Effect of 7-year application of a nitrification inhibitor, dicyandiamide (DCD), on soil microbial biomass, protease and deaminase activities, and the abundance of bacteria and archaea in pasture soils

Purpose: The nitrification inhibitor dicyandiamide (DCD) has been shown to be highly effective in reducing nitrate (NO3−) leaching and nitrous oxide (N2O) emissions when used to treat grazed pasture soils. However, there have been few studies on the possible effects of long-term DCD use on other soil enzyme activities or the abundance of the general soil microbial communities. The objective of this study was to determine possible effects of long-term DCD use on key soil enzyme activities involved in the nitrogen (N) cycle and the abundance of bacteria and archaea in grazed pasture soils. Materials and methods: Three field sites used for this study had been treated with DCD for 7 years in field plot experiments. The three pasture soils from three different regions across New Zealand were Pukemutu silt loam in Southland in the southern South Island, Horotiu silt loam in the Waikato in the central North Island and Templeton silt loam in Canterbury in the central South Island. Control and DCD-treated plots were sampled to analyse soil pH, microbial biomass C and N, protease and deaminase activity, and the abundance of bacteria and archaea. Results and discussion: The three soils varied significantly in the microbial biomass C (858 to 542 μg C g−1 soil) and biomass N (63 to 28 μg N g−1), protease (361 to 694 μg tyrosine g−1 soil h−1) and deaminase (4.3 to 5.6 μg NH4 + g−1 soil h−1) activity, and bacteria (bacterial 16S rRNA gene copy number: 1.64 × 109 to 2.77 × 109 g−1 soil) and archaea (archaeal 16S rRNA gene copy number: 2.67 × 107 to 3.01 × 108 g−1 soil) abundance. However, 7 years of DCD use did not significantly affect these microbial population abundance and enzymatic activities. Soil pH values were also not significantly affected by the long-term DCD use. Conclusions: These results support the hypothesis that DCD is a specific enzyme inhibitor for ammonia oxidation and does not affect other non-target microbial and enzyme activities. The DCD nitrification inhibitor technology, therefore, appears to be an effective mitigation technology for nitrate leaching and nitrous oxide emissions in grazed pasture soils with no adverse impacts on the abundance of bacteria and archaea and key enzyme activities