Global soil nitrogen cycle pattern and nitrogen enrichment effects: Tropical versus subtropical forests

Tropical and subtropical forest biomes are a main hotspot for the global nitrogen (N) cycle. Yet, our understanding of global soil N cycle patterns and drivers and their response to N deposition in these biomes remains elusive. By a meta‐analysis of 2426‐single and 161‐paired observations from 89 published 15 N pool dilution and tracing studies, we found that gross N mineralization (GNM), immobilization of ammonium (INH4) and nitrate (INO3), and dissimilatory nitrate reduction to ammonium (DNRA) were significantly higher in tropical forests than in subtropical forests. Soil N cycle was conservative in tropical forests with ratios of gross nitrification (GN) to INH4 (GN/INH4) and of soil nitrate to ammonium (NO3−/NH4+) less than one, but was leaky in subtropical forests with GN/INH4 and NO3−/NH4+ higher than one. Soil NH4+ dynamics were mainly controlled by soil substrate (e.g., total N), but climatic factors (e.g., precipitation and/or temperature) were more important in controlling soil NO3− dynamics. Soil texture played a role, as GNM and INH4 were positively correlated with silt and clay contents, while INO3 and DNRA were positively correlated with sand and clay contents, respectively. The soil N cycle was more sensitive to N deposition in tropical forests than in subtropical forests. Nitrogen deposition leads to a leaky N cycle in tropical forests, as evidenced by the increase in GN/INH4, NO3−/NH4+, and nitrous oxide emissions and the decrease in INO3 and DNRA, mainly due to the decrease in soil microbial biomass and pH. Dominant tree species can also influence soil N cycle pattern, which has changed from conservative in deciduous forests to leaky in coniferous forests. We provide global evidence that tropical, but not subtropical, forests are characterized by soil N dynamics sustaining N availability and that N deposition inhibits soil N retention and stimulates N losses in these biomes. We provide a comprehensive analysis of the patterns and drivers of soil gross N transformation rates in tropical and subtropical forests. We found that soil N cycle was conservative in tropical forests but was leaky in subtropical forests. The soil N cycle was more sensitive to N deposition in tropical forests than in subtropical forests. Nitrogen deposition leads to a leaky N cycle in tropical forests. We provide global evidence that tropical, but not subtropical, forests are characterized by soil N dynamics sustaining N availability and that N deposition inhibits so