Naoko Tokuchi

Intermittent denitrification: The application of a 15N natural abundance method to a forested ecosystem

Abstract The 15N natural abundance method was used to assess the intermittent occurrence of denitrification in the Kiryu watershed in Shiga Prefecture, Japan. The concentration and isotopic composition (° 15N) of NO3− −N, as well as some physical and chemical variables that potentially affect denitrification, were measured along the flow path from precipitation to stream water via soil solution and groundwater. High maximum groundwater level promoted a decrease in N03−−N concentration that was associated with an increase in the δ 15N of N03−−N with soil depth; i.e., denitrification occurred in the soil due to increasing soil moisture. While the maximum level of groundwater fell, no such changes were observed. Dissolved oxygen (DO) and Mn2+ concentrations in soil solutions indicated that strong anaerobic condition did not occur during the study period. These results suggested that denitrification was occurring temporarily in anaerobic microsites such as waterlogged soil aggregates. Upward expansion of groundwater zone thus appeared to play an important role in promoting such microanaerobic sites, resulting in the intermittent occurrence of denitrification. Based on these data, a schematic model for assessing denitrification was proposed based on a N03−−N/Cl− − δ 15S N of NO3−−N map for soil-water systems. Our data showed that variations of NO3−-N concentration and δ 15N value are useful indicators for elucidating nitrogen dynamics as affected by water mixing, plant uptake, nitrification, and denitrification in forested ecosystems.

Spatial variability of soil nitrogen transformation patterns along a forest slope in a Cryptomeria japonica D. Don plantation

Abstract Spatial variability of soil nitrogen (N) transformation patterns was investigated along a slope of a Cryptomeria japonica plantation. The controlling factors of the soil N transformation patterns were examined by multivariate analysis. Net N mineralization showed no clear gradient along the slope, while net nitrification and percent nitrification were high in the lower part, and very low in the upper part of the slope. The middle part was the transition zone corresponding to the patchiness of the lower and upper soils. Soil properties (e.g. C/N ratio and pH) except total N significantly correlated with slope position. Principal component (PC) analysis showed soil properties to be divided into three groups which were PC1 (water content and extractable organic C and N), PC2 (C/N ratio and pH), and PC3 (total C and N). Regressions of soil N transformation pattern against PCs scores suggested that net N mineralization was mainly regulated by PC3, while net nitrification and percent nitrification were mainly regulated by PC1 and PC2. The scatter plots of percent nitrification and the first two PCs scores suggest that PC2 regulates the potentiality, and PC1 affects specially in the middle of the slope where the effect of PC2 was moderate.

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.

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.

Natural 15 N Abundance of Plants and Soil N in a Temperate Coniferous Forest

Measurement of nitrogen isotopic composition (δ15N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of δ15N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of δ15N between soil N pools and plants, and by using an existing model. δ15N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO3– pool at two sites with different nitrification rates in a temperature forest in Japan. δ15N of plants was similar to that of soil NO3– in the high-nitrification site. Because of high foliar NRA and the large foliar NO3– pool at this site, we concluded that plant δ15N indicated a great reliance of plants on soil NO3– there. However, many δ15N of soil N overlapped each other at the other site, and δ15N could not provide definitive evidence of the N source. The existing model was verified by measured δ15N of soil inorganic N and it explained the variations of plant δ15N between the two sites in the context of relative importance of nitrification, but more information about isotopic fractionations during plant N uptake is required for quantitative discussions about the plant N source. The model applied here can provide a basis to compare δ15N signatures from different ecosystems and to understand N dynamics.

Nitrogen and phosphorus enrichment and balance in forests colonized by cormorants: Implications of the influence of soil adsorption

