Variation In δ15N Values Research Articles

Problems Associated with the AMS Dating of Small Bone Samples: The Question of the Arrival of Polynesian Rats to New Zealand

We have AMS dated samples of Pacific rat (Rattus exulans) bone “collagen” and filtered gelatin samples from the prehistoric site of Shag River Mouth, New Zealand. The age of occupation of this site has previously been determined based on 50 radiocarbon measurements. The site dates to the late Archaic phase of southern New Zealand prehistory (about 650–500 BP; 14th–15th century AD). The results of rat bones which we have dated produce a range in ages, from about 980–480 BP, a difference we attribute to a combination of effects. Pretreatment appears to be an important variable, with results showing differences in 14C age between the progressive “collagen” and filtered gelatin chemical treatment stages. Amino acid profiles suggest there is a proteinaceous but non-collagenous contaminant which is removed by the more rigorous pretreatment. Stable isotopes vary between pretreatments, supporting the removal of a contaminant, or contaminants. Variation in δ15N values imply a range in uptake of dietary protein, and might suggest a potential influence from the local aquatic environment or the consumption of marine-derived protein. Rats are opportunistic, omnivorous mammals, and, therefore, obtain carbon from a variety of reservoirs and so we ought to expect that in environments where there is a variety of reservoirs, these will be exploited. Taken together, the results show that rat bone AMS 14C determinations vary in comparison with the established age of the site, but are in notably better agreement with non-collagenous data than in previously published determinations (Anderson 1996).

MANGROVE ISOTOPIC (δ15N AND δ13C) FRACTIONATION ACROSS A NITROGEN VS. PHOSPHORUS LIMITATION GRADIENT

Mangrove islands in Belize are characterized by a unique switching from nitrogen (N) to phosphorus (P) limitation to tree growth from shoreline to interior. Fertilization has previously shown that Rhizophora mangle (red mangrove) fringe trees (5–6 m tall) growing along the shoreline are N limited; dwarf trees (≤1.5 m tall) in the forest interior are P limited; and transition trees (2–4 m tall) are co-limited by both N and P. Growth patterns paralleled a landward decrease in soil flushing by tides and an increase in bioavailable N, but P availability remained consistently low across the gradient. Stable isotopic composition was measured in R. mangle leaves to aid in explaining this nutrient switching pattern and growth variation. Along control transects, leaf δ15N decreased from +0.10‰ (fringe) to −5.38‰ (dwarf). The δ15N of N-fertilized trees also varied spatially, but the values were consistently more negative (by ∼3‰) compared to control trees. Spatial variation in δ15N values disappeared when the trees were fertilized with P, and values averaged +0.12‰, similar to that in control fringe trees. Neither variation in source inputs nor microbial fractionation could fully account for the observed patterns in δ15N. The results instead suggest that the lower δ15N values in transition and dwarf control trees were due to plant fractionation as a consequence of slower growth and lower N demand. P fertilization increased N demand and decreased fractionation. Although leaf δ13C was unaffected by fertilization, values increased from fringe (−28.6‰) to transition (−27.9‰) to dwarf (−26.4‰) zones, indicating spatial variation in environmental stresses affecting stomatal conductance or carboxylation. The results thus suggest an interaction of external supply, internal demand, and plant ability to acquire nutrients under different hydro-edaphic conditions that vary across this tree-height gradient. The findings not only aid in understanding mangrove discrimination of nitrogen and carbon isotopes, but also have implications for identifying nutrient loading and other stress conditions in coastal systems dominated by mangroves.

Stable isotope composition of walleye: 15N accumulation with age and area-specific differences in δ13C

Stable nitrogen and carbon isotope ratios were measured for walleye (Stizostedion vitreum) collected across Lake Champlain, Vermont, to determine relationships between isotopic composition and diet, location of capture, length, weight, and age. Variation in δ13C values reflected area-specific differences in isotopic composition of organisms collected across the lake. A critical assumption in the application of isotope techniques is that a predictable relationship exists between the diet and isotopic composition of an organism. Our results indicate that isotopic fractionation factors may not be independent of age as has largely been assumed. By combining stable nitrogen and carbon isotope analysis with conventional stomach content analysis, we documented significant age effects in the δ15N composition of adult walleye that were not attributable to observed changes in diet. Age accounted for 81% of the variation in δ15N values of walleye (ages 2–27, N = 65, δ15N range = 15.3–19.2‰), providing evidence supporting 15N accumulation over the life span of walleye. Therefore, the risk of making faulty inferences of trophic position and food web interactions based on δ15N values may be increased when age is unknown. Our results indicate that metabolic effects associated with age require greater consideration in applications of stable isotope analysis.

