Evidence from farm level studies indicates that there is potential to improve nitrogen (N) fertilizer efficiency of Australian dairy farms. Increasing N fertilizer application rates to drive pasture dry matter production beyond an agronomic or economical optimum has the potential to result in detrimental environmental outcomes. Our study, using the biophysical whole-farm systems model DairyMod, modelled a range of N fertilizer rates on total N loss for five dairy sites through south-eastern Australia, using 18 years of historical climate. Nitrogen accumulation in plant biomass and soil N accumulation within and below the rootzone were estimated. Total N loss, in the form of volatilization, leaching, runoff and denitrification lost to the environment were also estimated. The reduction in N fertilizer inputs required to achieve 90% of relative yield (Y90), relative to maximum pasture production (Ymax), was > 50% across all sites and seasons. The associated reduction in total N loss when fertilizer was reduced from Ymax to Y90, varied between 34% and 74%, depending on site and season. Nitrogen recovery (proportion of N recovered in biomass relative to N fertilizer applied) exceeded 100% with lower N fertilizer rates (< 30 kg N ha-1 month-1) for most sites and seasons. Demand for N was high during spring due to high pasture growth and this was supported via N mineralization and legacy N build-up in winter. Nitrate leaching risk was highest in winter for the four temperate sites and autumn at the subtropical site. This study demonstrated the benefits of developing site and seasonal-specific N fertilizer best management practice guidelines that are both economical and environmentally beneficial. When considering whether to add more fertilizer, the value of additional pasture production needs to be weighed up against environmental N losses and the cost of additional N fertilizer to achieve this. The relationship between seasonal soil and climatic conditions and N loss and recovery were also examined for one rainfed site. As this study does not consider the externalities associated with N loss, recommendations need to be considered and amended in the context of location specificity and seasonal climatic conditions.

中文翻译：

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澳大利亚东南部放牧牧场对氮肥的模拟季节性响应：氮损失和恢复

来自农场水平研究的证据表明，有可能提高澳大利亚奶牛场的氮 (N) 肥效率。增加氮肥施用率以推动牧场干物质生产超出农艺或经济最佳水平有可能导致有害的环境结果。我们的研究使用生物物理全农场系统模型 DairyMod，使用 18 年的历史气候模拟了澳大利亚东南部五个奶场的氮素总流失量的一系列氮肥施用率。估计了植物生物量中的氮积累和根区内外的土壤氮积累。还估算了以挥发、浸出、径流和反硝化形式损失到环境中的总氮损失。相对于最大牧草产量 (Ymax)，实现 90% 的相对产量 (Y90) 所需的氮肥投入减少在所有地点和季节均 > 50%。当肥料从 Ymax 减少到 Y90 时，总氮损失的相关减少在 34% 到 74% 之间变化，具体取决于地点和季节。大多数地点和季节的氮回收率（生物量中回收的氮相对于施氮肥的比例）超过 100%，施氮量较低（< 30 kg N ha-1 month-1）。由于牧场的高生长，春季对 N 的需求很高，这得到了冬季 N 矿化和遗留 N 积累的支持。四个温带地区冬季硝酸盐浸出风险最高，亚热带地区秋季硝酸盐浸出风险最高。这项研究证明了开发场地和季节性氮肥最佳管理实践指南的好处，这些指南既经济又环保。在考虑是否添加更多肥料时，需要权衡额外牧场生产的价值与环境氮损失以及为实现这一目标而额外添加氮肥的成本。还研究了一个雨养地点的季节性土壤和气候条件与氮损失和恢复之间的关系。由于本研究未考虑与 N 损失相关的外部性，因此需要在位置特殊性和季节性气候条件的背景下考虑和修改建议。需要权衡额外牧场生产的价值与环境氮损失和额外氮肥的成本来实现这一目标。还研究了一个雨养地点的季节性土壤和气候条件与氮损失和恢复之间的关系。由于本研究未考虑与 N 损失相关的外部性，因此需要在位置特殊性和季节性气候条件的背景下考虑和修改建议。需要权衡额外牧场生产的价值与环境氮损失和额外氮肥的成本来实现这一目标。还研究了一个雨养地点的季节性土壤和气候条件与氮损失和恢复之间的关系。由于本研究未考虑与 N 损失相关的外部性，因此需要在位置特殊性和季节性气候条件的背景下考虑和修改建议。