One-third of human-induced reactive nitrogen emissions can be traced to livestock supply chains. Highly variable nitrogen use efficiencies along these chains uncover opportunities for more sustainable nitrogen management.
Reactive nitrogen must be more sustainably managed in global food systems. The extent of fertilizer use over the past century has allowed higher agricultural productivity, but surplus reactive nitrogen in the environment has damaged air and water quality, led to biodiversity loss, and contributed to climate change and stratospheric ozone depletion1. Livestock supply chains contribute to these damaging nitrogen emissions when inorganic fertilizers and manure are used to enhance pasture productivity and produce feed crops, during on-farm accumulation and management of manure, and when feeds, products and manure are processed and transported. Yet, understanding the full scale of livestock nitrogen emissions and the opportunities to improve nitrogen management in the livestock sector has proved challenging given the diversity of livestock systems across the globe and the multiple pathways and forms of nitrogen emissions.
Writing in Nature Food, Uwizeye and colleagues2 report the results of a comprehensive data synthesis and modelling effort to uncover the patterns of reactive nitrogen emissions across global livestock supply chains for 2010. Through a spatially explicit analysis of livestock supply chains that considers the diversity of livestock species, systems, production intensities, and the origins and management of animal feeds, the authors reveal the large variability in nitrogen use efficiencies across global livestock systems and how emissions are embedded into internationally traded commodities.
Uwizeye and colleagues find that the livestock sector drives one-third of all nitrogen emissions from the entire global economy, and these emissions are heterogeneously distributed across nitrogen compounds, activity, livestock systems and regions (Fig. 1). Most nitrogen is emitted in just two forms: nitrate (NO3–, 45%), which degrades water quality in freshwater and coastal systems, and ammonia (NH3, 41%), which contributes to air pollution. Emissions predominantly occur during the production of feed crops and pastures (68%), while emissions across all production activities are dominated by livestock production in South and East Asia (63%). Ruminant livestock systems (mostly cattle and buffalo) that produce meat and milk comprise 71% of global emissions, and ruminant meat ranks the lowest in life-cycle nitrogen use efficiency. Even though ruminant meat tends to have higher environmental impacts per kilocalorie than pigs and poultry3, ruminants can make use of non-food biomass from grasslands and stovers.
These results highlight the need for ambitious efforts to reduce nitrogen emissions from livestock systems, taking into account the diversity of production systems and the global impact of local management decisions. Perhaps most useful, the quantification of nitrogen use efficiency levels associated with each livestock system and species reveals critical opportunities — hotspots of efficiency provide models for best practices, whereas hotspots of inefficiency can guide interventions. For example, the authors identify low nitrogen use efficiencies in many systems across East Asia related to high synthetic fertilizer application and unregulated manure disposal.
Many possibilities for improvement exist and, in some cases, are already being seized. For example, over the past few decades, the amount of feed required to produce a kilogram of animal-sourced food has declined in many systems4. More recently, lower protein content and better feed formulation have reduced nitrogen content in manure4. Many technologies to decrease manure nitrogen losses at the housing, storage, processing and field application stages have considerable potential to reduce pollution5.
Given the magnitude of the challenge, complementary structural and systemic solutions are needed. Reconnecting crop and livestock production is critical for enhancing nutrient recycling, decreasing the need for inorganic fertilization in isolated cropping systems, and decreasing potential overfertilization or waste of manure in areas of high livestock concentration6. Integrated management of crops and livestock can be facilitated by considering manure sources and sinks across landscapes within spatial units recently described as “manuresheds”7. Likewise, efforts towards optimal manure allocation can help maximize regional nitrogen use efficiency8. Relocation of livestock systems to enhance crop–livestock connectivity and reduce pollution has been proposed in Europe9 and is now occurring in China through ‘pig relocation’ policies10. Increasing the share of locally produced feed will also reduce reactive nitrogen emissions embedded in international trade, whereas more feeds that do not compete with human food production (such as food wastes) can boost circularity of nutrients and reduce overall feed demand4. Finally, reduced consumption of livestock products in high-income regions could help limit overall livestock nitrogen emissions.
A combination of approaches will be needed to tackle the livestock nitrogen challenge while ensuring a fair, healthy and environmentally friendly food system. The comprehensive analyses provided by Uwizeye and colleagues will help chart a roadmap towards achieving this in the future.
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The authors declare no competing interests.
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Mueller, N.D., Lassaletta, L. Nitrogen challenges in global livestock systems. Nat Food 1, 400–401 (2020). https://doi.org/10.1038/s43016-020-0117-7