Role of dissolved organic nitrogen in the nitrogen cycle of a mountainous forest ecosytem in Taiwan

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Dissolved organic nitrogen (DON) plays a prominent role in the N cycle of forest soils under conditions of low N deposition and often represents the dominant form of N in solution. Little is known about the sources, properties, genesis and fate of DON, especially in subtropical forests under high precipitation regime. In field and laboratory studies we plan to investigate these issues in a mountainous Chamaecyparis obtusa var. formosana forest in Taiwan. In previous studies we found the fluxes of dissolved organic carbon (DOC) and DON exceptionally high in this ecosystem. Concentrations and fluxes of DON, DOC and mineral elements will be measured in fortnightly resolution in throughfall, forest floor percolates and seepage. The release and fate of DOC and DON in the soil will be related empirically to temperature, precipitation and throughfall chemistry. Different forest floor layers and decomposing roots will be investigated in laboratory extractions for their role as sources of DON and DOC. The quality of DON from different sources will be evaluated using C/N ratios and the composition of amino acids and amino sugars. In laboratory studies we will investigate the potential formation of DON by reaction of DOC with nitrite, using solutions that contain different qualities of dissolved organic matter. Furthermore, the microbial use and net mineralization of soil derived DON and DOC will be studied in laboratory incubations. Here we use samples from the field and from the laboratory experiments on DON formation by reaction of nitrite. The results are expected to substantially improve our understanding of the N cycle in these forest ecosystems.