Nobuhito Ohte

Residence times and flow paths of water in steep unchannelled catchments, Tanakami, Japan

Apparatus for moulding and shaping the opening of a container in which a cap is inserted in such a fashion that a portion thereof is gripped by and embraces a particularly smooth and polished portion of the neck of the container to form an air-tight seal.

Sources, transformations, and hydrological processes that control stream nitrate and dissolved organic matter concentrations during snowmelt in an upland forest

We explored catchment processes that control stream nutrient concentrations at an upland forest in northeastern Vermont, USA, where inputs of nitrogen via atmospheric deposition are among the highest in the nation and affect ecosystem functioning. We traced sources of water, nitrate, and dissolved organic matter (DOM) using stream water samples collected at high frequency during spring snowmelt. Hydrochemistry, isotopic tracers, and end-member mixing analyses suggested the timing, sources, and source areas from which water and nutrients entered the stream. Although stream-dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) both originated from leaching of soluble organic matter, flushing responses between these two DOM components varied because of dynamic shifts of hydrological flow paths and sources that supply the highest concentrations of DOC and DON.

Comparative Evaluation on Nitrogen Saturation of Forest Catchments in Japan and Northeastern United States

To analyze the differences in the status and processes of nitrogen saturation in Japan and northeastern United States, we examined the hydrobiogeochemistry of nitrogen of forested watersheds in these regions. Two distinct differences were found between watersheds in Japan compared with those in US. 1) In Japanese watersheds, marked decreases of NO3− concentration in surface waters during the summer growing season were not found and NO3− concentrations sometimes increased especially in the summer at nitrogen saturated sites. This contrast with watersheds in US where decreases in NO3− concentration during the summer are commonly observed except those watersheds in advanced stages of nitrogen saturation. These differences in NO3− concentration relationships can be attributed to climatic differences, with Japan having high precipitation and high discharge during the summer, while in many regions of North America lowest discharges are found in the summer. The climatic regime in Japan leads to high rates of mineralization and the rapid transport of NO3− to streams in summer. 2) Japanese watersheds, even those with high NO3− concentrations in surface waters, show little evidence of acidification. This is in contrast to sites in US where increased NO3− concentrations, especially during episodic events, result in surface water acidification.

Hydrologic and geochemical influences on the dissolved silica concentration in natural water in a steep headwater catchment

The dissolved silica concentrations in groundwater, springwater, and streamwater were measured on an unchanneled hillslope in the Tanakami Mountains of central Japan. The effects of preferential flowpaths, including lateral and vertical flow in the soil layer and flow through bedrock fractures, on the variation in the dissolved silica concentrations in runoff and groundwater were examined, as were the effects of the mixing of water from geochemically diverse water sources on the dissolved silica concentrations. The mean dissolved silica concentrations in water sampled from 40 cm below the soil surface and in transiently formed groundwater above the soil-bedrock interface during rainfall events were relatively constant, independent of the variation in the mixing ratio of pre-event water and incoming throughfall. These waters were mostly supplied by the vertical infiltration of water in soil. The mean dissolved silica concentrations were similar, regardless of sampling depth, although the mean residence time of the water increased with depth. These results indicated that the dissolved silica concentrations in soil water and transient groundwater were independent of contact time between the water and minerals. The mean dissolved silica concentration in perennially saturated groundwater above the soil-bedrock interface, which was recharged by water infiltrating through soil, and water emerging from bedrock in an area near the spring was more than twice that of transient groundwater, and the variation was relatively large. The mean dissolved silica concentration increased significantly downslope, from perennial groundwater to spring from soil matrix to stream, and the spring and stream concentrations also showed large variations. The dissolved silica concentration was highest in the spring from a bedrock fracture and was relatively constant. The mixing of water from two geochemically diverse water sources, soil and bedrock, controlled the dissolved silica concentrations of the perennial groundwater, the spring from soil matrix, and the stream. Our results demonstrated that in most areas of this headwater catchment, the preferential flowpaths have only a small effect on the dissolved silica concentrations. In a small area, which was < 2% of the total catchment area near the spring, the dissolved silica concentration was controlled by the mixing of water from geochemically diverse water sources.

