at a significance level of p < 0.05.

Results and Discussion Based on the results, reducing the size of bone char to the nano-scale caused a significant effect on the amount of available-P and inorganic forms of phosphorus (Ca₂-P, Ca₈-P, Ca₁₀-P, Fe-P, and Al-P) compared to the control. The application of nano bone char, especially at the rate of 400 mg kg-1, increased the amount of available phosphorus, dicalcium phosphate, and octacalcium phosphate compared to other treatments. The treatment of nano cow bone char at 400 mg kg⁻¹ increased the amounts of available phosphorus (Available-P), dicalcium phosphate (Ca₂-P), and octacalcium phosphate (Ca₈-P) by 8.4, 3.3, and 3.8 times, respectively, compared to the control treatment. The pseudo-second-order kinetic model provided the best fit to the kinetic release data due to its high coefficient of determination (R²) and low standard error (SE). The maximum release capacity (qₑ) of phosphorus increased with the use of nano bone char at 400 mg kg-1. Therefore, it can be concluded that nano bone char at 400 mg kg⁻¹ resulted in the highest phosphorus release in the soil.

Conclusion The results of this research showed the effect of bone char particle size on the release and distribution of phosphorus in different forms. In general, the study findings indicated that nano bone char demonstrated improvement compared to macro-sized bone char treatments. Nano bone char applied at a rate of 400 mg kg-1 was the best treatment for reducing insoluble forms of phosphorus to soluble forms, due to its higher specific surface area and reactivity. Thus, nano bone char can be considered a promising and environmentally friendly strategy for sustainable agriculture.