Different effects of the application of urea combined with nitrification inhibitor on cadmium activity in the rice-rape rotation system

Cadmium (Cd) is one of the most harmful and widespread pollutants in agricultural soil, where it is readily taken up by plants and threatens human health through the food chain. Nitrification inhibitors (NIs) are usually used to reduce nitrogen (N) loss in soil and increase the nitrogen use efficiency of crops. However, information regarding the Cd transfer in soil and crops system with the application of urea combined with NIs is limited. Especially, the influences of NIs on Cd availability in the rice-rape rotation are unclear. Here, we studied the pH, N speciation, and Cd activity in soils, as well as Cd accumulation in rice and rapeseed tissues that resulted after the application of dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) under alternating redox conditions. Rice and rape experiments appeared to represent an opposite phenomenon in the treatments of urea + NIs. Addition of NIs increased the pH of paddy soil, but decreased the pH of rapeseed soil. The treatments of urea + DCD and urea + DMPP0.5% produced a significantly lower concentration of extractable Cd in the paddy soil, and reduced the accumulation of Cd in brown rice. For rapeseed, the urea + NI treatments enhanced the Cd activity and increased the accumulation of Cd in rapeseed. It is proposed that NIs could be used to regulate N transformation in agricultural soils and inhibited Cd uptake by rice in urea fertilization. Moreover, the application of NIs combined with urea would potentially favor phytoextraction of Cd by rape, which is a ideal candidate for phytoremediation in Cd-contaminated soil.