Cropping systems depend on external nitrogen (N) to produce food. However, we lack metrics to account for society’s fertilizer dependency, although excessive increases in N application damage human and environmental health. The objective of this study is to propose a novel indicator, N fertilizer dependency, calculated as the ratio between human-controllable external inputs and total N inputs. Nitrogen fertilizer dependency has a solid mathematical base being derived from closing the nitrogen use efficiency (NUE) equation. This study also tests the value of the N fertilizer dependency concept at the cropping system (plant-soil) scale and at different spatial scales, from field to country, as a complementary indicator to promote sustainable production. The field experiments conducted with grain cereals as a main crop showed that when replacing the barley precedent crop with a legume, N fertilizer dependency accounted for soil legacy and was reduced by 15% in fertilized treatments. In a farm population, N fertilizer dependency ranged from 47 to 95% and accounted for the relevance of biological fixation and irrigation water N inputs, adding pertinent information to performance indicators (i.e., NUE). At the country scale, N fertilizer dependency showed different temporal patterns, depending mainly on the relevance of biological atmospheric N fixation. Nitrogen fertilizer dependency of global cropping systems has risen to ≈83% in the last five decades, even though the N exchange among regions has increased. Nitrogen fertilizer dependency has great potential to monitor the achievements of efforts aiming to boost system autonomy, and within similar agricultural systems, it can be used to identify practices that lead to a reduction of fertilizer needs. In summary, N fertilizer dependency is a new indicator to evaluate the agroenvironmental sustainability of cropping systems across the scales and provides a complementary dimension to the traditional indicators such as NUE, N output, and N surplus.

耕作系统依靠外部氮 (N) 来生产食物。然而，尽管过量增加施氮会损害人类和环境健康，但我们缺乏衡量社会对肥料依赖程度的指标。本研究的目的是提出一个新的指标，即氮肥依赖性，计算为人类可控的外部投入与总氮投入之间的比率。氮肥依赖性具有坚实的数学基础，源自氮肥利用效率 (NUE) 方程的闭合。本研究还测试了种植系统（植物-土壤）尺度和不同空间尺度（从田地到国家）氮肥依赖概念作为促进可持续生产的补充指标的价值。以谷物为主要作物进行的田间试验表明，当用豆类替代大麦之前的作物时，氮肥依赖占土壤残留，并且在施肥处理下减少了15%。在农场群体中，氮肥依赖率为 47% 至 95%，并考虑了生物固定和灌溉水氮输入的相关性，为绩效指标（即 NUE）添加了相关信息。在国家尺度上，氮肥依赖表现出不同的时间模式，主要取决于生物大气固氮的相关性。尽管各地区之间的氮交换有所增加，但在过去 50 年中，全球种植系统对氮肥的依赖度已上升至约 83%。氮肥依赖性在监测旨在提高系统自主性的努力所取得的成就方面具有巨大潜力，并且在类似的农业系统中，它可以用来确定导致肥料需求减少的做法。 综上所述，氮肥依赖度是评价不同尺度耕作系统农业环境可持续性的新指标，为氮肥利用效率、氮肥产出和氮盈余等传统指标提供了补充。