Linda L. Handley

The N-15 natural abundance (delta N-15) of ecosystem samples reflects measures of water availability

We assembled a globally-derived data set for site-averaged foliar delta(15)N, the delta(15)N of whole surface mineral soil and corresponding site factors (mean annual rainfall and temperature, latitude, altitude and soil pH). The delta(15)N of whole soil was related to all of the site variables (including foliar delta(15)N) except altitude and, when regressed on latitude and rainfall, provided the best model of these data, accounting for 49% of the variation in whole soil delta(15)N. As single linear regressions, site-averaged foliar delta(15)N was more strongly related to rainfall than was whole soil delta(15)N. A smaller data set showed similar, negative correlations between whole soil delta(15)N, site-averaged foliar delta(15)N and soil moisture variations during a single growing season. The negative correlation between water availability (measured here by rainfall and temperature) and soil or plant delta(15)N fails at the landscape scale, where wet spots are delta(15)N-enriched relative to their drier surroundings. Here we present global and seasonal data, postulate a proximate mechanism for the overall relationship between water availability and ecosystem delta(15)N and, newly, a mechanism accounting for the highly delta(15)N-depleted values found in the foliage and soils of many wet/cold ecosystems. These hypotheses are complemented by documentation of the present gaps in knowledge, suggesting lines of research which will provide new insights into terrestrial N-cycling. Our conclusions are consistent with those of Austin and Vitousek (1998) that foliar (and soil) delta(15)N appear to be related to the residence time of whole ecosystem N.

A theory for 15N/14N fractionation in nitrate-grown vascular plants

Abstract. We present a theory describing how the δ15N values of the nitrogen (N) pools in a vascular plant depend on that of its source N (nitrate), on 15N/14N fractionations during N assimilation, and on N transport within and N loss from the plant. The theory allows measured δ15N values to be interpreted in terms of physiological processes. The δ15N values of various N pools are calculated using three rules: (1) when a pool divides without transformation, there is no change in the δ15N values of the N entering the resulting pools; (2) when nitrate is assimilated by nitrate reductase, the δ15N values of the resulting pools (product and residual substrate) are described by a Rayleigh equation; (3) when two N pools mix, the δ15N value of the mixture is a weighted average of the δ15N values of the component pools. The theory is written as a spreadsheet and solved numerically. Potentially, it has multiple solutions. Some contravene physiological reality and are rejected. The remainder are distinguished, where possible, using additional physiological information. The theory simulated independent measurements of δ15N in N pools of Brassica campestris L. var. rapa (komatsuna) and Lycopersicon esculentum Mill. cv. T-5 (tomato).

Spatial variability of soil total C and N and their stable isotopes in an upland Scottish grassland

As preparation for a below ground food web study, the spatial variability of three soil properties (total N, total C and pH) and two stable isotopes (δ13C and δ15N of whole soil) were quantified using geostatistical approaches in upland pastures under contrasting management regimes (grazed, fertilised and ungrazed, unfertilised) in Scotland. This is the first such study of upland, north maritime grasslands. The resulting patterns of variability suggest that to obtain statistically independent samples in this system, a sampling distance of ≥13.5 m is required. Additionally, temporal change (a decline of 1‰) was observed in whole soil δ15N for the grazed, fertilised plot. This may have been caused by new inputs of symbiotically-fixed atmospheric N2.

Natural abundance of 15N and 13C in earthworms from a wheat and a wheat-clover field

Abstract The natural abundances of the stable isotopes of nitrogen (δ 15 N) and carbon (δ 13 C) were measured in plant shoots and in seven earthworm (Lumbricidae) species from a wheat and a wheat-clover cropping system. Variations in earthworm δ 13 C were generally small in these systems containing only C 3 plants. Plant shoot δ 15 N ranged from −2.2‰ to −0.7‰ in white clover and from +0.9‰ to +6.6‰ in winter wheat. Intraspecific variability in earthworm δ 15 N was small. Earthworm δ 15 N was significantly related to the cropping system and to the ecological grouping of the earthworms. Mean δ 15 N values of all earthworm groups were significantly lower (by 3.5‰ to 4.4‰) in the wheat-clover field than in the wheat field. Within each of the two cropping systems, earthworm nitrogen isotope ratios differed significantly between ecological earthworm groups, with δ 15 N values decreasing in the order Allolobophora chlorotica and Aporrectodea caliginosa > Aporrectodea longa > Lumbricus spp. These results demonstrate for the first time that the δ 15 N signature of legumes can influence those of soil invertebrates. Because isotope measurements reflect assimilated tissue nitrogen, they offer novel insights into the feeding ecology and trophic positions of earthworms.

Stable isotope natural abundances of soil, plants and soil invertebrates in an upland pasture

Abstract In an exploratory study of below-ground trophic relations, natural abundances of the stable isotope pairs 13C/12C and 15N/14N (δ13C and δ15N) were measured on samples of plant shoots, whole soil and soil invertebrates taken in 1994 from two contrasting treatments of a pre-existing experiment: (1) continued grazing by sheep, with N:P:K fertiliser additions from 1990 onward; and (2) no added fertiliser, but sheep grazing removed entirely. Stepwise trophic increases were documented better by seasonal averages of δ13C and δ15N and by seasonal trends, composed of data collected on several occasions, than by instantaneous values. Seasonal changes in plant monocot vs dicot differences for shoot δ13C and δ15N were detected from patterns over several individual sampling dates; instantaneous samples were neither statistically significant nor qualitatively interpretable. Significant isotopic differences between treatments were evident in invertebrates only as seasonal averages or trends. Seasonal variations of δ13C and δ15N in earthworms and slugs may reflect previously unsuspected invertebrate behaviour. Whole soil δ13C was static through time and across treatments. Whole soil δ15N changed seasonally, an effect consistent with 15N/14N fractionation, e.g. during denitrification.

