Limited role of biochars in nitrogen fixation through nitrate adsorption

Nitrate cycling is essential in sustaining soil systems. Excessive application of N-fertilizers and the associated underground water contamination have attracted a great deal of research attention. Sorption is efficient and environmentally friendly in nitrate fixation. A debate was noted in literature regarding whether biochars have potential to fix nitrate through sorption. In this study, biochars produced from different biomasses as well as biomass compositions were chosen as the absorbents to evaluate their potential efficiencies in nitrate fixation. Increased sorption to nitrate was observed for biochars with increased pyrolysis temperature, but the increasing extent varied with biomass. The surface base functional groups and surface charges of biochars could not well explain nitrate sorption. The significant positive correlation between nitrate sorption and biochar surface areas suggested that surface area was the controlling parameter for nitrate sorption. The pre-coating of tannic acid (TA) on biochars decreased but did not completely inhibit nitrate sorption. This observation suggested that nitrate sorption on biochars may be further decreased after their interactions with natural organic matter. Nitrate sorption was compared among various adsorbents, including biochars, soil particles, clay minerals, engineered particles, as well as humic substances. Soil particles generally showed high sorption to nitrate over biochars. This result suggested that biochars investigated in this work may play a limited role in nitrate fixation through sorption after their massive application. Nitrogen fixation through nitrate adsorption on biochars should be carefully evaluated taking into consideration of biochar feedstocks and properties.