An overuse of nitrogen fertilizer in farming can lead to negative impacts on the environment. The resulting ammonia emissions can, e.g., contribute to air, water, and soil pollution. In China, for example, nitrogen fertilizers account for roughly a third of total ammonia emissions. Agriculture in China is still dominated by smaller farms. This can lead to a higher use of fertilizer and the use of less advanced methods for fertilization than in larger-scale farms.

Baojing Gu, Zhejiang University, Hangzhou, China, and colleagues have investigated the impact of farm size on ammonia emissions in croplands in over 2,800 counties in China. The team used data from a national pollution census, including farm sizes, crop types, fertilizer types and amounts, and fertilization methods. They developed an ammonia emission model based on the crop and fertilizer types, as well as fertilization and irrigation methods. Then they calculated an emission factor for each county that represents a weighted average of the emissions in the surveyed plots. They also estimated the changes in ammonia emissions for a scenario with more large-scale farming.

The team found that ammonia emissions per unit area generally decrease with an increase in farm size. Statistically, a 1 % increase in farm size was found to be associated with a 0.07 % decrease in ammonia emissions. According to the researchers, larger farms with the accompanying larger incomes can allow the use of more advanced and efficient fertilization techniques. For the different crop types, the largest reduction in ammonia emissions was found for maize. The team estimates that the total national ammonia emissions from croplands could be approximately halved in a large-scale farming scenario.

• Ammonia Emissions from Croplands Decrease with Farm Size in China,
  Chen Wang, Jiakun Duan, Chenchen Ren, Hongbin Liu, Stefan Reis, Jianming Xu, Baojing Gu,
  Environ. Sci. Technol. 2022.
  https://doi.org/10.1021/acs.est.2c01061