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Synthetic ammonia production by the Haber–Bosch process revolutionized agriculture by making relatively inexpensive nitrogen (N) fertilizer widely available and enabling a rise in global food production1,2. The Haber–Bosch process relies on fossil fuels (known as grey ammonia production) and emits more than 450 Mt of CO2 annually3. Green ammonia, which is produced using renewable energy, offers a pathway to decouple ammonia production from fossil fuels and reduce CO2 emissions. As a carbon-free fuel, green ammonia could partially replace fossil fuels to decarbonize hard-to-abate sectors such as maritime shipping4. However, the widespread use of green ammonia could have complex environmental and social consequences, as it threatens to add reactive N into the biosphere3 and could disrupt fertilizer markets. In this Comment, we identify opportunities, barriers and open questions related to green ammonia production and usage as a fertilizer and beyond. We then recommend research priorities to avoid unforeseen consequences through research, monitoring and adaptation in real time. 

Economic experiments have emerged as a powerful tool for agricultural policy evaluations. In this perspective, we argue that involving stakeholders in the design of economic experiments is critical to satisfy mandates for evidence-based policies and encourage policymakers' usage of experimental results. To identify advantages and disadvantages of involving stakeholders when designing experiments, we synthesize observations from six experiments in Europe and North America. In these experiments, the primary advantage was the ability to learn within realistic decision environments and thus make relevant policy recommendations. Disadvantages include complicated implementation and constraints on treatment design. We compile 12 recommendations for researchers.