LAUSR

Soil gross nitrification (GN) is a critical process in the global nitrogen (N) cycle that results in the formation of nitrate through microbial oxidation of ammonium or organic N, and can both increase N availability to plants and nitrous oxide emissions. Soil GN is thought to be mainly controlled by soil characteristics and the climate, but a comprehensive analysis taking into account the climate, soil characteristics, including microbial characteristics, and their interactions to better understand the direct and indirect controlling factors of GN rates globally is lacking. Using a global meta-analysis based on 901 observations from 330 15 N-labelled studies, we show that GN differs significantly among ecosystem types, with the highest rates found in croplands, in association with higher pH which stimulates nitrifying bacteria activities. Autotrophic and heterotrophic nitrifications contribute 63 and 37%, respectively, to global GN. Soil GN increases significantly with soil total N, microbial biomass, and soil pH, but decreases significantly with carbon (C) to N ratio (C: N). Structural equation modeling suggested that GN is mainly controlled by C: N and soil total N. Microbial biomass and pH are also important factors controlling GN and their effects are similar. Precipitation and temperature affect GN by altering C: N and/or soil total N. Soil total N and temperature drive heterotrophic nitrification, whereas C: N and pH drive autotrophic nitrification. Moreover, GN is positively related to nitrous oxide and carbon dioxide emissions. This synthesis suggests that changes in soil C: N, soil total N, microbial population size, and/or soil pH due to anthropogenic activities may influence GN, which will affect nitrate accumulation and gaseous emissions of soils under global climate and land-use changes.