Biogeochemical Influences on the Determination of Water Chemistry in a Temperate Forest Basin: Factors Determining the pH value

In order to clarify the mechanism of pH determination in a temperate forest watershed in Japan, intensive hydrochemical observations that included in situ measurement of dissolved pCO2 were carried out in 1991 and 1992. From the variations of observed pCO2 and pH and estimated alkalinity associated with the hydrological process, the factors determining pH were described. There were two hydrological processes which have different determining hydrochemical processes: (1) rainfall and throughfall to infiltration in the soil layer to stable groundwater and (2) stable groundwater to spring water to stream water. In the first process, pH is influenced by infiltration from the low pCO2 layer to the high CO2 layer and by an increase of alkalinity, which is mainly caused by an exchange reaction and chemical weathering. In the shallow soil layer the protons for alkalinity generation are supplied by acid deposits from rainfall and throughfall, microbial acid production, and CO2 dissolution reaction. In the deeper layer an increase of alkalinity caused by Na+ generation becomes remarkable as depth increases. This process is strongly controlled by chemical weathering. In the second process, pH increases with CO2 degassing around the spring point. The alkalinity is kept at the same level as that of the stable groundwater. These results suggests that the biochemically supplied CO2 in soil not only directly controls the pH determination, but also has influences on the alkalinity generation as another determining factor of pH.

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.

Nematomorph parasites indirectly alter the food web and ecosystem function of streams through behavioural manipulation of their cricket hosts

Nematomorph parasites manipulate crickets to enter streams where the parasites reproduce. These manipulated crickets become a substantial food subsidy for stream fishes. We used a field experiment to investigate how this subsidy affects the stream community and ecosystem function. When crickets were available, predatory fish ate fewer benthic invertebrates. The resulting release of the benthic invertebrate community from fish predation indirectly decreased the biomass of benthic algae and slightly increased leaf break-down rate. This is the first experimental demonstration that host manipulation by a parasite can reorganise a community and alter ecosystem function. Nematomorphs are common, and many other parasites have dramatic effects on host phenotypes, suggesting that similar effects of parasites on ecosystems might be widespread.

WATER UTILIZATION OF NATURAL AND PLANTED TREES IN THE SEMIARID DESERT OF INNER MONGOLIA, CHINA

We used stable isotope techniques to investigate water utilization of two native trees, Sabina vulgaris Ant. and Artemisia ordosica Krasch., and one introduced tree, Salix matsudana Koidz., in the semiarid Mu-Us desert, Inner Mongolia, China. The study site was in a region where there has been a decline in agricultural productivity, caused by severe desertification over the past several decades. S. matsudana is used extensively for reforestation to protect farms and cultivated lands from shifting sand dunes. We identified water sources for each tree species by comparing the stable isotopes δD and δ18O in water in stems, soil, and groundwater. We also measured δ13C levels in leaves to evaluate the intrinsic water-use efficiency (WUE) of each plant. Comparison of isotopes showed that S. vulgaris and S. matsudana consume relatively deep soil water as well as groundwater, whereas A. ordosica uses only shallow soil water. The δ13C measurements indicated that S. vulgaris has exclusively high WUE, whereas that of the other species was typical of temperate-region C3 plants. The water source data plus WUE data suggest that planted S. matsudana uses groundwater freely, whereas native plants conserve water. Thus, reforestation with S. matsudana might cause irreversible groundwater shortages. Corresponding Editor: E. A. Holland.

Ecological perspectives on microbes involved in N-cycling.

Nitrogen (N) cycles have been directly linked to the functional stability of ecosystems because N is an essential element for life. Furthermore, the supply of N to organisms regulates primary productivity in many natural ecosystems. Microbial communities have been shown to significantly contribute to N cycles because many N-cycling processes are microbially mediated. Only particular groups of microbes were implicated in N-cycling processes, such as nitrogen fixation, nitrification, and denitrification, until a few decades ago. However, recent advances in high-throughput sequencing technologies and sophisticated isolation techniques have enabled microbiologists to discover that N-cycling microbes are unexpectedly diverse in their functions and phylogenies. Therefore, elucidating the link between biogeochemical N-cycling processes and microbial community dynamics can provide a more mechanistic understanding of N cycles than the direct observation of N dynamics. In this review, we summarized recent findings that characterized the microbes governing novel N-cycling processes. We also discussed the ecological role of N-cycling microbial community dynamics, which is essential for advancing our understanding of the functional stability of ecosystems.

Biological proliferation of cesium-137 through the detrital food chain in a forest ecosystem in Japan

Radionuclides, including 137Cs, were released from the disabled Fukushima Daiichi Nuclear Power Plant and had been deposited broadly over forested areas of north-eastern Honshu Island, Japan. In the forest, 137Cs was highly concentrated on leaf litters deposited in autumn 2010, before the accident. Monitoring of the distribution of 137Cs among functional groups clearly showed the role of the detrital food chain as the primary channel of 137Cs transfer to consumer organisms. Although many studies have reported the bioaccumulation (or dilution) of radioactive materials through trophic interactions, the present results highlight the importance of examining multiple possible pathways (e.g., grazing vs. detrital chains) in the proliferation of 137Cs through food webs. These results provide important insight into the future distribution and transfer of 137Cs within forest ecosystems.