Cropland Acidification in China: China is experiencing increasing problems with acid rain, groundwater pollution, and nitrous oxide emissions. Rapid development of industry and transportation has accelerated nitrate (N) emissions to the atmosphere. Consequently, soil degradation, water shortage, and pollution, in addition to atmospheric quality decline are becoming major public concerns across China. Since the 1990s, China has become both the largest consumer of chemical N fertilizers and the highest cereal producer in the world, which has consequences for arable soil acidification. Guo et al. (p. 1008 , published online 11 February) present a meta-analysis of a regional acidification phenomenon in Chinese arable soils that is largely associated with higher N fertilization and higher crop production. Such large-scale soil acidification is likely to threaten the sustainability of agriculture and affect the biogeochemical cycles of nutrients and also toxic elements in soils.

Global potent greenhouse gas nitrous oxide (N₂O) emissions from soil are accelerating, with increases in the proportion of reactive nitrogen emitted as N₂O, i.e., N₂O emission factor (EF). Yet, the primary controls and underlying mechanisms of EFs remain unresolved. Based on two independent but complementary global syntheses, and three field studies determining effects of acidity on N₂O EFs and soil denitrifying microorganisms, we show that soil pH predominantly controls N₂O EFs and emissions by affecting the denitrifier community composition. Analysis of 5438 paired data points of N₂O emission fluxes revealed a hump-shaped relationship between soil pH and EFs, with the highest EFs occurring in moderately acidic soils that favored N₂O-producing over N₂O-consuming microorganisms, and induced high N₂O emissions. Our results illustrate that soil pH has a unimodal relationship with soil denitrifiers and EFs, and the net N₂O emission depends on both the N₂O/(N₂O + N₂) ratio and overall denitrification rate. These findings can inform strategies to predict and mitigate soil N₂O emissions under future nitrogen input scenarios.