Effectiveness of dicyandiamide as a nitrification inhibitor in biochar-amended soil

Overuse of nitrogen (N) fertilizers may lead to many environmental issues via N leaching into groundwater and agricultural runoff into surface water. Biochar, a sustainable soil amendment agent, has been widely studied because of its potential to retain moisture and nutrients. However, recent studies have shown that biochar has a very limited ability to improve the retention of negatively charged nitrite (NO−2) or nitrate (NO−3). Although positively charged ammonium (NH+4) can be better held by biochar, it is usually susceptible to nitrification and can be easily transformed into highly mobile NO−2 and/or NO−3. In practice, dicyandiamide (DCD) has been used to inhibit nitrification, preserving N in its relatively immobile form as NH+4. Therefore, it is likely that the effects of DCD and biochar in soils would be synergistic. In this study, the influences of biochar on the effectiveness of DCD as a nitrification inhibitor in a biochar-amended soil were investigated by combining the experimental results of incubation, adsorption isotherm, and column transport with the simulated results of different mathematical models. Biochar was found to stimulate the degradation of DCD, as the maximum degradation rate slightly increased from 1.237 to 1.276 mg kg−1 d−1 but the half-saturation coefficient significantly increased from 5.766 to 9.834 mg kg−1. Considering the fact that the availability of DCD for nitrification inhibition was continuously decreasing because of its degradation, a novel model assuming non-competitive inhibition was developed to simulate nitrification in the presence of a decreasing amount of DCD. Depending on the environmental conditions, if the degradation of DCD and NH+4 in biochar-amended soil is not significant, improved contact due to the mitigated spatial separation between NH+4 and DCD could possibly enhance the effectiveness of DCD.