Although much concern has been directed at nitrogen (N) cycling in terrestrial ecosystems with bird colonies, little has been clarified on the processes of phosphorus (P) cycling itself, and few comparisons between P and N cycling in bird colonies have been made. On the Isaki Headland and Chikubu Island, which are located on or near the shore of Lake Biwa, Central Japan, a dramatic increase in the population of cormorants has occurred since the 1980s. There has been a concomitant increase in the transport of nutrients from the lake to the waterside ecosystems. We compared the pools and dynamics of N and P in the cormorant-colony forests in order to clarify the effects of differences in soil N and P dynamics on the N–P balance of these colony forests. The total N concentration in the forest floor at excrement-influenced sites was not significantly different from that at sites without such influence, in spite of the heavy load of cormorant-derived N. In contrast to N, forest floor P concentration at the sites with excrement influence was significantly higher compared to sites without such influence, resulting in the lower forest floor N/P ratio at the excrement-influenced sites even after colony abandonment. The site pattern of total N and P concentrations and N/P ratio for mineral soil was similar to that for the forest floor. It seems that the leaky character for N and the accumulative character for P are due to the high mobility of nitrate in soils and the tight absorption of inorganic P to clay minerals, respectively. The site pattern of N/P ratios observed for Chamaecyparis obtusa Sieb. et Zucc. leaves is consistent with that for the forest floor and/or mineral soil, suggesting that the soil geochemical property was reflected in the foliar N/P ratio. The chemistry of throughfall and soil solution was also changed due to deposition of cormorant excrement, and the changes continued for a few years after abandonment of the colony. The quantitative analyses for N and P suggested that the major part of N and P transported by cormorants was not retained in plant matter and the surface soil beneath the colony but instead leached into deeper soil layers. The influence of cormorant excrement on nutrient balance of the whole colony ecosystem is also discussed.

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.

Topographical variations in a plant–soil system along a slope on Mt Ryuoh, Japan

The plant–soil system was studied at different topographic levels (i.e. ridge, backslope and footslope) along a slope in a Cryptomeria plantation. Soil solution chemistry at each representative topographic plot was investigated. Tree height and diameter of Cryptomeria decreased upslope. The understory species composition changed along the slope. The upper part of the slope with Oa horizon soil N transformation was characterized by ammonification, while most of the inorganic N in the lower part of the slope without Oa horizon was nitrified. The inorganic N form in the soil solution corresponded with soil N transformation. Ammonium was the dominant inorganic N at the ridge, while NO3 predominated at the foot of the slope. Soil solution chemistry was similar to throughfall at the ridge. At the foot slope, the chemical composition of the soil solution was different from throughfall due to high NO3− concentrations. This suggests that the inorganic N form regulated not only N concentration but also cation concentrations. The soil N transformation pattern is important in nutrient cycling.

An in situ lysimeter experiment on soil moisture influence on inorganic nitrogen discharge from forest soil

Abstract The influences of the soil moisture condition on ammonification, nitrification and denitrification were examined by in situ experiments on forest floors using soil column lysimeters. Distinct differences were detected in the discharge concentrations of NH4+ and NO3− from lysimeters with different soil water content. Three types of lysimeters (diameter 19.5 cm, depth 30.0 cm) were prepared by containing undisturbed soil samples. Inorganic nitrogen discharged from lysimeters under natural rainfall was measured approximately once a week from 5 July to 29 November 1991. Soil moisture conditions of the three lysimeters were controlled using an average pressure head at the bottom of each column at -268.6 hPa, 0 hPa, and 19.8 hPa. These three moisture conditions corresponded approximately to the conditions in the upper or mid-upper hillslopes with relatively dry soil, lower hillslopes with relatively wet soil, and riparian zones around spring points and streams with saturated soil, respectively. For these soil moisture conditions, three different types of inorganic nitrogen discharges were observed; (1) ammonium and nitrate nitrogen; (2) nitrate nitrogen; (3) neither ammonium nor nitrate nitrogen. The result clearly showed the restraint of nitrification by soil moisture deficit and the reduction of NO3− by denitrification under saturated conditions. This suggests that the different stages of the process in nitrogen dynamics could occur owing to only the influence of soil moisture variations even in the shallow soil layer. The nature of inorganic nitrogen discharge from each lysimeter helped to explain the spatial heterogeneity of the nitrogen dynamics caused by distributed soil moisture conditions within a watershed system.

Estimation of radioactive 137-cesium transportation by litterfall, stemflow and throughfall in the forests of Fukushima.

Since the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011, large areas of the forests around Fukushima have become highly contaminated by radioactive nuclides. To predict the future dynamics of radioactive cesium ((137)Cs) in the forest catchment, it is important to measure each component of its movement within the forest. Two years after the accident, we estimated the annual transportation of (137)Cs from the forest canopy to the floor by litterfall, throughfall and stemflow. Seasonal variations in (137)Cs transportation and differences between forests types were also determined. The total amount of (137)Cs transported from the canopy to the floor in two deciduous and cedar plantation forests ranged between 3.9 and 11.0 kBq m(-2) year(-1). We also observed that (137)Cs transportation with litterfall increased in the defoliation period, simply because of the increased amount of litterfall. (137)Cs transportation with throughfall and stemflow increased in the rainy season, and (137)Cs flux by litterfall was higher in cedar plantation compared with that of mixed deciduous forest, while the opposite result was obtained for stemflow.