Role of DIN and DON in nitrogen cycle in a humid subtropical forest catchment in northeast Taiwan

Although global models of nitrogen (N) cycling typically focus on nitrate of ecosystem N saturation, dissolved organic nitrogen (DON) is the dominant form of nitrogen export from many watersheds. In previous hypotheses, DON dynamics in the watersheds was treated as being functionally equivalent to inorganic N forms. However, unlike inorganic N, the dynamics of N contained within organic molecules is controlled not only by direct biological demand for N, but also by heterotrophic demand for the reduced C, to which N is attached. During 2016-2018, we evaluated the DON release hypothesis and the passive carbon vehicle hypothesis by comparing streamwater DON, DOC, and DIN concentrations across Fushan experimental forested watershed in the northeast Taiwan. We found that (1) the export of the Fushan Experimental Forest (FEF) is N saturated and (2) the altering nature of the DON release hypothesis and passive carbon vehicle hypothesis between non-event days and typhoon events. Results show that DON concentrations change systematically with increasing nitrate concentrations in all surveys. Among which, DON concentration correlates negatively with nitrate concentration in non-event days but positively during typhoon events. Our results support the coupling between DIN, DON, and DOC concentrations in forested watersheds that are subject to high rates of anthropogenic N loading. In non-event days, the N-containing dissolved organic matter may be in a labile form of carbon. Thus, alleviating heterotrophic N limitation may result in a decrease in DON export (passive carbon vehicle hypothesis), while during typhoon events, DON losses increase as demand for labile N forms attenuates (DON release hypothesis). These hypotheses are not mutually exclusive but represent the potentially contrasting roles of DON within C and N cycles. Our study suggests that bioavailability assays and addition experiments will present variations in the direct biological demand for N and heterotrophic demand for the reduced C, which is informative and necessary for characterizing the processes controlling DON export.

Keywords: DON, DIN, N saturation, DON release hypothesis, passive carbon vehicle hypothesis