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 (I-NH4) and nitrate (I-NO3) 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 I-NH4 and I-NO3. By compiling and analyzing 1966 observations from 274 N-15-labelled studies, we found a global average of I-NH4 and I-NO3 of 7.41 +/- 0.72 and 2.03 +/- 0.30 mg N kg(-1) day(-1) with a ratio of I-NO3 to I-NH4 (I-NO3:I-NH4) of 0.79 +/- 0.11. Soil I-NH4 and I-NO3 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 I-NH4 and I-NO3, respectively. The structural equation modeling showed that higher soil microbial biomass, total N, pH, and precipitation stimulate I-NH4 and I-NO3 through enhancing GNM and GN. However, higher temperature and soil bulk density suppress I-NH4 and I-NO3 by reducing microbial biomass and total N. Soil I-NH4 varied with terrestrial ecosystems, being greater in grasslands and forests, which have higher rates of GNM, than in croplands. The highest I-NO3:I-NH4 was observed in croplands, which had higher rates of GN. The global average of GN to I-NH4 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.