Global patterns of soil gross immobilization of ammonium and nitrate in terrestrial ecosystems

Microbial nitrogen (N) immobilization, which typically results in soil N retention but based on the balance of gross N immobilization over gross N production, affects the fate of the anthropogenic reactive N. However, global patterns and drivers of soil gross immobilization of ammonium (INH4 ) and nitrate (INO3 ) are still only tentatively known. Here, we provide a comprehensive analysis considering gross N production rates, soil properties, and climate and their interactions for a deeper understanding of the patterns and drivers of INH4 and INO3 . By compiling and analyzing 1966 observations from 274 15 N-labelled studies, we found a global average of INH4 and INO3 of 7.41 ± 0.72 and 2.03 ± 0.30 mg N kg-1 day-1 with a ratio of INO3 to INH4 (INO3 :INH4 ) of 0.79 ± 0.11. Soil INH4 and INO3 increased with increasing soil gross N mineralization (GNM) and nitrification (GN), microbial biomass, organic carbon, and total N and decreasing soil bulk density. Our analysis revealed that GNM and GN were the main stimulators for INH4 and INO3 , respectively. The structural equation modeling showed that higher soil microbial biomass, total N, pH, and precipitation stimulate INH4 and INO3 through enhancing GNM and GN. However, higher temperature and soil bulk density suppress INH4 and INO3 by reducing microbial biomass and total N. Soil INH4 varied with terrestrial ecosystems, being greater in grasslands and forests, which have higher rates of GNM, than in croplands. The highest INO3 :INH4 was observed in croplands, which had higher rates of GN. The global average of GN to INH4 was 2.86 ± 0.31, manifesting a high potential risk of N loss. We highlight that anthropogenic activities that influence soil properties and gross N production rates likely interact with future climate changes and land uses to affect soil N immobilization and, eventually, the fate of the anthropogenic reactive N.