Shoot δ15N correlates with genotype and salt stress in barley

Given a uniform N source, the δ15N of barley shoots provided a genotypic range within treatments and a separation between control and salt-stress treatments as great as did δ13C*. Plant δ15N has been represented in the literature as a bioassay of external source δ15N and used to infer soil N sources, thus precluding consideration of the plant as a major cause in determining its own 815N. We believe this to be the first report of plant δ15N as a genetic trait. No mechanistic model is needed for use of δ15N as a trait in controlled studies; however, a qualitative model is suggested for further testing.

THE 15N NATURAL ABUNDANCE PATTERNS OF FIELD-COLLECTED FUNGI FROM THREE KINDS OF ECOSYSTEMS

The variability of fungal δ15N values within one sample population can be large and unpredictable. The observation that fungi are often 15N-enriched relative to nearby plant foliage may reflect a mycorrhizal relationship or it may be fortuitous and chiefly arise from other causes. In a cultivated system (decorative flower bed), the 15N-enrichment of fungal caps of fruit bodies relative to stipes suggested that much of the observed variation was caused by internal cycling of fungal N. Controlled experiments are needed in order to identify the sources of observed isotopic discrimination. Of the systems sampled, the two natural ones had larger ranges of δ15N among all samples than the cultivated one.

Wild Rabbit Host and Some Parasites Show Trophic-Level Relationships for δ13C and δ15N: A First Report

Abstract We report the first isotopic study of an animal host-parasite system. Parasitic, intestinal nematodes, Graphidium strigosum and Passalurus ambiguus, were (15)N-enriched relative to their host, the European rabbit Oryctolagus cuniculus, while parasitic cestodes, Cittataenia denticulata and Mosgovoyia pectinata, were (15)N-depleted, suggesting different trophic relationships. Host embryos were more similar in their δ(13)C and δ(15)N values to maternal muscle than were any of the parasites. Coprophagy, the direct recycling of food by the rabbit eating its own faeces, did not lead to isotopic differences between stomach contents and faeces, suggesting that the major point for isotopic discrimination in lagomorph nitrogen metabolism is in the animal rather than in the gut. We conclude that bulk δ(13)C and δ(15)N can reveal valuable new information about host-parasite relationships, and these could be explored further at the biochemical level using compound-specific isotopic analyses.

Above-ground grazing affects floristic composition and modifies soil trophic interactions

Abstract There are few data on the functional inter-relationships between above- and below-ground components of soil ecosystems. Here, we report changes in below-ground soil invertebrate trophic relationships (manifested as alterations in stable isotope natural abundances, δ 13 C and δ 15 N) that arose in association with the removal of sheep grazing and from the resulting changes in above-ground floristic composition. Consequent to grazing removal, Lolium perenne L. (perennial rye-grass) was replaced as the dominant plant species in ungrazed treatments by Ranunculus repens L. (creeping buttercup), a species with more 13 C-enriched foliage. Consequently, all invertebrate functional groups studied, but not whole soil, were more 13 C-enriched in ungrazed treatments. Earthworms (detritivore) from grazed treatments were significantly 15 N-enriched compared with earthworms from ungrazed treatments. In contrast, slug (herbivore) δ 15 N exhibited no treatment effect. Reasons for this are unclear but may be related to the effects of above-ground grazing on the composition of below-ground microbial/microfaunal communities. Omnivores/carnivores (beetles and spiders), were more 15 N-enriched than primary producers in the grazed than in the ungrazed treatments (6 vs. 4‰) suggesting a longer below-ground foodchain in the grazed plots. The cessation of fertilizer application had no comparable effects on below-ground trophic relationships.

Comparisons of fatty acid and stable isotope ratios in symbiotic and non-symbiotic brittlestars from Oban Bay, Scotland

The bed-forming brittlestars Ophiothrix fragilis , Ophiocomina nigra and Amphiura chiajei from Oban Bay, Scotland were studied using methods previously employed to study chemoautotrophic symbioses. Ophiothrix fragilis and A. chiajei both contain symbiotic bacteria (SCB) while Ophiocomina nigra is non-symbiotic. Samples were taken of Ophiothrix fragilis at approximately two-week intervals for one year. Symbiotic bacteria numbers were determined by direct counting of homogenates of the arms of 50 individual brittlestars. Water samples were analysed for chlorophyll content. Stable isotope ratios for carbon and nitrogen were determined for each homogenate sample. Regular SCB counts were made on the infaunal brittlestar A. chiajei . Homogenate samples of Ophiothrix fragilis , A. chiajei and the non-symbiotic Ophiocomina nigra were analysed to produce fatty acid profiles for each species. Symbiotic bacteria count varied by up to one order of magnitude in both Ophiothrix fragilis and A. chiajei with no evidence of seasonality in this variation. Symbiotic bacteria number was inversely correlated with δ 15 N but no relationship was established with δ 13 C. 16:1ω7 and 18:1.ω7 fatty acids were used as putative bacterial markers. Both symbiotic species had higher percentages of 16:1ω7 than the non-symbiotic Ophiocomina nigra . However, only Ophiothrix fragilis appeared to receive appreciable quantities of 18:1ω7 from its SCB. The SCB are heterotrophic and may contribute to the nitrogen budget of the host. The two symbiotic species studied here derive the bulk of their nutrition from conventional feeding but SCB make significant, additional contributions.