The effects of urine nitrogen application rate on nitrogen transformations in grassland soils

Abstract Urine is a critical nitrogen (N) input in temperate grazed grasslands and can drive substantial nitrous oxide (N 2 O) production in soils. However, it remains unclear how differences in the N input rate affect N 2 O fluxes and vary between different grassland soils. The effect of increasing urine N application on ammonium (NH 4 + ), nitrite (NO 2 − ) and nitrate (NO 3 − ) concentrations and N 2 O production was tested in two grassland soils, a free-draining loam and an imperfectly drained sandy-loam. It was hypothesized that high-urine N application rates would lead to ammonia/ammonium (NH 3 /NH 4 + ) accumulation influencing N transformation rates and N 2 O production which differ between grassland soils. Fresh cattle urine was applied at rates equivalent to 300 and 1000 kg N/ha in an aerobic incubation experiment. Soils were destructively sampled over 80 days to measure changes in inorganic-N and pH. The higher N addition rate was associated with elevated NH 3 concentrations up to day 35 in soils, probably inhibiting NO 2 − to NO 3 − reduction. In contrast, there was no inhibition of nitrification in the 300 kg N/ha treatment. Cumulative N 2 O fluxes were greatest from the 300 kg N/ha treatment for the loam soil, but were greater for the sandy-loam under the 1000 kg N/ha treatment. The results also show that differences in soil properties, in particular carbon availability, can be important in regulating N transformation and N 2 O production. Collectively, these results demonstrate the proposed mechanism of nitrification inhibition at high-N input rates, driven by either high NH 3 /NH 4 and/or increased levels of NH 4 HCO 3 from urea hydrolysis.