Abstract—Biochar has the potential to retain soil nitrogen (N), but inconclusive results are reported. This study aimed to examine the adsorption behavior of ammonium (NH4⁺) and nitrate (NO3⁻) onto biochar and the pH effects to assess retention capability and elucidate the impact on N management. Laboratory experiments were conducted to determine adsorption capacity of NH4⁺ and NO3⁻ onto seven biochar products from different feedstocks and Pyrolysis Temperature (PT). All products exhibited adsorption capacity for NH4⁺ with most isotherms fit well by Langmuir and/or Freundlich equations except one by linear equation. Almond shell biochar from 550 °C PT showed the highest adsorption for NH4⁺ in the solution concentration range of 1–200 N mg L⁻¹. Almond shell biochar from 900 °C PT had the lowest adsorption capacity at low solution concentration, but adsorption increased linearly and surpassed all other products as concentration increased. Two softwood biochar products (500 and 540 °C PT) showed a similar trend. Further, NH4⁺ adsorption was highly pH dependent (peaked around pH 9 for all products but reduced by more than half at neutral pH). Except for the almond char at 900 °C PT, no adsorption of NO3⁻ was observed for other products at 5 or 50 mg N L⁻¹ initial solution concentration. The results concluded that the ability to retain N retention by biochar without modification is limited and varied among feedstocks and pH. The role of biochar to improve N availability or use efficiency should not be overstated to avoid mismanagement decision making.

Keywords—ammonium, nitrate, adsorption isotherm, adsorption envelope

Cite: Suduan Gao, Aileen Hendratna, and Tom Pflaum, "Assessment of Biochar Adsorption Capacity for Ammonium and Nitrate and Implications on Soil Nitrogen Management," Journal of Advanced Agricultural Technologies, Vol. 11, No. 2, pp. 17-27, 2024.

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