The application of nitrogen (N) fertilizer both underpins high productivity of agricultural systems and contributes to multiple environmental harms. The search for ways that farmers can optimize the N fertilizer applications to their crops is of global significance. A common concept in developing recommendations for N fertilizer applications is the “mass balance paradigm” – that is, bigger crops need more N, and smaller less – despite several studies showing that the crop yield at the optimum N rate (N_opt) is poorly related to N_opt. In this study we simulated two contrasting field experiments where crops were grown for 5 and 16 consecutive years under uniform management, but in which yield at N_opt was poorly correlated to N_opt. We found that N lost to the environment relative to yields (i.e., kg N t^-1) varied +/- 124 and 164 % of the mean in the simulations of the experiments. Conversely, N exported in harvested produce (kg N t^-1) was +/- 11 and 48 % of the mean. Given the experiments were uniformly managed across time, the variations result from crop-to-crop climatic differences. These results provide, for the first time, a quantitative example of the importance of climatic causes of the poor correlation between yield at N_opt and N_opt. An implication of this result is that, even if yield of the coming crop could be accurately predicted it would be of little use in determining the amount of N fertilizer farmers need to apply because of the variability in environmental N losses and/or crop N uptake. These results, in addition to previous empirical evidence that yield at N_opt and N_opt are poorly correlated, may help industry and farmers move to more credible systems of N fertilizer management.

氮肥的施用既支撑了农业系统的高生产力，又造成多种环境危害。寻找农民优化作物氮肥施用的方法具有全球意义。制定氮肥施用建议的一个常见概念是“质量平衡范式”——即较大的作物需要更多的氮，而较小的作物则需要较少的氮——尽管多项研究表明，最佳施氮量 (N _opt ) 下的作物产量较差与 N _opt相关。在这项研究中，我们模拟了两个对比性的田间实验，其中作物在统一管理下连续种植 5 年和 16 年，但其中 N _{opt 的产量与 N opt 的}相关性较差。我们发现，在实验模拟中，相对于产量（即 kg N t ^-1 ） ，环境中损失的 N变化为平均值的 +/- 124 % 和 164 %。相反，收获农产品中的氮输出量 (kg N t ^-1 ) 为平均值的 +/- 11 和 48%。鉴于这些实验在不同时间段内是统一管理的，因此差异是由作物与作物之间的气候差异造成的。这些结果首次提供了气候原因对 N _opt和 N _opt产量之间相关性较差的重要性的定量示例。这一结果的含义是，即使可以准确预测即将到来的作物的产量，由于环境氮损失和/或作物氮吸收的可变性，它对于确定农民需要施用的氮肥量也没有多大用处。除了之前的经验证据表明 N _opt和 N _opt产量相关性较差之外，这些结果可能有助于工业和农民转向更可靠的氮肥管理系统。