Stream Water Dissolved Organic Carbon Concentrations Research Articles

Biogeochemical and hydrological controls on carbon export from a forested catchment in central Japan

AbstractHere we review research on the links between hydrological processes and the biogeochemical environment controlling the dynamics of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in temperate forested catchments. In addition, we present the results of original experiments. The spatial and temporal changes in DIC and DOC concentrations were investigated in tandem with observations of elementary belowground hydrological processes for a forested headwater catchment in central Japan. The soil CO2 gas concentration, which is the source of DIC, increased with depth. The hydrological characteristics of groundwater also affected the spatial variation of partial pressure of dissolved CO2 (pCO2) in groundwater. The temporal variations in the soil CO2 gas concentration and the pCO2 values of groundwater suggested that the dynamics of DIC were strongly affected by biological activity. However, the geographical differences in DIC leaching were affected not only by the link between climatological conditions and biological activity, but also by other factors such as geomorphologic conditions. The DOC concentrations decreased with selective removal of hydrophobic acid during vertical infiltration. The major DOC‐removal mechanisms were retention of metal‐organic complexes to soil solids in the upper mineral soil layer and decomposition of DOC in the lower mineral soil layer. The responses of the DIC and DOC concentrations to changes in discharge during storm events were explained by the spatial variation in the DIC and DOC concentrations. Seasonal variation, which represents a long‐term change, in stream water DOC concentrations was affected not only by the temporal variation in DOC concentrations in the topsoil, which may be affected by biological activity, but also by water movement, which transports DOC from the topsoil to stream water. These results indicate that both a biogeochemical approach and a method for evaluating the hydrological effects on carbon dynamics are critical for clarifying the carbon accumulation‐and‐release processes in forested ecosystems.

Biogeochemistry of a Treeline Watershed, Northwestern Alaska

Since 1950, mean annual temperatures in northwestern Alaska have increased. Change in forest floor and soil temperature or moisture could alter N mineralization rates, production of dissolved organic carbon (DOC) and organic nitrogen (DON), and their export to the aquatic ecosystem. In 1990, we began study of nutrient cycles in the 800-ha Asik watershed, located at treeline in the Noatak National Preserve, northwestern Alaska. This paper summarizes relationships between topographic aspect, soil temperature and moisture, inorganic and organic N pools, C pools, CO2 efflux, growing season net N mineralization rates, and stream water chemistry. Forest floor (O2) C/N ratios, C pools, temperature, and moisture were greater on south aspects. More rapid melt of the soil active layer (zone of annual freeze-thaw) and permafrost accounted for the higher moisture. The O2 C and N content were correlated with moisture, inorganic N pools, CO2 efflux, and inversely with temperature. Inorganic N pools were correlated with temperature and CO2 efflux. Net N mineralization rates were positive in early summer, and correlated with O2 moisture, temperature, and C and N pools. Net nitrification rates were inversely correlated with moisture, total C and N. The CO2 efflux increased with temperature and moisture, and was greater on south aspects. Stream ion concentrations declined and DOC increased with discharge. Stream inorganic nitrogen (DIN) output exceeded input by 70%. Alpine stream water nitrate (NO3-) and DOC concentrations indicated substantial contributions to the watershed DIN and DOC budgets.

Modification of Snowmelt Chemistry by Forest Floor and Mineral Soil, Northern Michigan

AbstractPrecipitation, snowpack, snowmelt, forest floor percolate, soil water, and streamwater chemistry were studied throughout winter for 7 yr in a small (176 ha) Northern Michigan watershed vegetated by northern hardwoods. The objective was to see how precipitation and snowmelt chemistry is modified before it reaches the stream. Intermittent snowmelt occurred throughout winter, and 65 to 78% of total snowpack loss of H+, NH+4, NO−3, and SO2−4 occurred before peak snowpack and snowmelt. From 70 to 95% of snowmelt H+, NH6+4, and NO−3 was retained in the forest floor and shallow mineral soil by exchange, adsorption, or biological uptake. Snowmelt SO2−4 concentration was enriched in forest floor percolate and shallow soil water probably by organic mineralization, low adsorption, and perhaps desorption. The significant relationship between dissolved organic carbon (DOC) and SO2−4 concentration in forest floor percolate and shallow soil water indicates organics may contribute to reduced SO2−4 adsorption. Sulfate and organic anions were major contributors to base cation (CB) flux in the forest floor while SO2−4 was related to CB flux in shallow soils. A decline in mineral soil and streamwater DOC concentration and increase in streamwater HCO−3 relative to concentrations in forest floor percolate suggests the relative importance of organic decomposition decreased and soil mineral weathering increased with soil depth. High streamwater Ca2+, Mg2+, and HCO−3 concentrations indicated that soil mineral weathering was a major process altering snowmelt chemistry at the watershed level.

Variability in Removal of Dissolved Organic Carbon in Hyporheic Sediments

Dissolved organic carbon (DOC) is consumed by microbial metabolism as streamwater perfuses through a lateral gravel bar of the East Branch of the Wappinger Creek. The rate of DOC removal was estimated from the decline in DOC and travel time through the bar. Variability in DOC removal together with potential regulatory factors was determined for 14 dates spanning more than a 2-y period. DOC removal was not correlated with temperature, availability of oxygen, or residence time within hyporheic sediments. Hyporheic DOC could be predicted (r² = 0.68) from streamwater DOC concentrations, with a surprisingly constant 57% (±9% [1 SD]) removal of DOC. This pattern suggests an initial concentration-dependent adsorption of DOC onto surfaces. This mechanism allows for efficient retention of DOC within hyporheic sediments even under conditions (low temperature, high interstitial velocity) that might be expected to minimize biotic consumption of DOC.