Background/Question/Methods Nitrogen (N) deposition is known to impact carbon (C) dynamics in temperate ecosystems, but less is know about the effects of added N in tropical forests, where N is not generally limiting to plant growth. We examined changes in soil C dynamics with N fertilization in two tropical forest types (premontane and montane) in the Luquillo Mountains, Puerto Rico. We hypothesized that increased N would accelerate the decomposition of labile C pools, while decreasing losses of more recalcitrant C compounds. We measured bulk soil C, C fractions, C:N, and ¹³C NMR as measures of C content and chemical properties in fertilized and control plots. Fluxes of dissolved organic C (DOC) and soil respiration were measured in the field and in a lab incubation. As indices of microbial activity, we measured hydrolytic enzymes, which degrade labile C substrates, and oxidative enzymes that degrade more recalcitrant compounds. 
Results/Conclusions After 3.5 years of N fertilization, plots with added N had higher C content (42.3 ± 6.8 and 40.7 ± 4.7 g/cm², premontane and montane respectively) than control plots (34.2 ± 5.9 and 34.3 ± 1.3 g/cm²) at 0 – 10 cm depth. We measured higher DOC production in the top 10 cm of fertilized soils for both forest types, yet lower DOC fluxes at 40 cm, indicating increased movement of C into soils from the litter layer. Field soil respiration measurements indicated suppressed respiration in fertilized plots for both forest types, with the strongest difference in the montane forest. In the laboratory incubation, only the premontane forest maintained lower CO₂ fluxes from fertilized soil cores. Soil enzyme activities increased in fertilized plots of both forest types for hydrolytic enzymes, and there was a trend toward decreased oxidative enzyme activity (n.s.). Analysis of soil C fractions showed a decrease in the total mass of labile and occluded C fractions of fertilized soils, which concurs with the increase in hydrolytic enzyme activity. We measured increased C content of the mineral-associated pool, perhaps due to increased adsorption of C on mineral surfaces. The ¹³C – NMR analysis of the labile and occluded C pools demonstrated an increase in the alkyl:O-alkyl ratio for the montane forest, indicating increased humification of these pools. Together, these results indicate that the retention of more recalcitranct soil C compounds outweighs increased decomposition of labile C with N additions in tropical forests, leading to net increases in soil C content.