Ayato Kohzu

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.

Biogenic Phosphorus Compounds in Sediment and Suspended Particles in a Shallow Eutrophic Lake: A 31P-Nuclear Magnetic Resonance (31P NMR) Study

Differences in biogenic phosphorus (P) compounds between sediment and suspended particles in aquatic environments are important for understanding the mechanisms of internal P loading, but these differences are still unknown. We used solution-state (31)P-nuclear magnetic resonance spectroscopy ((31)P NMR) with NaOH-ethylenediaminetetraacetic extraction to detect the multiple P compounds in suspended particles and sediment in the eutrophic Lake Kasumigaura, including orthophosphate monoesters, orthophosphate diesters, pyrophosphate, and polyphosphate. We tested the hypothesis that there is a significant difference between these groups in suspended particles and sediment. Biogenic P other than orthophosphate was found in significantly higher proportions in suspended particles (74.3% of total P) than in sediment (25.6%). Orthophosphate monoesters were comparatively more abundant in suspended particles, as indicated by the ratio of orthophosphate diesters to monoesters (average, 0.31 for suspended particles; 1.05 for sediment). The compounds identified as orthophosphate monoesters by (31)P NMR spectroscopy originated mainly from phospholipids (α-glycerophosphate and β-glycerophosphate) and ribonucleic acid (RNA-P), whereas the orthophosphate diesters included mostly DNA (DNA-P). These results suggest that the dynamics of orthophosphate diesters, the production of DNA-P, or the degradation of phospholipids, play an important role in P cycling in Lake Kasumigaura.

Nitrogen isotope fractionation during nitrogen transport from ectomycorrhizal fungi, Suillus granulatus, to the host plant, Pinus densiflora

Abstract The 15N natural abundance in Pinus densiflora Sieb. et Zucco that had been inoculated and not inoculated with ectomycorrhizal fungi (Suillus granulatus (L.:Fr.) O. Kuntze) was compared. The inoculated pine needles showed a lower δ15N value, while the uninoculated ones showed a higher δ15N value. Higher δ15N values in the mycelial mat of the ectomycorrhizal fungi compared to those of the inoculated pine needles were also observed. These facts indicate that nitrogen isotope fractionation occurred during the nitrogen transport from mycorrhizal fungi to the host plants.

Significance of rooting depth in mire plants: Evidence from natural ^15N abundance

Variation in stable nitrogen isotope ratios (δ15N) was assessed for plants comprising two wetland communities, a bog-fen system and a flood plain, in central Japan. δ15N of 12 species from the bog-fen system and six species from the flood plain were remarkably variable, ranging from −5.9 to +1.1‰ and from +3.1 to +8.7‰, respectively. Phragmites australis exhibited the highest δ15N value at both sites. Rooting depth also differed greatly with plant species, ranging from 5 cm to over 200 cm in the bog-fen system. There was a tendency for plants having deeper root systems to exhibit higher δ15N values; plant δ15N was positively associated with rooting depth. Moreover, an increasing gradient of peat δ15N was found along with depth. This evidence, together with the fact that inorganic nitrogen was depleted under a deep-rooted Phragmites australis stand, strongly suggests that deep-rooted plants actually absorb nitrogen from the deep peat layer. Thus, we successfully demonstrated the diverse traits of nitrogen nutrition among mire plants using stable isotope analysis. The ecological significance of deep rooting in mire plants is that it enables those plants to monopolize nutrients in deep substratum layers. This advantage should compensate for any consequential structural and/or physiological costs. Good evidence of the benefits of deep rooting is provided by the fact that Phragmites australis dominates as a tall mire grass.

Nitrogen-stable isotopic signatures of basal food items, primary consumers and omnivores in rivers with different levels of human impact

We examined how nitrogen-stable isotopic signatures of food web components (basal resources, primary and lower consumers, and omnivores) in rivers change with increasing levels of human population density (HPD) in their watersheds. Samples were collected from 22 rivers flowing in the Lake Biwa basin, Japan. Among three potential resources at the base of food webs (epilithon, benthic and suspended particulate organic matter), the mean isotopic values (δ15N) of the epilithon (4.5–7.8%) were consistently higher than those of other items (1.9–4.2%) and displayed the most pronounced elevation (by 3.3%) with increasing HPD. The mean δ15N values of the individual taxa of lower consumers (bivalve, snail and caddisfly) tended to increase with increasing HPD, although the pattern and the extent of the elevation were highly variable among the taxa. These results suggest a taxon-specific feature in the N source (or sources) of lower consumers. Our data suggested that human activities (e.g. nutrient loading) potentially induce changes in the N baselines of river food webs. The major N source of bivalves appeared to be shifted from suspended particulate organic matter to other items with increasing HPD. Trophic levels of goby fish (Rhinogobius sp. OR) and shrimp (Palaemon paucidens), being estimated to be at 2.4–3.8 and 2.1–3.4, respectively, did not differ significantly among rivers with different HPD levels.

Fast and precise method for HPLC-size exclusion chromatography with UV and TOC (NDIR) detection: importance of multiple detectors to evaluate the characteristics of dissolved organic matter.

A new type of high-performance liquid chromatography (HPLC)-size exclusion chromatography (SEC) system with ultraviolet (UV) absorbance detection and non-dispersive infrared (NDIR) detection of total organic carbon is described. The introduction of an online degassing tube and a low-volume HPLC column helped to reduce the analytical time and increase the sensitivity of the SEC system. This study is the first in which linear calibration curves (R(2)>0.99) were obtained for both UV absorbance and NDIR data for polystyrene sulfonate standards, which are the most suitable standards for molecular size analysis of aquatic humic substances as well as dissolved organic matter (DOM). Using the calibration curves, the molecular size distribution of DOM in water collected from Lake Kasumigaura and in pore water from lake sediments was estimated. Most of the DOM had a molecular weight less than 4000 Daltons (Da), and the amount of low-molecular-weight DOM (∼ 2000 Da) with low UV absorbance increased with depth in the sediment pore water. This result shows the importance of combining quantitative analysis by NDIR detection with qualitative analysis by UV detection to determine the chemical and physical properties of DOM. The possible sources and reactivity of DOM in Lake Kasumigaura and its sediment pore water are also discussed.

