Interaction of Nitrapyrin and Dicyandiamide with Soil Humic Compounds

Abstract

Abstract Sorption to organic matter is a primary factor influencing the effectiveness of nitrification inhibitors applied to soils. Interaction of the nitrification inhibitors nitrapyrin [2‐chloro‐6‐(trichloromethyl)‐pyridine] and dicyandiamide (DCD) with humic acid (HA) and fulvic acid (FA) was studied to compare the adsorptivity of the humic materials and elucidate adsorption mechanisms. Ten milligrams of HA or FA were suspended in aqueous solutions (10 mL) of either nitrapyrin (2, 4, 6, 8, and 12 mg L −1 ) or DCD (5, 10, 20, 40, and 80 mg L −1 ). The quantity of nitrification inhibitor adsorbed was measured after shaking the suspension of nitrapyrin for 24 h or DCD for 48 h. Results of adsorption experiments along with Freundlich K values indicate that FA adsorbed more DCD than HA, and HA was a better adsorbent for nitrapyrin. Infrared spectra provided evidence that adsorption occurred predominantly through an ionic‐bonding mechanism involving the protonated amino group of DCD or the N of the pyridine ring of nitrapyrin and the negatively charged functional groups of the humic materials. The contribution of hydrophobic forces in the adsorption of the nitrification inhibitors was minimal.