The nitrification inhibitor dicyandiamide increases mineralization-immobilization turnover in slurry-amended grassland soil

Nitrification inhibitors are used in agriculture for the purpose of decreasing nitrogen (N) losses, by limiting the microbially mediated oxidation of ammonium (NH4+) to nitrate (NO3-). Successful inhibition of nitrification has been shown in numerous studies, but the extent to which inhibitors affect other N transformations in soil is largely unknown. In the present study, cattle slurry was applied to microcosms of three different grassland soils, with or without the nitrification inhibitor dicyandiamide (DCD). A solution containing NH4+ and NO3-, labelled with N-15 either on the NH4+ or the NO3- part, was mixed with the slurry before application. Gross N transformation rates were estimated using a N-15 tracing model. In all three soils, DCD significantly inhibited gross autotrophic nitrification, by 79-90%. Gross mineralization of recalcitrant organic Nincreased significantly with DCD addition in two soils, whereas gross heterotrophic nitrification from the same pool decreased with DCD addition in two soils. Fungal to bacterial ratios were not significantly affected by DCD addition. Total gross mineralization and immobilization increased significantly across the three soils when DCD was used, which suggests that DCD can cause non-target effects on soil N mineralization- immobilization turnover.