Soil aeration affects the degradation rate of the nitrification inhibitor dicyandiamide

Dicyandiamide (DCD) is a nitrification inhibitor of variable efficacy. In soils, DCD biodegradation rate is known to be a function of temperature; however, microbial activity can also be affected by soil aeration and substrate availability. Studies determining the effects of soil aeration on DCD degradation are few. We tested the null hypothesis that the rate of degradation of DCD in soil would be the same under aerobic and anaerobic conditions. Soils from two sites with different organic matter concentrations but the same parent material were sampled to the same depth, sieved, and repacked into tubes (soil cores'). These were saturated with a DCD solution (30 mu gmL(-1)) and placed under controlled aeration conditions by imposing five levels of matric potential (0, -1, -3, -6, and -10kPa) at a constant temperature (22 degrees C). The relative O-2 diffusivity (O-2 diffusion coefficient in soil/O-2 diffusion coefficient in air, Dp/Do) was measured, along with periodic destructive sampling of soil cores over 40 days, to assess the DCD concentrations. Fitting first-order exponential functions to plots of soil DCD concentration v. time showed that the DCD degradation rate was greater (P<0.05) when the soil was aerobic (Dp/Do 0.01). Consequently, the null hypothesis was rejected. These results show that soil aeration determines the degradation rate of DCD.