Interactive Effects of Copper and Organic Matter on Dicyandiamide Efficacy in Suppressing Soil Nitrification and N 2 O and CO 2 Emissions

Nitrification is a key process governing nitrogen (N) loss and greenhouse gas emissions in agricultural soils, and its regulation is strongly influenced by both chemical inhibitors and soil properties. Copper (Cu), a metal cofactor that is crucial for the function of ammonia monooxygenase (AMO), plays an important role in ammonia oxidation, whereas dicyandiamide (DCD) suppresses nitrification and may interact with Cu to inhibit AMO activity. However, the extent to which Cu availability and soil organic matter (SOM) jointly regulate DCD efficiency remains poorly understood. In this study, an incubation experiment was conducted using tropical paddy soils with contrasting SOM contents to explore how varying Cu levels (10 and 200 mg Cu kg −1 soil) impact DCD efficiency in regulating the nitrification process and controlling nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) emissions. Our results showed that DCD generally suppressed nitrification, as indicated by reduced NO 3 − accumulation and lower NO 3 − /NH 4 + ratios. However, the response to Cu was strongly SOM-dependent. Under low SOM, Cu addition was associated with a partial restoration of nitrification activity, suggesting a potential reduction in DCD efficiency, whereas under high SOM, this effect appeared to be attenuated, likely due to Cu complexation and reduced bioavailability. Increasing Cu levels further weakened DCD inhibition, particularly in low SOM soils. DCD significantly reduced N 2 O emissions, but this mitigation effect declined with Cu addition, suggesting a Cu-mediated influence on nitrification–denitrification pathways. On the other hand, CO 2 emissions were reduced under DCD application and appeared to be further reduced under Cu treatments. Changes in enzyme activities and nitrifier gene abundances supported these patterns, suggesting distinct responses of AOA and AOB communities under varying SOM and Cu conditions. This study provided evidence that the interaction of Cu availability and SOM may play an important role in governing the efficacy of nitrification inhibitors. This highlights the importance of considering soil-specific chemical environments when optimizing N management strategies to reduce environmental N losses.