Understanding the mechanisms for the lower nitrous oxide emissions from fodder beet urine compared with kale urine from dairy cows

Nitrous oxide (N₂O) is a potent greenhouse gas and is produced in the soil by nitrification and denitrification processes. Previous studies have reported apparent lower N₂O emissions from dairy cow urine from fodder beet plants than from kale plants, although the underpinning mechanisms were poorly understood. Here, we report a laboratory incubation study to improve understanding of possible mechanisms underpinning different N₂O emissions from two different forage plants, fodder beet (FB) and kale. Materials and methods: The treatments included two urine sources from cows grazing FB or kale and two soils (FB soil and kale soil): FB soil control, FB soil + FB urine, FB soil + kale urine, kale soil control, kale soil + FB urine and kale soil + kale urine. The incubation temperature was 10 °C to simulate New Zealand autumn/winter soil temperatures. Results and discussion: Results showed that the total N₂O emissions from FB urine treatment and from urine treatments in the FB soil were lower than those from kale urine in the kale soil (P < 0.05). There was a delay in the ammonia-oxidising bacteria (AOB) growth in the FB soil + urine compared with that in kale soil with urine and this led to slower ammonia oxidation in the FB soil. Denitrifier communities were not significantly affected by the urine or soil treatments. Conclusions: These results suggest that there are plant secondary metabolites (PSMs) in the FB urine or root exudates in the FB soil, which might have inhibited the AOB growth and the nitrification process which led to lower N₂O emissions in the FB soil or urine. Further research is needed to determine the chemical composition of urine from different forage plants and to identify potential PSMs or root exudates in the urine or soil from different forage plants