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 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.
