『Abstract
Anthropogenic nitrogen (N) deposition id 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 addition, and can drive increase in bulk soil C stocks in tropical
forests.
Keywords: Aboveground biomass; Dissolved organic carbon; Nutrient
limitation; Roots; Soil density fractions; Soil respiration』
Introduction
Methods
Study site
Aboveground carbon
Belowground carbon
Soil carbon fractions
Statistical analyses
Results
Aboveground responses to N fertilization
Response of soil C pools and fluxes
Changes in soil C and N concentrations
Bulk soil C content
Soil density fractions
Soil respiration and fine roots
Dissolved organic carbon
Discussion
Conclusions
Acknowledgements
Open access
Appendix 1
Appendix 2
Appendix 3
References