A preliminary study on the synthesis and characterization of novel biochar-based fertilizers

Negative environmental impacts of intensive agriculture are often associated with low nitrogen (N) use efficiency. Different slow-release fertilizers have been proposed to improve N use efficiency and reduce associated losses, like the case of biochar-based fertilizers. The main objective of the present study was to produce and characterize novel biochar-based fertilizers that could reduce the total N-losses in agricultural soils. A common residue in Mediterranean cropping systems, olive tree pruning, was chosen as feedstock to produce slow pyrolysis biochars at 400 and 800°C. Three different activation treatments were applied to both biochars: 1) ultrasonic treatment at room temperature, 2) 5% H₂O₂ (1:25 ratio) during 1,5 h at 80°C and 3) 40% HNO₃ (1:25 ratio) during 1 h at 80°C. Activated biochars were supplemented with N either by 2 M urea or 2 M ammonium sulfate at 80°C. After each treatment N-enriched biochars were washed three times with deionized water and dried overnight. The results showed that biochar-based fertilizers from 400°C biochar presented more nitrogen retention capacity than 800°C biochar. Furthermore, HNO₃ activation was the most effective treatment to facilitate the incorporation of N from urea and ammonium sulfate. Moreover, urea was more effective than ammonium sulfate, reaching N values in the biochar-based fertilizers up to 7.0%. Further studies are being carried out to chemically and agronomically characterize the new biochar-based fertilizers and their suitability as slow-release fertilizers.