June 1 2010

12:00 BSRB 154

Ecolunch: Daniela Cusack
UC Santa Barbara

Effects of Chronic Nitrogen Additions on Carbon Cycling in Two Tropical Forests


Anthropogenic nitrogen deposition is increasing in tropical regions, and has the potential to alter the carbon biogeochemistry in these ecosystems. Driving questions of this study were: 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? The study utilized a long-term nitrogen fertilization experiment in a lower elevation rainforest and an upper elevation cloud forest in Puerto Rico. Carbon and nitrogen pools were measured in plants and soils in treatment and control plots over time. Microbial community characteristics, extracellular enzyme activities, and soil respiration were measured in the field and in laboratory incubations to estimate patterns of, and controls on, carbon 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. The data presented here clearly indicate that 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.















































































































































































































































































































































































































































































































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