Biochar acidification increased sorption and reduced leaching of nitrate

Area of Science:

• Soil Science
• Environmental Chemistry
• Agricultural Science

Background:

• Biochar application can mitigate nitrate (NO₃^-) leaching but often requires uneconomic rates.
• High nitrate sorption capacity in biochar is crucial for effectiveness.
• Nitrate sorption is reportedly higher under acidic conditions, but mechanisms and acidification potential are unexplored.

Purpose of the Study:

• Quantify NO₃^- sorption on woody biochars at soil-relevant pH (5 and 7).
• Investigate the influence of biochar production temperature on NO₃^- sorption.
• Explore the potential of biochar acidification to reduce NO₃^- leaching.

Main Methods:

• NO₃^- sorption was measured on biochars produced at highest treatment temperatures (HTT) from 400-900 °C at pH 5 and 7.
• Sorption data were correlated with surface functional groups and biochar aromaticity.
• Biochar acidification was tested in aqueous suspension and soil columns using combined HNO₃+KNO₃ enrichment.

Main Results:

• NO₃^- sorption capacities varied significantly with pH and HTT, ranging from 700-6200 mg NO₃^--N kg^-1 at pH 5 and 200-2500 mg NO₃^--N kg^-1 at pH 7.
• Maximized sorption occurred with biochars produced at HTT ≥750 °C.
• Acidified biochar reduced NO₃^- leaching by up to 23% in acidic soil (pH 5.7) and 12% in neutral soil (pH 7.4).

Conclusions:

• Nitrate sorption to biochar is pH-dependent and can be enhanced by producing biochars at higher temperatures (≥750 °C).
• Acidification of biochar, particularly under nitrate enrichment, effectively reduces nitrate leaching in both laboratory and soil column experiments.
• Biochar acidification presents a promising strategy for developing controlled-release fertilizers to minimize nitrate loss from agricultural soils.