A Hierarchical Framework for Unpacking the Nitrogen Challenge

Abstract

To feed the world population while mitigating pressing nitrogen (N) pollution problems, tremendous efforts have been devoted to developing and implementing N-efficient technologies in crop or livestock production, but limited progress has been made. The N management improvement on a farm does not necessarily translate to N pollution reduction on a broader scale due to complex responses of natural and human systems and lack of coordination among stakeholders. Consequently, it is imperative to develop an N management framework that encompasses the complex N dynamics across systems and spatial scales, yet simple enough to guide policies and actions of various stakeholders. Here, we propose a new framework, CAFE, that defines four N management systems (Cropping, Animal-crop, Food, and Ecosystem) in a hierarchical manner, and apply it to 13 representative countries to partition N surpluses across systems in a simple and consistent manner, thereby facilitating the identification and prioritization of systems-based intervention strategies. Surprisingly, the Cropping system contributes less than half of the total N surplus within its Ecosystem for most countries, highlighting the importance of N management beyond croplands. This framework reveals that the relevant priorities and key stakeholders for enhanced N management vary among countries, such as improving the Cropping-system efficiencies in China, adjusting the animal-crop portfolio in the Netherlands, reducing food wastage in the U.S., and lowering crop storage losses and increasing overall production capacities in African countries. As N surplus increases along the CAFE hierarchy, systems-based intervention strategies are revealed: (a) coupling chemical fertilizers with other N sources by maintaining half of the N from manure and biological N fixation; (b) coupling animal-crop production by reducing animal density to lower than 1.2 livestock units per hectare, and increasing self-sufficiency of animal feed to above 50%; (c) coupling food trade with domestic demand and production; and (d) coupling population needs for economic opportunities with environmental capacity of the region. This novel framework can help unpack the “wicked” N management challenges across systems to provide new insights and tools for improving N management on and beyond farms.