Introduction
Iron deficiency is a major nutritional problem in calcareous soils because high pH and carbonate content reduce Fe solubility and limit its uptake by plants. Although synthetic chelates such as Fe-EDDHA are effective in correcting Fe deficiency, their high cost and limited sustainability have encouraged the search for alternative Fe sources. Iron-loaded biochar has attracted attention as a slow-release amendment due to its porous structure, high surface area, and reactive functional groups. In addition, converting agricultural wastes into biochar provides an environmentally sound strategy for recycling residues and improving soil fertility. Therefore, this study was conducted to identify a suitable biomass for preparing Fe-loaded biochar and to compare its effect with Fe-EDDHA on soil Fe availability and Fe concentration in sorghum grown in calcareous soil.
Materials and Methods
Tea waste was collected from Lahijan, Guilan Province, and citrus residues including lemon, orange, and bitter orange were collected from Babol, Mazandaran Province, Iran. The biomasses were washed, oven-dried, ground, and sieved to less than 2 mm. Biochar was produced by slow pyrolysis at 450°C for 2 h with a heating rate of 10°C min−1. The biochars were then crushed and sieved to obtain particles of 0.5-1 mm. Iron loading was performed by wetness impregnation method using FeSO4.7H2O solution containing 1000 mg L−1 Fe(II) at a solid-to-solution ratio of 1:50 and pH 6. The suspension was stirred for 8 h, filtered, dried, and calcined at 600°C for 1 h. To select the best biochar, total Fe and DTPA-extractable Fe were measured, and tea-waste biochar was selected for further study. An incubation experiment was carried out as a factorial in a completely randomized design with three replications. Treatments included control, Fe-loaded biochar at 10 mg Fe kg−1 soil, and Fe-EDDHA at 10 mg Fe kg−1 soil, sampled after 3, 360, and 720 h. DTPA-extractable Fe was measured at each time. A greenhouse experiment was also conducted in a completely randomized factorial design with three replications. The first factor was foliar application at two levels: distilled water and Fe-EDTA at 3%. The second factor was soil Fe application including 0, 5, and 10 mg Fe kg−1 from Fe-loaded biochar or Fe-EDDHA. Sorghum was grown in 5 kg soil pots under greenhouse conditions. After four months, shoot Fe concentration and post-harvest soil DTPA-extractable Fe were determined. Data were analyzed using SAS software, and means were compared by LSD at the 5% level.
Results and Discussion
Among the tested feedstocks, tea-waste biochar had the highest total Fe content, about 373 mg kg−1, and was therefore selected as the superior biochar. The results also showed that pyrolysis reduced DTPA-extractable Fe compared with the original biomasses, indicating transformation of Fe into more stable and less soluble forms during biochar production. In the incubation experiment, soil Fe treatment, incubation time, and their interaction had significant effects on DTPA-extractable Fe at the 1% probability level. Both Fe-loaded biochar and Fe-EDDHA increased soil Fe availability compared with the control. In the Fe-loaded biochar treatment, DTPA-extractable Fe increased by about 3.1%, 13%, and 30% over the control after 3, 360, and 720 h, respectively. In the Fe-EDDHA treatment, the corresponding increases were about 12.5%, 58%, and 87%. These results indicate that Fe-EDDHA was more effective in maintaining Fe in a readily available form in the short term, whereas Fe-loaded biochar showed a slower but progressive release pattern. In the greenhouse experiment, foliar application, soil Fe source, and their interaction significantly affected shoot Fe concentration and soil DTPA-extractable Fe. The lowest shoot Fe concentration was observed in the untreated control 126 mg kg−1. Soil application of Fe-loaded biochar increased shoot Fe concentration to 169.39 and 280 mg kg−1 at 5 and 10 mg Fe kg−1, respectively. The corresponding values for Fe-EDDHA were 205 and 270 mg kg−1. Foliar Fe-EDTA further improved plant Fe status in all treatments. The highest shoot Fe concentration, 321 mg kg−1, was recorded in the treatment receiving 10 mg Fe kg−1 as Fe-loaded biochar plus foliar Fe-EDTA. Soil DTPA-extractable Fe after harvest ranged from 3.18 to 5.49 mg kg−1, with the highest value also observed in the same treatment. These findings suggest that Fe-loaded biochar can act as a slow-release Fe source and that its combined use with foliar Fe application enhances both soil Fe availability and plant Fe nutrition
Conclusion
The results showed that tea waste is a suitable feedstock for producing Fe-loaded biochar. Although Fe-EDDHA was more effective in rapidly increasing DTPA-extractable Fe in calcareous soil, Fe-loaded tea-waste biochar significantly improved both soil Fe availability and Fe concentration in sorghum, especially at the higher application rate. The combined use of Fe-loaded biochar and foliar Fe-EDTA produced the best overall response. Therefore, Fe-loaded tea-waste biochar can be considered a promising and sustainable amendment for improving Fe nutrition in calcareous soils and reducing dependence on synthetic Fe chelates.