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.
Published Version
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