Effect of treated farm dairy effluents, with or without animal urine, on nitrous oxide emissions, ammonia oxidisers and denitrifiers in the soil.

Purpose: In New Zealand, the application of farm dairy effluent (FDE) on pasture soils is the third largest source of nitrous oxide (N₂O) emissions from grazed grassland. Recently, new FDE treatment technologies have been developed to produce clarified water (CW) and treated effluent (TE) to recycle water and reduce the volume of fresh water used at the farm dairy. The aim of this study was to compare the effects of CW and TE with those of FDE on N₂O emissions and the growth of ammonia-oxidising bacteria (AOB), ammonia-oxidising archaea (AOA) and denitrifiers, when the effluents were applied to a grazed pasture soil. Materials and methods: A microcosm incubation study was carried out to determine the effects of applying CW, TE and untreated FDE, with or without animal urine, on N₂O emissions, and the abundance of AOB, AOA and the denitrifying functional genes, including nirS, nirK and nosZ. The soil used was a Templeton silt loam (Udic Haplustepsts). The effluents were applied at nitrogen (N) rates equivalent to 100 kg N ha⁻¹ and the animal urine at 700 kg N ha⁻¹. The soils were incubated at 12 °C to simulate autumn/winter soil temperatures in New Zealand, and the soil moisture was maintained at field capacity. Results and discussion: Results showed that the application of all the different effluents significantly increased the total N₂O emissions (0.21–0.28 kg N₂O-N ha⁻¹) compared with that in the control (0.18 kg N₂O-N ha⁻¹). However, there were no significant differences in total N₂O emissions between the different effluent treatments. Similarly, although the application of animal urine together with the different effluents further increased N₂O emissions (7.7–8.8 kg N₂O-N ha⁻¹) above that from the urine only treatment (5.8 kg N₂O-N ha⁻¹), there were no significant differences among the different effluent plus urine treatments. These N₂O results corresponded with the changing trends of the abundance of AOB, AOA, nirS, nirK and nosZ, that is the application of the CW, TE and FDE, with or without animal urine, had a similar impact on the growth dynamics of these microbial populations. Conclusions: These results indicate that the application of the CW and TE to dairy pasture soils would have a similar effect on N₂O emissions, ammonia oxidisers and denitrifiers as that of the untreated FDE, with or without animal urine. The treated effluent or clear water from the new effluent treatment technology would therefore not increase N₂O emissions nor adversely affect the key microbial populations involved in N cycling in soil. [ABSTRACT FROM AUTHOR]