Masanori Katsuyama

Determining the sources of stormflow from the fluorescence properties of dissolved organic carbon in a forested headwater catchment

Abstract Concentrations of SiO 2 and dissolved organic carbon, as well as the fluorescence properties of the latter were used as tracers of stormflow sources in a forested headwater catchment in Japan. Separate analyses were made of throughfall and of groundwater in the saturated and transient saturated zones, using three-dimensional fluorescence spectrometry. Groundwater in the saturated zone showed almost no fluorescence, whereas groundwater from the transient saturated zone showed fluorescence patterns characteristic of fulvic acid. Throughfall showed fluorescence of a non-fulvic character. Stormflow water showed fulvic-type fluorescence, showing that groundwater from the transient saturated zone contributed 62.7% of the total discharge. The source area for transient saturated zone groundwater accounted for less than 1% of the catchment area during the storm event. The contribution of the riparian zone to the storm runoff was important, although its source area was also very small.

Distribution and characteristics of molecular size fractions of freshwater-dissolved organic matter in watershed environments: its implication to degradation

Distributions of molecular size and fluorescence properties of dissolved organic matter (DOM) in the Lake Biwa and Lake Baikal watersheds were investigated using the cross-flow ultrafiltration technique and three-dimensional fluorescence measurements. From the fluorescence properties, protein-like substances were usually found in the 0.1 μm-GF/F fraction (the Durapore membrane retentate of the GF/F filtrate) of the lake DOM. The results indicated autochthonous production of protein-like organic-matters in the lake environment. Fulvic acid (FA)-like components were composed of two fractions with respect to fluorescence properties and molecular size. Two FA-like fluorescence peaks, which showed different fluorescence peak positions in the excitation-emission matrix (EEM), were partly fractionated by the molecular size of 5000 daltons (5 kDa). The FA-like fluorescence peak position of the <5-kDa fraction was observed at the shorter wavelength region compared with that of the fraction between 5 kDa and 0.1 μm (5 kDa20.1 μm fraction). A blue shift of the FA-like fluorescence peak position as well as a decrease in the molecular size of the DOM was observed in lake samples. The relative contribution of the <5 kDa fraction to the DOC concentration was high in lake waters (68%–79%) compared with river waters (44%–68%), suggesting characteristic changes in molecular size between riverine and lacustrine DOM. DOM of the 5 kDa–0.1 μm fraction was relatively higher in river waters than in lake waters. These findings coincided with in situ distributions of the fluorescence properties and molecular size of DOM found in both stream and lake environments. These results indicate that FA-like substances from forested watersheds are decomposed qualitatively and quantitatively in the river-lake environment by photochemical and biological processes.

Elucidation of the relationship between geographic and time sources of stream water using a tracer approach in a headwater catchment

[1] Tracer approaches have been used worldwide to clarify time and geographic sources of stream water in catchments, although the relationship between these two sources is poorly discussed. We considered the mean residence time (MRT) and its spatial distribution to determine the relationship between geographic source components and time source components. There were clear differences in solute concentrations and MRT among shallow, middle, and bottom layers along the vertical profile of the riparian groundwater body. Those in the stream water were intermediate compared to those in the shallow and middle layers; thus, the consistent geographic sources were groundwater in these layers. In the context of end-members mixing analysis (EMMA), however, the end-members were rainfall, hillslope groundwater, and riparian groundwater in the bottom layer. The other riparian groundwaters were a mixture of end-members. The discrepancy between the geographic sources and the end-members was resolved by considering the MRT, the time required to move the end-member or geographic source within the catchment. Our approach clarified the relationships among the geographic sources, time sources, and hydrological pathways, which are the essential factors of runoff generation processes and hydrochemical processes. Therefore, to go beyond previous applications of EMMA on the basis of systematic learning from observed data, it is insightful to combine the common approaches to analyze landscape heterogeneity and process complexity and to reconsider the framework of the hydrobiogeochemical models in various regions and at multiple spatial scales.

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

Here 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.

Interactive responses of dissolved sulfate and nitrate to disturbance associated with pine wilt disease in a temperate forest

Abstract To examine the effects of pine wilt disease on SO4 2− dynamics in a forested ecosystem, we analyzed the soil solution, groundwater, and streamwater in the Kiryu Experimental Basin, central Japan. The NO3 − concentrations in the soil surface layer showed a remarkable seasonality, with peak concentration in the fall. The SO4 2− concentrations in an area affected by pine wilt disease increased after NO3 − concentrations peaked. The delay between maximum SO4 2− to NO3 − concentrations may be explained by anion adsorption on variable charges in humus under low pH conditions resulting from nitrification. Concentrations of SO4 2− increased with groundwater depth, while the levels of NO3 − tended to decrease with groundwater depth. The vertical distribution of the SO4 2− and NO3 − concentrations in groundwater affected the seasonal changes in stream SO4 2− and NO3 − concentrations, as groundwater levels changed. It is reasonable to assume that nitrogen dynamics and hydrological processes play important roles in the retention and discharge of SO4 2− from disturbed forest soil systems.

