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.

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