Nitrogen availability in biochar-based fertilizers depending on activation treatment and nitrogen source.

Different activation and N-doping treatments were used to produce biochar-based fertilizers (BBFs) with increased N concentration and slow N release. Pristine biochars were produced by pyrolysis of olive tree pruning feedstock at low and high temperatures (400 and 800 °C). These biochars were activated either by ultrasonication, or oxidation with hydrogen peroxide (H ₂ O ₂ ) or nitric acid (HNO ₃ ) to increase their N retention potential. Subsequently biochars were enriched with N with either urea or ammonium sulfate. The activation of low-temperature biochars with HNO ₃ was the most effective treatment leading to new surface carboxylic groups that facilitated the later enrichment with N. When treated with urea, BBFs reached 7.0 N%, whereas the H ₂ O ₂ activation only allowed an increase up to 2.0 N%. The use of urea as the external N source was the most efficient for incorporating N. Urea treated biochars had a water-soluble fraction that represented up to 14.5 % of the total N. The hydrolyzable N fraction, composed by amides and simple N heterocycles originated by the N-doping treatments, and nitro groups generated from HNO ₃ activation, represented up to 60 % of the total N. This study relates the N chemical forms in the new BBFs to potential N availability in soil. The presence of water-soluble, hydrolyzable and non-hydrolyzable N implied that these BBFs may supply N that would be progressively available for plants, acting as slow-release fertilizers.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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