Effects of nitrogen application rate and a nitrification inhibitor dicyandiamide on ammonia oxidizers and N₂O emissions in a grazed pasture soil

Abstract

Ammonia oxidizers, including ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) are important drivers of a key step of the nitrogen cycle — nitrification, which affects the production of the potent greenhouse gas, nitrous oxide (N₂O). A field experiment was conducted to determine the effect of nitrogen application rates and the nitrification inhibitor dicyandiamide (DCD) on the abundance of AOB and AOA and on N₂O emissions in a grazed pasture soil. Nitrogen (N) was applied at four different rates, with urea applied at 50 and 100 kg N ha⁻¹ and animal urine at 300 and 600 kg N ha⁻¹. DCD was applied to some of the N treatments at 10 kg ha⁻¹. The results showed that the AOB amoA gene copy numbers were greater than those of AOA. The highest ratio of the AOB to AOA amoA gene copy numbers was 106.6 which occurred in the urine-N 600 treatment. The AOB amoA gene copy numbers increased with increasing nitrogen application rates. DCD had a significant impact in reducing the AOB amoA gene copy numbers especially in the high nitrogen application rates. N₂O emissions increased with the N application rates. DCD had the most significant effect in reducing the daily and total N₂O emissions in the highest nitrogen application rate. The greatest reduction of total N₂O emissions by DCD was 69% in the urine-N 600 treatment. The reduction in the N₂O emission factor by DCD ranged from 58% to 83%. The N₂O flux and NO₃⁻-N concentrations were significantly correlated to the growth of AOB, rather than AOA. This study confirms the importance of AOB in nitrification and the effect of DCD in inhibiting AOB growth and in decreasing N₂O emissions in grazed pasture soils under field conditions.