Particulate carbon and nitrogen dynamics in a headwater catchment in Northern Thailand: hysteresis, high yields, and hot spots

Abstract

Abstract:Rivers of South and Southeast Asia disgorge large suspended sediment loads, reflecting exceptionally high rates of erosion promoted by natural processes (tectonic and climatic) and anthropogenic (land-use change) activities that are characteristic of the region. While particulate carbon and nitrogen fluxes have been characterized in some large Asian rivers, less is known about the headwater systems where much sediment and organic material are initially mobilized. This study, conducted in the 74-km 2 Mae Sa Experimental Catchment in northern Thailand, shows that the Sa River is an important source for particulate organic carbon (POC) and particulate organic nitrogen (PON) transported to larger river systems and downstream reservoirs. However, the yields during three years of investigation varied greatly: 5.0-22.3 Mg POC km À2 y À1 and 0.48-2.02 Mg PON km À2 y À1. The 22.3 Mg POC km À2 y À1 yield is the highest reported for any river on the Asian continent. Stream samples collected during 12 storms showed that almost 3% of the total suspended solid load is POC 0.7 μm to 2.0 mm in size. This percentage is higher than other values for most large rivers on the continent. Further, we documented a strong pulse hysteretic behaviour in the stream, whereby peak fluxes of POC and PON are often delayed (anticlockwise hysteresis) or accelerated (clockwise hysteresis) relative to stream flow peaks (or are complex), complicating the prediction of storm-based or annual particulate carbon and nitrogen fluxes. Stream turbidity and total suspended sediment are reasonable proxies for POC and PON concentrations, while stream discharge is not a good predictor variable. Observed C:N ratios for measured particulate samples range from 3 to 83, with the high-end values likely associated with fresh (non-decomposed) vegetative material greater than 2 mm in diameter. The C:N ratio (weighted based on three sediment sizes) for 12 events ranges from 7.5 to 15.3. These modest values reflect the relatively low C:N ratios for small size fractions (0.7-0.63 μm) that comprise 50-90% of the TSS load in the events. Overall, organic material <0.63 μm contribute about 75% of the total POC load and 80% of the PON load. The annual C:N ratio for the river is approximately 10-11. Collectively, our findings indicate the occasionally high yields make the Sa River-and potentially other similar headwater rivers-a hot spot for POC and PON transported to downstream water bodies. Complex hysteresis patterns and high year-to-year variability hinders our ability to calculate and predict these yields without continuous, automated monitoring of discharge and turbidity.

Dynamics and fluxes of organic carbon and nitrogen in two Guiana Shield river basins impacted by deforestation and mining activities

“…Similarly, the PN concentrations correlate significantly with POC in the Oyapock ( r = 0.51, p < .05) and in the Maroni ( r = 0.69, p < .001). Although the range of PN concentrations was greater in the Maroni than that found in the Oyapock; both rivers present very low concentrations of PN in comparison to the range (0.2–29.7 mgN L −1 ) observed in rivers of Southern and South‐eastern Asia (Ziegler et al, ), reflecting the exceptionally high erosion rates promoted by natural processes (tectonic and climatic events) and anthropogenic activities (land use change) in these regions of Asia. Indeed, the fluvial PN reveals the quality of the water as a signal of land‐cover/land‐use changes and other anthropogenic activities that accelerate erosion rates and affect soil quality.…”

Particulate Organic Matter Composition in Stream Runoff Following Large Storms: Role of POM Sources, Particle Size, and Event Characteristics

“…The composition of stream POM during storms is dictated by the types of sources and their composition and the mobilization of the sources by hydrologic flow paths. Storm event characteristics such as magnitude, intensity, antecedent moisture conditions, and seasonal timing exert a strong control on the hydrology and flow paths and the subsequent sources that are intercepted/mobilized (Dhillon & Inamdar, ; Goñi et al, ; Jung et al, ; Ziegler et al, ). In this study, summer storms were convective systems with high intensity and short duration, while winter events were generally frontal storm systems with longer duration, though they also produced periods of high intensity.…”

Particulate Organic Carbon in the Tropical Usumacinta River, Southeast Mexico: Concentration, Flux, and Sources

“…Similar findings have been cited in other tropical rivers (e.g., [15,18]). For example, some Asian rivers draining through highly eroded regions exhibited a POC/TSS ranging between 0.2 and 3.6%, reflecting, in the same way, the terrestrial sedimentary origin [13,57,61].…”

“…Seasonal hydrologic variation often guides POC supply and flux in tropical rivers. While low-water discharge conditions enhance autochthonous production and decrease POC flux [12], high-water discharge conditions increase POC flux with the allochthonous terrestrial matter via erosion [13,14]. However, the seasonal effect on the flux and source of POC is not always clear in tropical rivers, as in some mountainous watersheds [15].…”