Inhibition of ammonia oxidisers to control nitrification rate under simulated winter dairy forage grazing conditions: An incubation study

Abstract

The microbial process of nitrification plays a key role within the soil nitrogen cycle. Nitrification is the process where ammonia is oxidised to nitrite and then to nitrate and this process can have major negative environmental effects. It has previously been determined that both ammonia oxidising bacteria (AOB) and ammonia oxidising archaea (AOA) mediate the first step of the nitrification process, i.e. the ammonia oxidation process in soil, but it is unclear which group is important under wet winter forage grazing conditions. The reduction of nitrification rates is an important factor in reducing NO₃⁻ leaching from winter forage systems and a key mitigation tool is the use of the nitrification inhibitor dicyandiamide (DCD). The aim of this study was to investigate the effect of cow urine and dicyandiamide (DCD), on AOA and AOB population abundance in dairy winter forage grazed soils. While this study indicated that AOA were present within the soil, it was AOB that played the dominant role in ammonia oxidation in the urine treated soil. In the urine only treatment (applied at 500 kg N ha⁻¹), the AOB amoA gene copy numbers were 11.7 times that of the control on day 21. The urine plus DCD treatment applied at 10 kg DCD ha-1 and urine plus DCD treatment applied at 20 kg DCD ha⁻¹, respectively, showed a 91.3 % and 96.6 % reduction in AOB amoA gene copy numbers at the same point in time. By day 112, the nitrate concentration for the urine only treatment was 8.4 times the control. Whereas, the urine plus DCD (10 kg ha⁻¹) and urine plus DCD (20 kg ha⁻¹) treatments had an 84.4 % and an 88.5 % reduction in nitrate concentration, respectively, compared to the urine only treatment. These results illustrate that while both AOA and AOB were present within the soil only one group of microbes was actively involved in the ammonia oxidation process. The results also show that using the nitrification inhibitor DCD is a highly effective way to inhibit the growth of AOB, leading to a reduction in NO₃⁻ concentration in the soil under a winter forage grazing system.