Stream Runoff and Nitrate Recovery Times After Forest Disturbance in the USA and Japan

To understand mechanisms of long-term hydrological and biogeochemical recovery after forest disturbance, it is important to evaluate recovery times (i.e., time scales associated with the return to baseline or predisturbance conditions) of stream runoff and nitrate concentration. Previous studies have focused on either the response of runoff or nitrate concentration, and some have specifically addressed recovery times following disturbance. However, controlling factors have not yet been elucidated. Knowing these relationships will advance our understanding of each recovery process. The objectives of this study were to explore the relationship between runoff and nitrate recovery times and identify potential factors controlling each. We acquired long-term runoff and stream water nitrate concentration data from 20 sites in the USA and Japan. We then examined the relationship between runoff and nitrate recovery times at these multiple sites and use these relationships to discuss the ecosystem dynamics following forest disturbance. Nitrate response was detected at all study sites, while runoff responses were detected at all sites with disturbance intensities greater than 75% of the catchment area. The runoff recovery time was significantly correlated with the nitrate recovery time for catchments that had a runoff response. For these catchments, hydrological recovery times were slower than nitrate recovery times. The relationship between these two recovery times suggests that forest regeneration was a common control on both recovery times. However, the faster recovery time for nitrate suggests that nitrogen was less available or less mobile in these catchments than water.

N2O production by denitrification in an urban river: evidence from isotopes, functional genes, and dissolved organic matter

Rivers are important sources of N2O emissions into the atmosphere. Nevertheless, N2O production processes in rivers are not well identified. We measured concentrations and isotopic ratios of N2O, NH4+, NO2−, and NO3− in surface water to identify the microbial processes of N2O production along the Tama River in Japan. We also measured the functional gene abundance of nitrifiers and denitrifiers (amoA-bacteria, nirK, nirS, nosZ clade I, nosZ clade II) together with concentrations of dissolved organic carbon (DOC) and fluorescence intensities of protein and humic components of dissolved organic matter (DOM) to support the elucidation of N2O production processes. The observed nitrogen (δ15N) and oxygen (δ18O) of N2O were within the expected isotopic range of N2O produced by nitrate reduction, indicating that N2O was dominantly produced by denitrification. The positive significant correlation between N2ONet concentration and nirK gene abundance implied that nitrifiers and denitrifiers are contributors to N2O production. Fluorescence intensities of protein and humic components of DOM and concentrations of DOC did not show significant correlations with N2O concentrations, which suggests that DOC and abundance of DOM components do not control dissolved N2O. Measurement of isotope ratios of N2O and its substrates was found to be a useful tool to obtain evidence of denitrification as the main source of N2O production along the Tama River.

Evapotranspiration and water source of a tropical rainforest in peninsular Malaysia

Abstract To evaluate water use and the supporting water source of a tropical rainforest, a 4‐year assessment of evapotranspiration (ET) was conducted in Pasoh Forest Reserve, a lowland dipterocarp forest in Peninsular Malaysia. The eddy covariance method and isotope signals of rain, plant, soil, and stream waters were used to determine forest water sources under different moisture conditions. Four sampling events were conducted to collect soil and plant twig samples in wet, moderate, dry, and very dry conditions for the identification of isotopic signals. Annual ET from 2012 to 2015 was quite stable with an average of 1,182 ± 26 mm, and a substantial daily ET was observed even during drought periods, although some decline was observed, corresponding with volumetric soil water content. During the wet period, water for ET was supplied from the surface soil layer between 0 and 0.5 m, whereas in the dry period, approximately 50% to 90% was supplied from the deeper soil layer below 0.5‐m depth, originating from water precipitated several months previously at this forest. Isotope signatures demonstrated that the water sources of the plants, soil, and stream were all different. Water in plants was often different from soil water, probably because plant water came from a different source than water that was strongly bound to the soil particles. Plants showed no preference for soil depth with their size, whereas the existence of storage water in the xylem was suggested. The evapotranspiration at this forest is balanced and maintained using most of the available water sources except for a proportion of rapid response run‐off.