The soil food web of two beech forests (Fagus sylvatica) of contrasting humus type: stable isotope analysis of a macro- and a mesofauna-dominated community.

The structure of the soil food web in two beech (Fagus sylvatica) forests, the Göttinger Wald and the Solling forest (Northern Germany), was investigated using variations in tissue 15N concentrations of animal species or taxa. The Göttinger Wald is located on a limestone plateau and characterized by mull humus with high macrofauna activity, particularly of Lumbricidae, Diplopoda and Isopoda. In contrast, the Solling forest is located on a sandstone mountain range and characterized by moder humus. The soil fauna of this forest is dominated by mesofauna, particularly by Collembola, Enchytraeidae and Oribatida. In June 1995 soil fauna was sampled using heat extraction. Three soil layers were analysed at each of the sites. 15N/14N ratios of bulk material increased strongly with soil depth in both forests. This also applied to the water-soluble fraction at the Göttinger Wald, but not at the Solling. Generally, the water-soluble fraction was more enriched in 15N than the bulk materials. For most animals studied 15N/14N ratios varied little with soil depth. In both forests soil animals could be classified either as saprophages, including microphytophages, or predators. On average, the δ15N of predatory taxa (Chilopoda, Araneida, Gamasina, Staphylinidae) exceeded that of saprophagous or microphytophagous taxa (Lumbricidae, Isopoda, Diplopoda, Collembola, Oribatida, Enchytraeidae) by 4.4 and 3.9‰ for the Göttinger Wald and the Solling, respectively. We assume that most of the saprophagous or microphytophagous taxa studied consist of primary and secondary decomposers and hypothesize that predators prey more on secondary than primary decomposers. Generally, average δ15N values differed little between saprophagous (Lumbricidae, Diplopoda, Isopoda) and microphytophagous taxa (Collembola, Oribatida). The variations in δ15N values of species within these taxa consistently exceeded the variation between them, indicating that the species of each of these taxa form a continuum from primary to secondary decomposers. Also, variations in δ15N values within predatory taxa in most cases exceeded that between taxa excluding top predators like Sorex. We conclude that using higher taxonomic units in soil food web analysis is problematic and in general not consistent with nature. Higher taxonomic units may only be useful for depicting very general trophic groupings such as predators or microbi-detritivores.

Spatial variability in Mussels used to assess base level nitrogen isotope ratio in freshwater ecosystems

Freshwater mussels have been used to establish base level nitrogen isotope ratio values (δ15N) used in trophic position and food web studies in freshwater ecosystems. In this study, we assess the variability introduced when using unionid mussels in this manner by investigating the spatial variation in δ15N values in mussels from different locations in shallow freshwater ponds, and also differences in δ15N in different tissue types in the mussels. Results from the analysis of adductor, foot and a section of the mantle tissues from mussels across all ponds showed that adductor tissue was consistently enriched by about 1‰ versus the mantle and foot. The foot showed the least variability which, coupled with ease in obtaining consistent samples, led us to select this tissue type for subsequent analysis. The six ponds included in the study had average mussel δ15N values ranging from 4.9‰ to 11.9‰. Four of the six ponds showed no significant within pond differences between δ15N values from mussels collected at different sampling sites. The range of mussel δ15N values obtained from within ponds showed that using data from a single mussel to assess baseline δ15N values would result in the introduction of a maximum error of 0.56 of a trophic level to subsequent trophic position calculations.

15N natural abundances and N use by tundra plants.

