C. M. Scrimgeour

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.

Mechanistic interpretation of carbon isotope discrimination by marine macroalgae and seagrasses

The literature, and previously unpublished data from the authors laboratories, shows that the δ13C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from -3 to -35‰. While some marine macroalgae have δ13C values ranging over more than 10‰ within the thallus of an individual (some brown macroalgae), in other cases the range within a species collected over a very wide geographical range is only 5‰ (e.g. the red alga Plocamium cartilagineum which has values between -30 and -35‰). The organisms with very negative δ13C (lower than -30‰) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δ13C values (higher than -10‰) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δ13C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δ13C values lower than -30‰ have pyrenoids. Previous work showed a good correlation between δ13C values lower than -30‰ and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low δ13C values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher δ13C values if diffusive conductance was relatively low. The photosynthesis of organisms with δ13C values more positive than -10‰ (i.e. more positive than the δ13C of CO2 in seawater) must involve HCO3- use.

A Comparison of Soil Microbial Community Structure, Protozoa and Nematodes in Field Plots of Conventional and Genetically Modified Maize Expressing the Bacillus thuringiens is CryIAb Toxin

Field trials were established at three European sites (Denmark, Eastern France, South-West France) of genetically modified maize (Zea mays L.) expressing the CryIAb Bacillus thuringiensis toxin (Bt), the near-isogenic non-Bt cultivar, another conventional maize cultivar and grass. Soil from Denmark was sampled at sowing (May) and harvest (October) over two years (2002, 2003); from E France at harvest 2002, sowing and harvest 2003; and from SW France at sowing and harvest 2003. Samples were analysed for microbial community structure (2003 samples only) by community-level physiological-profiling (CLPP) and phospholipid fatty acid analysis (PLFA), and protozoa and nematodes in all samples. Individual differences within a site resulted from: greater nematode numbers under grass than maize on three occasions; different nematode populations under the conventional maize cultivars once; and two occasions when there was a reduced protozoan population under Bt maize compared to non-Bt maize. Microbial community structure within the sites only varied with grass compared to maize, with one occurrence of CLPP varying between maize cultivars (Bt versus a conventional cultivar). An overall comparison of Bt versus non-Bt maize across all three sites only revealed differences for nematodes, with a smaller population under the Bt maize. Nematode community structure was different at each site and the Bt effect was not confined to specific nematode taxa. The effect of the Bt maize was small and within the normal variation expected in these agricultural systems.

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).

Disentangling the impact of AM fungi versus roots on soil structure and water transport

The relative importance of roots and AM-fungi on soil physical processes was investigated by controlling the presence of roots and AM fungi in pot experiments using a mycorrhiza-defective tomato mutant and a wild-type tomato (Solanum lycopersicum L.). Root-Zone and Bulk Soil sections were established by splitting pots into two lengthwise halves using a nylon mesh that contained roots whilst allowing the free movement of fungal hyphae. Post-incubation microbial populations and fungal biomass were measured and related to soil stability, pore structure and water repellency. Unplanted controls consistently had the least fungal biomass, fatty acids, water-stable aggregates (WSA) and water repellency. Wild-type-planted treatments had significantly more WSA than mycorrhiza-defective treatments (Pu2009<u20090.01). Fluctuations in water content induced by transpiration caused significant changes in soil pore structure, measured using high-resolution X-Ray computer tomography. Porosity and mean pore size increased in soil aggregates from planted treatments, which had larger more heterogeneous pores than those in the unplanted soils. AM fungi accentuated soil stability. However, changes were not linked to repellency and fungal biomass. The presence of plants, regardless of AM fungi, appears to have the greatest impact on increasing soil stability.

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.

Intraspecific transfer of carbon between plants linked by a common mycorrhizal network

To quantify the involvement of arbuscular mycorrhiza (AM) fungi in the intraspecific transport of carbon (C) between plants we fumigated established Festuca ovina turf for one week with air containing depleted 13C. This labelled current assimilate in a section of mycorrhizal or non-mycorrhizal turf. Changes in the 13/12C ratio of adjacent, unfumigated plants, therefore, allowed the movement of C between labelled and unlabelled plants to be estimated. In mycorrhizal turves, 41% of the C exported to the roots from the leaves was transported to neighbouring plants. The most likely explanation of this is was the transport of C via a common hyphal network connecting the roots of different plants. No inter-plant transport of C was detected in non-mycorrhizal turves. There was no evidence that the C left fungal structures and entered the roots of receiver plants. Mycorrhizal colonisation increased carbon transport from leaves to root from 10% of fixed carbon when non-mycorrhizal to 36% in mycorrhizal turves. These results suggest that AM fungi impose a significantly greater C drain on host plants than was previously thought.