How does copper influence the nitrification inhibition efficiency of dicyandiamide under varying soil moisture levels?

The nitrification process in agricultural systems leads to nitrogen (N) loss and related pollution. The efficiency of dicyandiamide (DCD) towards nitrification inhibition increases when combined with copper (Cu). Yet, the mechanisms by which Cu influences the nitrification inhibition efficiency of DCD under varying soil moisture levels remain unclear. Therefore, our study evaluated the effect of DCD and Cu, solely and in combination, in comparison to urea-only or the CK (soil only), on nitrification inhibition and nitrous oxide (N₂O) emission under different moisture levels (saturated and unsaturated conditions). The results showed that urea-only treatment had significantly lower ammonium retention efficiency compared with the CK, DCD, Cu, and DCD with Cu by 9, 13, 28 and 46 %. Nitrification inhibition rate increased by 122, 71 and 52 % under DCD with Cu, Cu and DCD compared to urea-only treatment. The N₂O emissions were significantly reduced underDCD, Cu, DCD with Cu, and CK treatments by 21, 24, 41 and 62 %, respectively, compared with the urea-only treatment. The increased nitrification inhibition by DCD and Cu is due to suppressed activities and reduced abundance of nitrifier enzymes and genes. Nitrification inhibition was significantly higher in saturated soil than in unsaturated soil under Cu, DCD, DCD with Cu, and CK treatments by 203, 91, 21 and 33 %, respectively. However, N₂O emissions reduction was significantly lower by 133, 99 and 62 % under Cu, CK, and DCD with Cu, respectively, in saturated than in unsaturated soils. Soil moisture at saturation limits oxygen availability, which affects soil microbial activities, hence reducing nitrification inhibition. The understanding of these mechanisms will allow for improvement in nitrification inhibition approaches towards reducing N-related losses and pollution.