Plant species collected from tundra ecosystems located along a north-south transect from central Alaska to the north coast of Alaska showed large and consistent differences in 15N natural abundances. Foliar δ15N values varied by about 10% among species within each of two moist tussock tundra sites. Differences in 15N contents among species or plant groups were consistent across moist tussock tundra at several other sites and across five other tundra types at a single site. Ericaceous species had the lowest δ15N values, ranging between about -8 to -6‰. Foliar 15N contents increased progressively in birch, willows and sedges to maximum δ15N values of about +2‰ in sedges. Soil 15N contents in tundra ecosystems at our two most intensively studied sites increased with depth and δ15N values were usually higher for soils than for plants. Isotopic fractionations during soil N transformations and possibly during plant N uptake could lead to observed differences in 15N contents among plant species and between plants and soils. Patterns of variation in 15N content among species indicate that tundra plants acquire nitrogen in extremely nutrient-poor environments by competitive partitioning of the overall N pool. Differences in plant N sources, rooting depth, mycorrhizal associations, forms of N taken up, and other factors controlling plant N uptake are possible causes of variations in δ15N values of tundra plant species.

Glacial‐interglacial migration of an upwelling field in the Western Equatorial Pacific recorded by sediment 15N/14N

Nitrogen isotope ratios (δ15N) of surface sediment in the equatorial ocean reflect the mode of upper water circulation pattern. We applied this knowledge to reconstruct glacial‐interglacial variations of an upwelling field in western equatorial Pacific using δ15N from sediment cores. Distinct minima in δ15N values were found corresponding to glacial periods on the northern and eastern edges of the upwelling field (Mindanao Dome), indicating glacial enlargement of upwelling due to enhanced winter monsoon activity. However, at a site on southern side of the upwelling region, minima in δ15N values were obtained during interglacial periods. These opposing variations in δ15N values could be interpreted as a northward shift of the upwelling zone, during glacial periods. The glacial‐interglacial migration of the circulation pattern in this region may reflect a modal change in the global thermohaline circulation (conveyor belt) during glaciation.

Research Note:The use ofLolium multiflorumand uninoculatedTrifolium subterraneumas reference crops in nitrogen fixation studies

Abstract The 15N natural abundance and15 N dilution methods were used to determine the viability of using either Lolium multiflorum (annual ryegrass) or uninoculated Trifolium subterraneum (subterranean clover), as non‐fixing reference crops in N2‐fixation studies. The δ13N values for the15 N natural abundance method and atom% 15N values for the 15N dilution method for plant and soil samples are presented. The ryegrass pastures showed the least variation in δ15N and atom% I5N values between experimental plots. This crop appeared to be a more appropriate choice in such studies, but further investigation is needed to confirm this observation. The use of uninoculated subterranean clover as a reference crop seemed problematic, since inoculation with effective rhizobia could not be prevented. Relatively high variation in δ15N values for inoculated subterranean clover was obtained compared to the atom% 15N values. This result indicated that the use of the 15N natural abundance method should be evaluated for oth...

15N natural abundance of plant and soil components of a Banksia woodland ecosystem in relation to nitrate utilization, life form, mycorrhizal status and N2‐fixing abilities of component species

ABSTRACTStudies of the variation in δ15N values for plants from a fire‐prone Banksia woodland in South West Australia showed that pioneer herbaceous, non‐mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81%c). A detailed study of one such species Ptilotus polystachus demonstrated a close correspondence between the δ15N values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive δ15N values and moderately active in nitrate reduction and storage. Nitrogen‐fixing species had the lowest δ15N values (–0.36‰), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had δ15N values which fell within the range of values obtained for the miscellaneous assemblage of N2‐fixing species. Consequently, 15N abundance values failed to distinguish N2 fixing from non‐fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N2 fixation. The study demonstrated complex patterns of nitrogen utilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by component species.

Spatial Variability of Nitrogen‐15 and Total Nitrogen in Some Virgin and Cultivated Soils

AbstractSoil cores were taken from six locations representing three virgin and three cultivated soils in increments of 15 or 30 cm down to depths ranging from 120 to 300 cm. The number of samples of an individual soil profile varied from 20 to 96, though smaller numbers of some subsurface horizons were obtained at two locations where very rocky conditions were encountered. Frequency distribution analysis of δ15N values and of total N showed that they were log‐normally distributed in about half the groups of samples. Assumption of normal distribution in all samples would result in a maximum error of 37% in total N and 0.1% in 15N content if the real distribution were in fact log‐normal. Coefficients of variation of total N were somewhat lower in cultivated soils than in virgin soils, but C.V. values for atom %15N were comparable in the two groups of soils. It is concluded that the magnitude of natural variation in δ15N values at a given sampling site, both laterally and vertically, is so great as to preclude tracing biological events by means of natural abundances.