Effects of chronic nitrogen additions on above- and belowground carbon dynamics in two tropical forests

TitleEffects of chronic nitrogen additions on above- and belowground carbon dynamics in two tropical forests
Publication TypeJournal Article
Year of Publication2011
AuthorsCusack, DF, Silver, WL, Torn, MS, William H. McDowell
Accession NumberLUQ.1018
KeywordsAboveground biomass, Dissolved organic carbon, nutrient limitation, Roots, Soil density fractions, soil respiration

Anthropogenic nitrogen (N) deposition is increasing rapidly in tropical regions, adding N to ecosystems that often have high background N availability. Tropical forests play an important role in the global carbon (C) cycle, yet the effects of N deposition on C cycling in these ecosystems are poorly understood. We used a field N-fertilization experiment in lower and upper elevation tropical rain forests in Puerto Rico to explore the responses of above- and belowground C pools to N addition. As expected, tree stem growth and litterfall productivity did not respond to N fertilization in either of these N-rich forests, indicating a lack of N limitation to net primary productivity (NPP). In contrast, soil C concentrations increased significantly with N fertilization in both forests, leading to larger C stocks in fertilized plots. However, different soil C pools responded to N fertilization differently. Labile (low density) soil C fractions and live fine roots declined with fertilization, while mineral-associated soil C increased in both forests. Decreased soil CO2 fluxes in fertilized plots were correlated with smaller labile soil C pools in the lower elevation forest (R2 = 0.65, p < 0.05), and with lower live fine root biomass in the upper elevation forest (R2 = 0.90, p < 0.05). Our results indicate that soil C storage is sensitive to N deposition in tropical forests, even where plant productivity is not N-limited. The mineral-associated soil C pool has the potential to respond relatively quickly to N additions, and can drive increases in bulk soil C stocks in tropical forests.