Temperature alters dicyandiamide (DCD) efficacy for multiple reactive nitrogen species in urea-amended soils: Experiments and modeling

Dicyandiamide (DCD) is a nitrification inhibitor (NI) used to reduce reactive nitrogen (N) losses from soils. While commonly used, its effectiveness varies widely. Few studies have measured DCD and temperature effects on a complete set of soil N variables, including nitrite (NO₂⁻) measured separately from nitrate (NO₃⁻). Here the DCD reduction efficiencies (RE) for nine N availability metrics were quantified in two soils (a loam and silt loam) using aerobic laboratory microcosms at 5–30 °C. Both regression analysis and process modeling were used to characterize the responses. Four metrics accounted for NO₃⁻ production and included total mobilized N, net nitrification, maximum nitrification rate, and cumulative NO₃⁻ (cNO₃^-). The REs for these NO₃⁻-associated production variables decreased linearly with temperature, and in all cases were below 60% at temperatures ≥22 °C, except for cNO3^- in one soil. In contrast, REs for NO₂⁻ and nitric oxide (NO) gas production were less sensitive to temperature, ranging from 80 to 99% at 22 °C and 50–95% at 30 °C. Addition of DCD suppressed nitrous oxide (N₂O) production in both soils by 20–80%, but increased ammonia volatilization by 36–210%. The time at which the maximum reduction efficiency occurred decreased exponentially with increasing temperature for most variables. The two-step nitrification process model (2SN) was modified to include competitive inhibition coupled to first-order DCD decomposition. Model versus data comparisons suggested that DCD had indirect effects on NO₂⁻ kinetics that contributed to the greater suppression of NO₂⁻ and NO relative to NO₃⁻. This study also points to the need for NIs that are more stable under increased temperature. The methods used here could help to assess the efficacy and temperature sensitivity of other NIs as well as new microbial inhibitors that may be developed.