CONTEXT

Circular resource use in agriculture and food systems could play an important role when aiming for sufficient food output with limited environmental impact and resource depletion. Circularity, however, is not a goal in itself. With respect to nutrient use and emissions, agricultural system sustainability is currently commonly assessed by nutrient output/input ratio (O/I, nutrient use efficiency) or surplus per ha (IO).

OBJECTIVE

Our aim is to assess how these sustainability indicators are related to nutrient cycling.

METHODS

Starting from basic circularity concepts, a set of equations (frame) is presented that relates nitrogen (N) and phosphorus (P) cycling to food product output, or to food use by human consumers. Circularity indicators express how many times a nutrient input cohort completes a full cycle (CyCt), or passes through the system's top trophic compartment (UseCt). Examples of such compartments are the crop (arable systems), the herd (livestock farms), and the human population (regional food systems). UseCt governs export in useful product. The frame allows to predict equilibrium O/I from system properties, and to attribute parts of O/I to direct (linear) and cycled flow. CyCt_R quantifies how many cycles could be completed by nutrients in absence of product export. CyCt_R allows to assess the efficacy of returning waste from exported products. Above indicators are compared against Finn cycling index and Figge circularity index, more commonly used in ecological and industrial research respectively. All indicators are calculated for systems of increasing complexity: (i) a UK wheat field, (ii) a Dutch dairy farm, and (iii) the Flanders regional food system. Their responses to changes in system properties are analysed for examples ii and iii.

RESULTS AND CONCLUSIONS

Nutrient flows in UK arable field and Flanders are almost linear. In UK arable field, O/I equals 0.74 (N) and 0.66 (P), with small contributions from cycled flow (9% for N, 5% for P). In Flanders, cycled flow constitutes only 2% of total N and P flows that reach the human consumer in Flanders. The dairy farm shows largest contributions of cycled flow: 35% (N) and 60% (P) of O/I comes from cycled flow, but O/I itself is only 0.28 (N) and 0.72 (P).

SIGNIFICANCE

The presented frame allows to assess the impacts of system changes on productivity, nutrient cycling, resource use and nutrient emissions. This is useful for ex-ante assessment of measures that reduce nutrient losses from the system or increase the retrieval of external waste flows.

中文翻译：

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循环指标及其与农业和粮食系统中养分利用效率的关系

语境

在以有限的环境影响和资源枯竭为目标时，农业和粮食系统中的循环资源利用可以发挥重要作用。然而，循环本身并不是目标。关于养分使用和排放，农业系统可持续性目前通常通过养分产出/投入比（O/I，养分利用效率）或每公顷盈余（I O）来评估。

客观的

我们的目标是评估这些可持续性指标如何与养分循环相关。

方法

从基本的循环概念出发，提出了一组方程式（框架），将氮 (N) 和磷 (P) 循环与食品产量或人类消费者的食物使用联系起来。循环指标表示营养输入队列完成一个完整周期 ( CyCt ) 或通过系统顶部营养室 ( UseCt )的次数。这种隔间的例子是作物（耕地系统）、畜群（畜牧场）和人口（区域粮食系统）。UseCt管理有用产品的出口。该框架允许根据系统属性预测平衡O / I，并归因于O / I的部分直接（线性）和循环流。CyCt _R量化了在没有产品输出的情况下营养素可以完成多少个循环。CyCt _R允许评估从出口产品中回收废物的效率。将以上指标与生态和工业研究中更常用的芬兰循环指数和菲格循环指数进行比较。所有指标都是针对越来越复杂的系统计算的：(i) 英国麦田，(ii) 荷兰奶牛场，以及 (iii) 法兰德斯区域食品系统。在示例 ii 和 iii 中分析了它们对系统属性变化的响应。

结果和结论

英国耕地和佛兰德斯的养分流动几乎是线性的。在英国耕地中，O / I等于 0.74 (N) 和 0.66 (P)，循环流的贡献很小（N 为 9%，P 为 5%）。在法兰德斯，循环流量仅占到达法兰德斯人类消费者的总氮和磷流量的 2%。奶牛场循环流贡献最大：35%（N）和60%（P）的O / I来自循环流，但O / I本身仅为0.28（N）和0.72（P）。

意义

所呈现的框架允许评估系统变化对生产力、养分循环、资源利用和养分排放的影响。这对于事前评估减少系统养分损失或增加外部废物流回收的措施很有用。