Daniela Cusack and Whendee Silver
Anthropogenic nitrogen deposition is increasing in tropical regions and has the potential to alter the carbon and nitrogen biogeochemistry in these ecosystems. This work focuses on three main questions related to the effects of increased nitrogen availability: 1) Does increased mineral nitrogen have the potential to alter carbon storage in tropical forests? 2) How do biotic (plant and microbe) and abiotic (mineral adsorption, pH) mechanisms interact to influence net changes in soil carbon storage with nitrogen additions? 3) How will nitrogen deposition impact natural sources of nitrogen in nitrogen-rich tropical forests? The study utilized a long-term nitrogen fertilization experiment in a lower elevation rainforest and a lower montane wet forest in Puerto Rico that differ in climate. Carbon and nitrogen pools were measured in plants and soils in treatment and control plots over time. Soil carbon pools were measured using three density fractions. Microbial community characteristics and enzyme activities, soil respiration, and biological nitrogen fixation were measured in the field and in laboratory incubations to estimate patterns and controls on carbon and nitrogen fluxes.
Nitrogen fertilization increased bulk soil carbon storage for both of these tropical forests. The majority of additional carbon was stored in the mineral-associated soil fraction. The increase in carbon storage was accompanied by decreased losses of carbon via respiration from soils, and decreased live fine root biomass. The microbial community responded to nitrogen additions differently in the two forests. In the lower elevation forest, bacterial dominance of the microbial community increased, whereas fungal biomass and abundance increased in the upper elevation forest. A laboratory soil incubation experiment showed that long-term nitrogen fertilization led to smaller biologically active soil carbon pools, and longer turnover times of slowly-cycling carbon pools. Finally, nitrogen additions displaced natural inputs of nitrogen via biological nitrogen fixation to these nitrogen-rich tropical forests. The data presented here clearly indicate that nitrogen and carbon biogeochemical cycles can be highly sensitive to increased nitrogen availability in nitrogen-rich tropical forests. Nitrogen deposition in tropical forests has the potential to increase soil carbon storage, with implications for global carbon cycling and climate change.