Transport of riverine dissolved carbon (including DOC and DIC) is a crucial process linking terrestrial and aquatic C reservoirs, but has rarely been examined in subtropical small mountainous rivers (SMRs). This study monitored DOC and DIC concentrations on a biweekly basis during non-event flow periods and at 3 h intervals during two typhoon events in three SMRs in southwestern Taiwan between January 2014 and August 2016. Two models, HBV (the Hydrologiska Byråns Vattenbalansavdelning model) and a three-endmember mixing model, were applied to determine the quantities of DOC and DIC transport from different flow paths. The results show that the annual DOC and DIC fluxes were 2.7–4.8 and 48.4–54.3 t C km⁻² yr⁻¹, respectively, which were approx. 2 and 20 times higher than the global mean of 1.4 and 2.6 t C km⁻² yr⁻¹, respectively. The DIC ∕ DOC ratio was 14.08, which is much higher than the mean of large rivers worldwide (1.86), and indicates the high rates of chemical weathering in this region. The two typhoons contributed 12 %–14 % of the annual streamflow in only 3 days (about 1.0 % of the annual time), whereas 15.0 %–23.5 % and 9.2 %–12.6 % of the annual DOC and DIC flux, respectively, suggested that typhoons play a more important role in DOC transport than DIC transport. The endmember mixing model suggested that DOC and DIC export was mainly from surface runoff and deep groundwater, respectively. The unique patterns seen in Taiwan SMRs characterized by high dissolved carbon flux, high DIC ∕ DOC ratio, and large transport by intense storms should be taken into consideration when estimating global carbon budgets.

Human-induced excess nitrogen outflowing from land through rivers to oceans has resulted in serious impacts on terrestrial and coastal ecosystems. Oceania, which occupies < 2.5% of the global land surface, delivers 12% of the freshwater and dissolved materials to the ocean on a global scale. However, there are few empirical data sets on riverine dissolved inorganic nitrogen (DIN) fluxes in the region, and their dynamics are poorly understood. In this study, a river monitoring network covering different types of land uses and population densities was implemented to investigate the mechanism of DIN export. The results show that DIN concentration/yield varied from ∼20 μM/∼300 kg-N km⁻² yr⁻¹ to ∼378 μM/∼10 000 kg-N km⁻² yr⁻¹ from the relatively pristine headwaters to the populous estuary. Agriculture and population density control DIN export in less densely populated regions and urban areas, respectively, and runoff controls DIN at the watershed scale. Compared to documented estimates from global models, the observed DIN export from the Danshui River is 2.3 times larger, which results from the region-specific response of DIN yield to dense population and abundant runoff. The dominating DIN species change gradually from NO₃⁻ in the headwaters (∼97%) to NH₄⁺ in the estuary (∼60%) following the urbanization gradient. The prominent existence of NH₄⁺ is probably the result of the anaerobic water body and short residence time, unlike in large river basins. Given the analogous watershed characteristics of the Danshui River to the rivers in Oceania, our study could serve as a first example to examine riverine DIN fluxes in Oceania.

Increases in nitrogen (N) availability and mobility resulting from anthropogenic activities have substantially altered the N cycle, both locally and globally. Taiwan characterized by the subtropical montane landscape with abundant rainfall, downwind of the most rapidly industrializing eastern coast of China, can be a demonstration site for extremely high N input and riverine DIN (dissolved inorganic N) export. We used 49 watersheds with similar climatic and landscape settings but classified into low, moderate, and highly disturbed categories based on population density to illustrate their differences in nitrogen inputs (through atmospheric N deposition, synthetic fertilizers, and human emission) and DIN export ratios. Our results showed that the island-wide average riverine DIN export is ∼ 3800 kg N km⁻² yr⁻¹, approximately 18 times the global average. The average riverine DIN export ratios are 0.30–0.51, which are much higher than the averages of 0.20–0.25 of large rivers around the world, indicating excessive N input relative to ecosystem demand or retention capacity. The low disturbed watersheds have a high N retention capacity and DIN export ratios of 0.06–0.18 in spite of the high N input (∼ 4900 kg N km⁻² yr⁻¹). The high retention capacity is likely due to effective uptake by secondary forests in the watersheds. The moderately disturbed watersheds show a linear increase in DIN export with increases in total N inputs and mean DIN export ratios of 0.20 to 0.31. The main difference in land use between low and moderately disturbed watersheds is the greater proportion of agricultural land cover in the moderately disturbed watersheds. Thus, their greater DIN export could be attributed to N fertilizers used in the agricultural lands. The greater export ratios also imply that agricultural lands have a lower proportional N retention capacity and that reforestation could be an effective land management practice to reduce riverine DIN export. The export ratios of the highly disturbed watersheds are very high, 0.42–0.53, suggesting that much of the N input is transported downstream directly, and urges the need to increase the proportion of households connected to a sewage system and improve the effectiveness of wastewater treatment systems. The increases in the riverine DIN export ratio along the gradient of human disturbance also suggest a gradient in N saturation in subtropical Taiwan. Our results help to improve our understanding of factors controlling riverine DIN export and provide empirical evidence that calls for sound N emission/pollution control measures.

Riverine dissolved organic carbon (DOC), responsible for riverine productivity, is rarely documented in subtropical small mountainous rivers (SMRs) where high rainfall and steep slopes are the main features. This study investigated the DOC export at eight sites in three Taiwan SMRs to characterize the dynamics and controlling factors of DOC transport. Results showed that the mean DOC concentration of ~0.78 mg L⁻¹ is much lower than the global average of ~5.29 mg L⁻¹. However, the mean DOC yield, ~22.51 kg-C ha⁻¹ yr⁻¹, is higher than the global average of 14.4–19.3 kg-C ha⁻¹ yr⁻¹. Comparing with worldwide rivers from literature, the annual discharge, slope, and SOC (soil organic carbon) are controlling factors as expected, though they influence in different ways. SOC stock likely regulated by elevation-dependent biomes dominate the DOC supply, while slope restrains the DOC generation due to shallow soil depth and fast runoff velocity. However, the abundant discharge flushing this persistent low supply leads to a large DOC export in the SMRs. Furthermore, the DOC dynamics during typhoon periods showed a clockwise hysteresis, suggesting that the DOC is mainly from the riparian zone or downslope area during the rising limb of the hydrograph. This study elucidates the DOC transport in SMRs and provides an atypical yet significant piece of understanding on DOC transport in a global context.