Abstract

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 Nrich
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.

Citation

Cusack, D. F., Silver, W. L., and McDowell, W. H. (2011): Effects of chronic nitrogen additions on above- and belowground carbon dynamics in two tropical forests. Biogeochemistry. DOI: 10.1007/s10533-010-9496-4