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$$ {I}_{{\mathrm{NH}}_4} $$) and nitrate (INO3$$ {I}_{{\mathrm{NO}}_3} $$), 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$$ {I}_{{\mathrm{NH}}_4} $$ (GN/INH4$$ {I}_{{\mathrm{NH}}_4} $$) and of soil nitrate to ammonium (NO3-/NH4+) less than one, but was leaky in subtropical forests with GN/INH4$$ {I}_{{\mathrm{NH}}_4} $$ 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$$ {I}_{{\mathrm{NH}}_4} $$ were positively correlated with silt and clay contents, while INO3$$ {I}_{{\mathrm{NO}}_3} $$ 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$$ {I}_{{\mathrm{NH}}_4} $$, NO3-/NH4+, and nitrous oxide emissions and the decrease in INO3$$ {I}_{{\mathrm{NO}}_3} $$ 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.

Automated summary

Tropical and subtropical forest biomes have different soil nitrogen cycle patterns, with tropical forests being more conservative and subtropical forests being more leaky. Soil nitrogen dynamics in tropical forests are more sensitive to nitrogen deposition, which leads to increased nitrogen losses and decreased soil nitrogen retention. Dominant tree species also affect soil nitrogen cycling patterns in these biomes.