Linking aboveground and belowground food webs through carbon and nitrogen stable isotope analyses

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) have been used for more than two decades in analyses of food web structure. The utility of isotope ratio measurements is based on the observation that consumer δ13C values are similar (<1‰ difference) to those of their diet, while consumer δ15N values are about 3‰ higher than those of their diet. The technique has been applied most often to aquatic and aboveground terrestrial food webs. However, few isotope studies have examined terrestrial food web structure that includes both above- and belowground (detrital) components. Here, we review factors that may influence isotopic signatures of terrestrial consumers in above- and belowground systems. In particular, we emphasize variations in δ13C and δ15N in belowground systems, e.g., enrichment of 13C and 15N in soil organic matter (likely related to soil microbial metabolism). These enrichments should be associated with the high 13C (~3‰) enrichment in belowground consumers relative to litter and soil organic matter and with the large variation in δ15N (~6‰) of the consumers. Because such enrichment and variation are much greater than the trophic enrichment generally used to estimate consumer trophic positions, and because many general predators are considered dependent on energy and material flows from belowground, the isotopic variation in belowground systems should be taken into account in δ13C and δ15N analyses of terrestrial food webs. Meanwhile, by measuring the δ13C of key predators, the linkage between above- and belowground systems could be estimated based on observed differences in δ13C of primary producers, detritivores and predators. Furthermore, radiocarbon (14C) measurements will allow the direct estimation of the dependence of predators on the belowground systems.

Carbon and nitrogen stable isotope ratios of macroinvertebrates in the littoral zone of Lake Biwa as indicators of anthropogenic activities in the watershed

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) of macroinvertebrates inhabiting littoral zones of lakes can serve as useful indicators of material loading from the watershed. We collected snails (Semisulcospira spp.) and bivalves (Unio douglasiae biwae Kobelt) from 29 littoral sites in Lake Biwa near the mouths of river tributaries with various human population density (HPD) and land-use patterns. The δ13C and δ15N signatures were determined for three potential food sources: particulate organic matter in the pelagic zone (PPOM), riverine particulate organic matter from tributaries (RPOM) and epilithic organic matter in the littoral zone (EOM). The stable isotope mixing model revealed that snails relied mainly on EOM, and bivalves on PPOM and RPOM. Multiple regression analysis showed that intersite variation in δ15N for snails was best explained by HPD, while variation in δ15N of EOM and nitrate was explained to a lesser extent by HPD. Comparison with isotope signatures of their food sources and riverine nutrients revealed that snails assimilated anthropogenic nitrogen from wastewater in the watershed. Our results also showed that the δ13C value of bivalves was marginally related to the fraction of paddy fields in the watersheds. In conclusion, the isotope signatures of macroinvertebrates inhabiting the littoral zone can be useful indicators of anthropogenic impacts from the watershed.

Evaluation of wastewater nitrogen transformation in a natural wetland (Ulaanbaatar, Mongolia) using dual-isotope analysis of nitrate.

The Tuul River, which provides water for the daily needs of many residents of Ulaanbaatar, Mongolia, has been increasingly polluted by wastewater from the citys sewage treatment plant. Information on water movement and the transformation of water-borne materials is required to alleviate the deterioration of water quality. We conducted a synoptic survey of general water movement, water quality including inorganic nitrogen concentrations, and isotopic composition of nitrogen (δ(15)N-NO(3)(-), δ(18)O-NO(3)(-), and δ(15)N-NH(4)(+)) and water (δ(18)O-H(2)O) in a wetland area that receives wastewater before it enters the Tuul River. We sampled surface water, groundwater, and spring water along the two major water routes in the wetland that flow from the drain of the sewage treatment plant to the Tuul River: a continuous tributary and a discontinuous tributary. The continuous tributary had high ammonium (NH(4)(+)) concentrations and nearly stable δ(15)N-NH(4)(+), δ(15)N-NO(3)(-), and δ(18)O-NO(3)(-) concentrations throughout its length, indicating that nitrogen transformation (i.e., nitrification and denitrification) during transit was small. In contrast, NH(4)(+) concentrations decreased along the discontinuous tributary and nitrate (NO(3)(-)) concentrations were low at many points. Values of δ(15)N-NH(4)(+), δ(15)N-NO(3)(-), and δ(18)O-NO(3)(-) increased with flow along the discontinuous route. Our results indicate that nitrification and denitrification contribute to nitrogen removal in the wetland area along the discontinuous tributary with slow water transport. Differences in hydrological pathways and the velocity of wastewater transport through the wetland area greatly affect the extent of nitrogen removal.

Dynamics of 15N natural abundance in wood-decomposing fungi and their ecophysiological implications

Nine species of basidiomycota and one species of ascomycota were grown in an ammonium sulphate media and on beech wood; and the general 15N dynamic patterns of the hyphae were examined. The fungal body initially became depleted in 15N in both the types of incubation. However, the underlying mechanisms were quite different, that is, significant fungal 15N drop on the beech wood is associated with the fungal N reallocation and the uptake of atmospheric ammonia and/or NOx, in addition to isotope fractionation during assimilation. Although the 15N values of the wood-decomposing basidiocarps were generally close to the 15N values of the wood, it does not always indicate that the wood derived N was the sole N source for the fungi throughout the growth periods as shown in our wood-decomposing experiment.