The industrial production of synthetic fertilizers has substantially increased reactive nitrogen (Nx) inputs into terrestrial ecosystems, rising from approximately 100 TgNyr⁻¹ in 1860 to 286 TgNyr⁻¹ currently, with 110 TgNyr⁻¹ originating from synthetic fertilizers. This surge in Nx has triggered various environmental problems, including climate change, ozone depletion, biodiversity loss, and eutrophication. To return to a safe operating space, anthropogenic Nx creation needs to be reduced to 60–100 TgNyr⁻¹, potentially up to 130 TgNyr⁻¹. The challenge lies in reconciling the need for reduced environmental Nx pollution with the imperative to feed a growing global population—a challenge the study terms the “Nx challenge”. This challenge is particularly acute concerning nitrous oxide (N2O) emissions (contributing about 6% of radiative forcing) and ammonia (NH3) and nitrate (NO3⁻) pollution, which degrade ecosystems. Maize, wheat, and rice, accounting for over 60 TgNyr⁻¹ of synthetic N fertilizer use (60% of the global total), are central to this challenge. They provide ~50% of globally consumed calories and ~12% of livestock feed. Reducing Nx input to cereal agroecosystems while maintaining or increasing yields is crucial to mitigating the Nx challenge, especially considering the projected 35–56% rise in crop demand between 2010 and 2050. However, this demand could be lower with widespread adoption of plant-based diets and reduced food waste.

Existing research highlights the significant impact of increased nitrogen fertilizer use on global food production and environmental degradation. Studies have shown that closing yield gaps through improved nutrient and water management can increase crop yields. However, these studies often neglect to consider the environmental consequences of increased nitrogen inputs, particularly the impact on greenhouse gas emissions (N2O) and water pollution (NO3⁻ leaching). Other research focuses on establishing planetary boundaries for nitrogen pollution, but often relies on exogenous assumptions for future developments in nitrogen use efficiency. This study distinguishes itself by providing a quantified scheme for improving global efficiency in converting nitrogen fertilizer into cereal production, considering both yield increases and the environmental impact of nitrogen losses.

This research employed the process-based biogeochemical model LandscapeDNDC (LDNDC) at a 0.5° resolution to simulate cereal cropping systems (maize, wheat, and rice). The model simulates nitrogen, carbon, and water flows within and between soil and plants, considering exchanges with the atmosphere and hydrosphere. Sub-models used include MeTr for soil carbon and N turnover and PlaMo for plant growth. Crop yields are determined by grain biomass at harvest, influenced by photosynthesis (Farquhar et al., Ball et al. approaches), N and water availability, and temperature. The model simulates N2O production during nitrification and denitrification, with the rate influenced by soil conditions and microbial populations. NO3⁻ leaching is modeled using a cascading bucket model, dependent on soil properties and water flow. NH3 production is modeled as an equilibrium reaction with NH4⁺, and its movement through the soil is modeled as diffusion. The model was calibrated and validated against field-scale measurements from various locations and land uses, ensuring robust simulation of crop yields and N losses across a range of N inputs. Global modeling involved grid-cell-specific crop cultivar selection, matching growing degree days to regional data. The model’s outputs, including cereal yields, N2O emissions, NO3⁻ leaching, and NH3 volatilization, were used to create a multidimensional dataset linking N fertilizer usage to cereal production and N losses. The study evaluated multiple scenarios relative to a 2015 baseline: (1) a low emission scenario prioritizing N2O emission reduction, (2) a high yield scenario prioritizing cereal production increase, and (3) a scenario maintaining regional production. A stochastic minimisation procedure was employed to spatially redistribute N fertilizer for optimal outcomes.

Simulations of current agricultural practices (baseline) showed a total global cereal production of 2570 TgYr⁻¹ from 62 TgNyr⁻¹ of synthetic fertilizer and 8 TgNyr⁻¹ of manure. N2O emissions were 1.0 TgNyr⁻¹, and NO3⁻ leaching losses were 14.5 TgNyr⁻¹. A significant portion of current production (53%) is concentrated in North America, East Asia, and Europe, which also account for high environmental N losses. Sub-Saharan Africa, in contrast, shows low fertilizer use and minimal contributions to N losses. The low emission scenario showed that maintaining current cereal production is achievable with a 32% reduction in total global N fertilizer usage (to 42 TgNyr⁻¹), resulting in a 29% reduction in N2O emissions and a 57% reduction in NO3⁻ leaching. Sub-Saharan Africa and Eurasia experienced yield increases despite modest fertilizer increases, while East Asia and North America saw minor yield reductions but substantial pollution decreases. The high yield scenario demonstrated that increasing cereal production by 15% by 2030 is feasible without increasing global fertilizer usage, achieving a 5% reduction in N2O emissions and a 46% reduction in NO3⁻ leaching. East Asia experienced a production decrease, though with substantial pollution reduction. Sub-Saharan Africa and Eurasia again showed significant production gains. A trade-off frontier analysis demonstrated that the efficiency of converting nitrogen fertilizer to cereal yields decreases with increasing production, implying that even small reductions in cereal demand could significantly affect fertilizer usage and losses. A scenario maintaining East Asian production with reduced N pollution showcased the significant challenges in balancing local food security with environmental protection.

The findings reveal the significant potential of spatially optimizing nitrogen fertilizer application to mitigate the environmental impacts of agriculture while ensuring global food security. The study's scenarios highlight the trade-offs between yield maximization and pollution reduction, suggesting that strategic redistribution can achieve both objectives. Redistribution shifts production away from current breadbasket regions and fosters self-sufficiency in regions like Sub-Saharan Africa. Reducing yields in high-yielding areas, although potentially politically challenging, is shown to be crucial for reducing pollution. The improved efficiency and reduced reliance on breadbasket regions enhance resilience to weather events and geopolitical instability. The results underscore that yield-gap closing strategies in sparsely fertilized regions must be coupled with fertilizer reductions in high-use areas. Although the study focuses on cereal crops, the findings are relevant to addressing broader nitrogen challenges. Further research can explore the interplay between spatial redistribution and other strategies for improving nitrogen use efficiency.

This research demonstrates that spatially redistributing nitrogen fertilizer in cereal agroecosystems can significantly mitigate the nitrogen challenge by 2030. This strategy offers a substantial step toward bringing nitrogen use within safe operating spaces, leading to a more equitable distribution of cereal production. Even partial implementation could provide substantial benefits, enhancing food security while mitigating the adverse effects of nitrogen on climate change and the environment. Future research could refine the model's structure and parameters, considering aspects such as crop rotations, multi-cropping, and the impact of diseases and pests, to improve prediction accuracy and inform policy decisions.

The study's reliance on a single-year model (2015) and the exclusion of crop rotations and multi-cropping practices could limit the accuracy of future projections. The model does not explicitly simulate the impacts of diseases, pests, or weeds, which could affect crop yields. The assumption of constant irrigation and management practices, except for fertilizer use, is a simplification. The study's projections assume commensurately supplied phosphorus, potassium, and other plant nutrients. Political and socioeconomic factors influencing fertilizer adoption and yield reductions in high-yielding areas are not explicitly modeled. Finally, the study’s projections do not include the impacts of other nitrogen sources such as livestock production. While the study focuses on cereal crops, this does not represent the entirety of agricultural activities.