Methanotroph abundance not affected by applications of animal urine and a nitrification inhibitor, dicyandiamide, in six grazed grassland soils

Abstract

Methanotrophs are an important group of methane (CH4)-oxidizing bacteria in the soil, which act as a major sink for the greenhouse gas, CH4. In grazed grassland, one of the ecologically most sensitive areas is the animal urine patch soil, which is a major source of both nitrate (NO3 −) leaching and nitrous oxide (N2O) emissions. Nitrification inhibitors, such as dicyandiamide (DCD), have been used to mitigate NO3 − leaching and N2O emissions in grazed pastures. However, it is not clear if the high nitrogen loading rate in the animal urine patch soil and the use of nitrification inhibitors would have an impact on the abundance of methanotrophs in grazed grassland soils. The purpose of this study was to determine the effect of animal urine and DCD on methanotroph abundance in grazed grassland soils. A laboratory incubation study was conducted to determine the effect of urine and DCD applications on the abundance of methanotrophs in six grazed grassland soils sampled from across New Zealand, using real-time PCR targeting the functional pmoA gene. Results showed that the pmoA gene copy numbers were low in these soils, mostly below 2.36 × 104 g−1 soil except in the West Coast soil where pmoA gene copy number reached 8.95 × 105 g−1 soil. Most of the clones identified were aligned to the type II methanotrophs. There was no significant effect (P < 0.05) on the abundance of methanotrophs by the applications of urine at 1,000 kg N ha−1 or DCD at 10 kg ha−1. These results suggest that the abundance of methanotrophs is not affected by urine deposition or the application of DCD to mitigate NO3 − leaching and N2O emissions in grazed grassland soils.