Zhejiang University reveals a new mechanism of global farmland nitrogen pollution control

On January 5, 2023, Beijing time, Professor Xu Jianming’s team from the School of Environment and Resources of Zhejiang University, Professor Gu Baojing and Dr. Zhang Xiuming, cooperated with universities and research institutes at home and abroad to publish a research result entitled “Cost-effective mitigation of nitrogen pollution from global croplands” in the journal Nature.

The results report how to reveal the new mechanism of nitrogen pollution in global farmland by constructing a cost-benefit analysis system, find that the mismatch of emission reduction costs and social benefits is the difficulty of nitrogen pollution control, and take part of the social benefits from the establishment of a nitrogen credit system to oversubsidize farmers, solve the global problem of agricultural non-point source pollution control, and ensure that global food security, environmental and health benefits can be truly realized.

The corresponding authors of the paper are Gu Baojing and Xu Jianming; The first author is Gu Baojing.

The world’s growing and affluent population demands continued increases in farmland yields, nitrogen is an essential nutrient for plant growth and development, and nitrogen fertilizer application is a key measure to achieve high yield and high quality crops. The annual application of nitrogen fertilizer in global farmland is as high as 120 million tons, coupled with organic fertilizer and biological nitrogen fixation, the total nitrogen input of farmland is as high as 200 million tons, but because the nitrogen use efficiency is less than 50%, more than half of the applied nitrogen is lost to the environment, bringing serious environmental and health effects, such as water eutrophication, soil acidification, air pollution, biodiversity reduction, greenhouse effect, etc. Reducing nitrogen pollution in farmland has attracted global attention. Compared with the point source emissions of industrial and domestic pollution, farmland pollution is a non-point source discharge, involving hundreds of millions of farmers around the world, and it is difficult to govern and implement pollution control measures. To that end, are there farmland management measures globally that can control nitrogen pollution, what are the potential and cost of implementing these measures, what are the benefits of reducing emissions, and do farmers have incentives to implement these reductions? Answering the above questions is of great significance to solving the global control of nitrogen pollution in farmland.

Through a systematic analysis of more than 1,500 field observations over the past 20 years, the study identified a package of emission reduction measures, including 11 nitrogen management measures including Synergistic Fertilizers (EEFs), 4R nutrient management, irrigation and pulse rotation (Figure 1). These measures can reduce nitrogen loss on farmland around the world, while also increasing yields and nitrogen fertilizer efficiency. However, the barriers and costs of implementing different emission reduction measures vary widely around the world. There are three levels of emission reduction measures: the first level includes nitrogen enhancement methods (organic amendments such as synergistic fertilizers, organic fertilizers and straw), crop pulses rotation and farmland buffer zones; The second level is 4R nutrient management, that is, the correct fertilization rate, fertilization type, fertilization time and location; The third level is the introduction of new varieties, optimal irrigation and optimized farming practices. In general, the higher the level of the measure, the more knowledge and additional inputs are required, and therefore the higher the cost of implementation, and the more difficult it is for farmers to take the measure. Overall, these measures can effectively reduce total nitrogen loss by 30-70%, while increasing crop yields by 10-30% and nitrogen use efficiency by 10-80%, respectively.

Figure 1: Impact of management measures on global agricultural nitrogen use and loss.

Using 2015 as a baseline, an additional 17 million tonnes of crop nitrogen could be harvested per year (+20%), 22 million tonnes of nitrogen fertilizer use could be reduced and 26 million tonnes of nitrogen pollution (-32%) could be reduced if the optimal package of mitigation measures were implemented on farmland around the world. These changes could bring total societal benefits of up to $476 billion, mainly food supply ($196 billion), human health ($130 billion), ecosystem services ($152 billion) and climate effects ($-2 billion) (Figure 2). Since reductions in Nr emissions in some regions may exacerbate global warming by reducing carbon sequestration in natural ecosystems, reflecting the climate trade-off effect of nitrogen emissions (Figure 2). The implementation of emission reduction measures requires additional investment in equipment, labor, materials and services, and the total implementation cost is estimated to be about 34 billion US dollars, because nitrogen management measures can save the application of nitrogen fertilizer on farmland and reduce fertilizer costs by 15 billion US dollars, offsetting 44% of the total reduction costs, so the final net implementation cost is only 19 billion US dollars, or about 4% of the total social benefit. These results suggest that the social benefits of optimizing nitrogen management in farmland far outweigh the costs of implementation.

Figure 2: Costs and social benefits of implementing nitrogen management measures in the global subregion in 2015.

To inform policymaking, the study explores the input, utilization and loss of nitrogen in farmland under a combination of measures at different levels from 2020 to 2050. Tier 1 measures have the greatest application potential globally and can contribute half of the nitrogen reduction potential of farmland. Net economic benefits are provided to farmers as fertilizer savings and legume rotation save fertilizer costs, combined with the economic benefits of increased crop yields that offset the cost of implementing measures (Figure 3). The additional cost of implementing Level 2 measures globally is $18 billion, and the total cost of Level 3 measures is estimated at $25 billion, but results in $7 billion in fertilizer cost savings. While implementing Tier 2 and Tier 3 measures has huge benefits for society as a whole, it comes at high cost for farmers, which will severely hinder the implementation of emission reduction measures.

Figure 3: Cost-benefit situation under the mix of measures at different levels in 2050. (a) Costs and benefits of abatement at the global scale; (b) Fertilizer savings in the subregion; (c) Changes in NUE in the subregion.

While cost-benefit analysis reveals the potential social benefits of taking these measures, how to support the implementation of these measures through policies to achieve win-win is an open research question. One suggested policy approach is to socialize the cost of abatement and provide farmers with access to financial support, namely the Agricultural Nitrogen Credit System (NCS), which focuses on the responsibilities and obligations of every participant in the food production-supply-consumption chain, including farmers, suppliers, processors, retailers, consumers and governments. The system advocates that the whole society that benefits from the optimal management of nitrogen should raise the budget to incentivize and subsidize farmers who implement best management practices to reduce pollution and increase yields, thereby reducing nitrogen pollution. The lower limit of nitrogen budget subsidies to farmers is the net implementation cost of all measures within a certain boundary (including transition costs such as training and opportunity costs such as farmers’ non-farm income), and the upper limit is the total social benefit of reducing nitrogen loss (excluding yield benefits). In fact, agricultural subsidies are commonly used in many countries to sustain agricultural practices that indirectly benefit food security and the environment, and NCS can be seen as a reinforcement to better link costs and benefits to society as a whole.

The project is supported by the National Natural Science Foundation of China Innovation Research Group and International Cooperation Project, and the United Nations Environment Programme Fund (International Nitrogen Management System, INMS). (Source: Science Network)

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