Background/Question/Methods
Nitrogen (N) deposition is known to impact decomposition in temperate ecosystems, but less is know about the effects of added N in tropical forests, where background soil N availability is relatively high. We examined changes in litter decomposition and microbial community characteristics with N fertilization in two tropical forest types (lower and upper elevation montane) in the Luquillo Mountains, Puerto Rico. We hypothesized that increased N would accelerate initial decomposition rates, while slowing later stages of decomposition. We predicted a decline in microbial biomass with N fertilization, with decreased abundance and activity of fungal decomposers. We measured decomposition rates over two years for mixed native litter and a common substrate. As indices of microbial activity, we measured hydrolytic enzymes that degrade simpler C substrates, and oxidative enzymes that degrade more complex compounds, in decomposing litter over the two years. Microbial community characteristics were assessed in litter and soils using 32 common microbial lipids from phospholipid fatty acid analysis (PLFA). Results/Conclusions
Decomposition rates for the common substrate were higher in fertilized versus control plots in the lower elevation forest. In this forest, decomposition rate constants (k values) for the common substrate were 1.6 ± 0.2 in fertilized plots versus 0.7 ± 0.2 in control plots (per year, mean ± one s.e., n = 3). In the upper elevation forest, k values were 0.5 ± 0.1 for both treatments. Decomposition rates for mixed native litter were more variable and did not show trends with fertilization treatment. In general, hydrolytic enzyme activities were elevated in N addition plots, while oxidative enzyme activities did not change or declined. The PLFA analysis showed evidence of a shift in microbial community characteristics with fertilization. Background microbial biomass was 50 % higher in the lower elevation forest than the upper forest, and there was a trend toward increased microbial biomass with N fertilization at both sites. A number of specific lipids increased in biomass with fertilization in the montane forest, including 16:1 ω5c (mychorrizal) and  18:1 ω7c (fungal), while the most notable change in the lower elevation was an increase in 16:0 (general microbial marker). These results indicate that microbial community and decomposer activity are sensitive to increased N in these tropical forests, and may be linked to increased decomposition rates.