Biochar acidification increased sorption and reduced leaching of nitrate

Biochar can reduce nitrate (NO₃^-) leaching from soil, yet typically only at high, uneconomic application rates. Therefore, biochar should be produced and modified to achieve high sorption capacity. Increased NO₃^- sorption at acidic pH has been reported. However, the sorption mechanisms and potential of biochar acidification to reduce nitrate leaching remain unexplored. Here, NO₃^- sorption was quantified at soil-relevant pH of 5 and 7 on woody biochars produced at highest treatment temperatures (HTT) of 400-900 °C. Sorption capacities ranged between 700 and 6200 mg NO₃^--N kg^-1 at pH 5 and between 200 and 2500 mg NO₃^--N kg^-1 at pH 7. Thus, NO₃^- sorption to biochar might be higher in acidic compared to neutral soils and can be maximized by producing biochars at HTT ≥750 °C. Correlation of sorption data with content of surface functional groups and proxies for biochar aromaticity suggest that NO₃^- is sorbed to protonated aromatic structures in biochars produced at >500 °C, while NO₃^- might sorb to protonated functional groups in biochar produced at 400 °C. Further, biochar acidification under NO₃^- enrichment was tested to reduce NO₃^- leaching. Both in aqueous suspension and in soil columns, full NO₃^- release from KNO₃-enriched industrial biochar was prevented by biochar acidification, i.e., combined HNO₃+KNO₃ enrichment. In soil with pH 5.7 or pH 7.4, only acidified biochar reduced NO₃^- leaching by up to 23 % and 12 %, respectively, compared to the soil-only control. Thus, acidification could be an interesting approach to produce biochar-based fertilizers with controlled NO₃^- release.