Cost-effective mitigation of nitrogen pollution from global croplands

The growing global population and increasing wealth drive intensified agriculture, leading to greater use of nitrogen (N) fertilizers and manure. However, over half of these N inputs are lost to the environment, causing air and water pollution, soil acidification, climate change, and biodiversity loss. Reducing N loss from croplands offers economic benefits through lower fertilizer needs, and improves human health and ecosystem services. While best management practices exist (e.g., 4R nutrient stewardship), their implementation is hindered by high heterogeneity, costs, and limited farmer knowledge. This study identifies a package of cost-effective on-farm mitigation measures to address this challenge.

Existing literature highlights the negative impacts of nitrogen pollution from croplands, and the potential benefits of improved nitrogen management. Studies have explored best management practices such as the 4R approach (right source, right rate, right time, right place) and soil testing. However, limitations on full implementation due to economic and knowledge constraints are also documented. This research builds upon this existing body of work by conducting a comprehensive meta-analysis to identify the most effective and cost-effective mitigation measures.

This research employed a meta-analysis of 1,521 field observations from the past two decades to assess the effectiveness of various nitrogen mitigation measures. Eleven key measures were selected based on criteria such as detailed information availability. These measures were categorized into three tiers based on implementation challenges: Tier 1 (enhanced-efficiency fertilizers, organic amendments, crop legume rotation, buffer zones); Tier 2 (4R nutrient stewardship); and Tier 3 (new cultivars, optimal irrigation, tillage). The study then integrated the reduction potential of these measures into nitrogen budget models (CHANS, MAgPIE, IMAGE) to estimate changes in the global cropland N budget for 2015. Finally, the implementation costs and societal benefits (including crop yield increases, human health improvements, ecosystem services, and climate change impacts) of these measures were estimated and analyzed. The uncertainties related to cost-benefit analysis are also considered throughout.

The meta-analysis revealed that the selected 11 measures can reduce total nitrogen losses by 30–70%, increase crop yields by 10–30%, and improve nitrogen use efficiency (NUE) by 10–80%. Enhanced-efficiency fertilizers, 4R stewardship, irrigation, and legume rotation showed particularly strong performance. Modeling results indicated that adopting these measures globally would increase crop nitrogen production by 20%, reduce fertilizer use by 21%, and decrease nitrogen pollution by 32% in 2015. This would lead to a 476 ± 123 billion USD societal benefit, exceeding the net mitigation cost of 19 ± 5 billion USD (with fertilizer savings offsetting 44% of the gross cost). The largest reductions in nitrogen input and losses were observed in East and South Asia and Southeast Asia, while high-income regions showed smaller reduction potential. Analysis by region showed significant variation in implementation costs and benefits, highlighting the need for targeted policies. A projection to 2050 under different scenarios based on the three tiers of mitigation measures shows that Tier 1 measures are most likely to offer net financial benefits to farmers while Tier 2 and 3 measures are cost effective to the whole society.

The findings address the research question by identifying a practical and cost-effective package of measures to reduce nitrogen pollution from croplands. The significant societal benefits far outweigh the implementation costs, highlighting the economic and environmental imperative for action. The regional variations in cost-benefit ratios underscore the importance of tailoring policy interventions to specific contexts. The study's success in quantifying the benefits and costs of implementing these measures provides crucial evidence for policymakers.

This study demonstrates the significant potential for cost-effective mitigation of nitrogen pollution from global croplands. The identified 11 measures offer substantial environmental and economic benefits. Policy instruments like a nitrogen credit system or taxes on nitrogen surplus could incentivize adoption, particularly in regions with different economic and technological capabilities. Future research could focus on refining regional cost-benefit analyses, exploring the effectiveness of various policy mechanisms, and investigating the integration of nitrogen management with other sustainable development goals.

The study relies on a meta-analysis of existing field observations, which might not capture the full range of possible mitigation outcomes. The cost-benefit analysis involves several assumptions and uncertainties, particularly in monetizing societal benefits such as human health improvements. The regionalization of the analysis based on three tiers might oversimplify the complexity of the real-world implementation challenges. The interaction effects between the identified measures are not completely considered in the analysis.