Water-energy-food nexus in the sustainable management of crop-livestock coupled systems

The recoupling of livestock and crop production by recycling of animal manure in croplands is urgently needed to achieve a sustainable agricultural development. However, an insufficient understanding of the resources and environmental performance of recoupling and the synergies and trade-offs among multiple sustainable development goals hinders the implementation of best management practices. Here, by combining footprint and lifecycle analyses, we distinguished two coupling degrees (semi-coupling, in which only solid manure is returned to croplands, and full coupling, which involves the return of both solid and liquid manure) and three coupling modes to quantitatively assess the influence of recoupling management measures on nitrogen (N) utilization, water and energy consumption, and greenhouse gas (GHG) emissions. Our results showed that conventional full coupling (FCPC) enhanced the system's N use efficiency and reduced reactive N (Nr) loss and the water footprint by 14 and 29 %, respectively, compared with semi-coupling (SCP), the status quo in most of China's livestock farms. However, FCPC slightly increased GHG emissions, mainly because of energy-water tradeoffs. An enhanced full coupling mode designed based on the water-energy-food nexus (FCPN) reduced the energy footprint and GHG emissions by 74 and 12 %, respectively, compared with SCP. Because FCPN additionally alleviated the pollution shift effect, the water footprint was further reduced by 34 % compared with SCP. Transforming from SCP to FCPC and FCPN offered a total social benefit, which integrates resource and environmental performances, of approximately 537 and 694 yuan t(-1) milk at an implementation cost of 396 and 401 yuan t(-1) milk, respectively. Our research fills a knowledge gap related to the quantification of recoupling impacts on the waterenergy-food nexus and provides a solution for the collaborative optimization of multiple objectives in China and other countries facing the same challenges in livestock revolution.