Urine is a critical nitrogen (N) input in temperate grazed grasslands and can drive substantial nitrous oxide (N2O) production in soils. However, it remains unclear how differences in the N input rate affect N2O fluxes and vary between different grassland soils. The effect of increasing urine N application on ammonium (NH4+), nitrite (NO2−) and nitrate (NO3−) concentrations and N2O 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 (NH3/NH4+) accumulation influencing N transformation rates and N2O 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 NH3 concentrations up to day 35 in soils, probably inhibiting NO2− to NO3− reduction. In contrast, there was no inhibition of nitrification in the 300 kg N/ha treatment. Cumulative N2O 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 N2O production. Collectively, these results demonstrate the proposed mechanism of nitrification inhibition at high-N input rates, driven by either high NH3/NH4 and/or increased levels of NH4HCO3 from urea hydrolysis.

中文翻译：

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尿氮施用量对草地土壤氮素转化的影响

尿液是温带放牧草原的关键氮 (N) 输入，可驱动大量一氧化二氮 (N2O) 在土壤中生产。然而，目前尚不清楚 N 输入速率的差异如何影响 N2O 通量和不同草地土壤之间的变化。增加尿氮施用量对铵（NH）的影响4+), 亚硝酸盐 (NO2-) 和硝酸盐 (NO3-) 浓度和 N2O 产量在两种草地土壤中进行了测试，一种是排水良好的壤土，另一种是排水不完全的沙壤土。假设高尿氮施用率会导致氨/铵（NH3/NH4+) 累积影响 N 转化率和 N2草地土壤之间的 O 产量不同。在好氧孵化实验中，新鲜牛尿的施用量相当于 300 和 1000 kg N/ha。在 80 天内对土壤进行破坏性采样，以测量无机氮和 pH 值的变化。较高的 N 添加率与 NH 升高有关3土壤中 35 天的浓度，可能抑制 NO2-到否3-减少。相反，在 300 kg N/ha 处理中没有抑制硝化作用。累积 N2300 kg N/ha 处理的壤土 O 通量最大，但在 1000 kg N/ha 处理下的沙壤土更大。结果还表明，土壤性质的差异，特别是碳的有效性，对于调节 N 转化和 N2Ø 生产。总的来说，这些结果证明了在高 N 输入速率下提出的硝化抑制机制，由高 NH 驱动3/NH4和/或 NH 水平升高4碳酸氢钠3尿素水解。