Ammonia Emissions from Croplands Decrease with Farm Size in China.

Farm size affects nitrogen fertilizer input and agricultural practices, which are key determinants of ammonia (NH 3 ) emissions from croplands. However, the degree to which NH 3 emissions are associated with changes in farm size is not well understood yet despite its crucial role in achieving agricultural sustainability in China, where agricultural production is still dominated by smallholder farms. Here we provide a first analysis of the relationship between farm size and NH 3 emissions based on 863 000 surveys conducted in 2017 across China. Results show that NH 3 emissions (kg ha -1 ) on average decrease by 0.07% for each 1% increase in average farm size. This change occurs mainly due to a reduction in nitrogen fertilizer use and the introduction of more efficient fertilization practices. The largest reduction in NH 3 emissions is found in maize, with less pronounced changes in rice cultivation, and none for wheat production. Overall lower NH 3 emissions factors can be observed in the north of China with increasing farm size, especially in the northeast, the opposite pattern was found in the south. National total NH 3 emissions could be approximately halved (1.5 Tg) in a scenario favoring a conversion to large-scale farming systems. This substantial reduction potential highlights the potential of such a transition to reduce NH 3 emissions, including benefits from a socioeconomic point of view as well as for improving air quality.