Changes in Denitrification Rate, Bacterial Denitrifier Community Structure and Abundance in Dairy-grazed Pasture Soils Treated with Cattle Urine and Dcd

Urine excreted by cattle can produce very high concentrations of available N in relatively small volumes of soil and lead to high nitrous oxide (N2O) emissions. Application of the nitrification inhibitor dicyandiamide (DCD) can inhibit nitrification. DCD application results in lower nitrate (NO3 ) concentrations and N2O emissions from denitrification in urine affected soils. However, the effect of urine and DCD on denitrification may vary depending on the soil’s inherent capacity to denitrify. We assessed the effect of DCD additions on N2O emissions, denitrifier community structure of nirS, nirK and nosZ genes and their abundance in urine affected soils in order to gain insight into how DCD affects the denitrification process within soils to which urine had been applied. We used surface samples from 3 New Zealand dairy pasture soils with contrasting denitrification enzyme activities (DEA) (Manawatu Fine Sandy Loam, Tokomaru Silt Loam and Otorohonga Silt Loam) with the highest DEA in Manawatu and the lowest in Otorohonga soil. The treatments applied were; cattle urine (700 mg N kg -1 soil), cattle urine + DCD (10 mg DCD kg -1 soil) and control (deionised water). Soils were saturated with water and incubated at 25 o C for four weeks. Gas samples and soil extracts collected during the incubation were analyzed to determine denitrification rate (DR), N2O and N2 emissions, pH, and mineral N contents in soils. We also determined the denitrifier community structure of nirS, nirK and nosZ genes and their abundance using molecular techniques. We observed increased DR and denitrifier genes abundances after 24 hours of incubation in soils that had urine applied. The DCD was ineffective in controlling denitrification after 24 hours of incubation. The results of the longer incubation time are under analysis.