Soil δ15N Research Articles

Widespread foliage δ15N depletion under elevated CO2: inferences for the nitrogen cycle

AbstractLeaf 15N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO2 concentration. We tested the hypothesis that elevated CO2 significantly changes foliage δ15N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ15N of foliage of 27 field‐grown plant species from six free‐air CO2 enrichment (FACE) experiments representing desert, temperate forest, Mediterranean‐type, grassland prairie, and agricultural ecosystems. We found that within species, the δ15N of foliage produced under elevated CO2 was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ15N by life form and growth habit revealed that the CO2 effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO2 concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO2 and leaf δ15N. In a select number of species, we measured bulk soil δ15N at a depth of 10 cm, and found that the observed depletion of foliage δ15N in response to elevated CO2 was unrelated to changes in the soil δ15N. While the data suggest a strong influence of elevated CO2 on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO2 alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ15N responses to elevated CO2 reported here resulted from a general drop in δ15N of the source N, caused by soil‐driven processes. There is a stronger possibility, however, that the general depletion of foliage δ15N under high CO2 may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.

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.

Global patterns of the isotopic composition of soil and plant nitrogen

We compiled new and published data on the natural abundance N isotope composition (δ15N values) of soil and plant organic matter from around the world. Across a broad range of climate and ecosystem types, we found that soil and plant δ15N values systematically decreased with increasing mean annual precipitation (MAP) and decreasing mean annual temperature (MAT). Because most undisturbed soils are near N steady state, the observations suggest that an increasing fraction of ecosystem N losses are 15N‐depleted forms (NO3, N2O, etc.) with decreasing MAP and increasing MAT. Wetter and colder ecosystems appear to be more efficient in conserving and recycling mineral N. Globally, plant δ15N values are more negative than soils, but the difference (δ15Nplant‐δ15Nsoil) increases with decreasing MAT (and secondarily increasing MAP), suggesting a systematic change in the source of plant‐available N (organic/NH4+ versus NO3−) with climate. Nitrogen isotopes reflect time integrated measures of the controls on N storage that are critical for predictions of how these ecosystems will respond to human‐mediated disturbances of the global N cycle.

Nitrogen dynamics in alpine ecosystems of the northern Caucasus

Net N mineralization, nitrification, microbial biomass N and 15N natural abundance were studied in a toposequence of representative soils and plant communities in the alpine zone of the northern Caucasus. The toposequence was represented by (1) low-productive alpine lichen heath (ALH) of wind-exposed ridge and upper slope; (2) more productive Festuca varia grassland (FG) of middle slope; (3) most productive Geranium gymnocaulon/Hedusarum caucasicummeadow (GHM) of lower slope; (4) low-productive snowbed community (SBC) of the slope bottom. N availability, net N mineralization and nitrification were higher in soils of alpine grassland and meadow of the middle part of the toposequence compared with soils of lichen heath and snowbed community of extreme habitats in the alpine zone. There was no correlation between intensities of N transformation processes and favorable (low soil acidity, low C/N ratio, long vegetation period, relatively high temperature, absence of hydromorphic features) and unfavorable (opposite) factors, indicating that the intensity of N mineralization and nitrification in the alpine soils is controlled by a complex combination of these factors. Potential net N mineralization and nitrification in alpine soils determined in the short-term laboratory incubation were considerably higher than those determined in the long-term field incubation. The differences of potential nitrification between soils of various plant communities did not correspond to the field determined pattern indicating the importance of on-site climatic conditions for control of nitrification in high mountains. The result of comparison of N transformation potentials in incubated and native soils indicated that nitrification potential was significantly increased after long-term soil incubation. It means that net nitrification determined in the field was probably overestimated, especially in the meadow soils. A soil translocation experiment indicated that low temperature was an important factor limiting net N mineralization and nitrification in alpine soils: net N mineralization and especially nitrification increased when alpine soils were translocated into the subalpine zone and mean annual temperature increased by about 3 °C. Additional N input increased N availability (NH4+-N) and potential nitrification in soils of the lower part of the toposequense (GHM and SBC), and potential net N mineralization in two soils of extreme habitats (ALH and SBC). A positive correlation was found between soil δ15N and net N mineralization and nitrification; the relative 15N enrichment was characteristic of grassland and meadow ecosystems. δ15N of total soil N pool increased during the field mineralization experiment; there was a positive tendency between the change in δ15N and net N mineralization and nitrification, however the relationship was not significant. Foliar δ15N of dominant plant species varied widely within community, however, a tendency of higher foliar δ15N for species growing on the soils with higher net N mineralization, nitrification and δ15N was observed.

Influence of former agricultural land use on net nitrate production in forest soils

SummaryExcept where nitrate is added to the soil artificially, nitrate is leached from forest soils only if it is produced. Although the factors influencing nitrification have been widely studied, nitrification activity still cannot be simply predicted from ecosystem characteristics. In France, about half of the present forest area was agricultural in 1850. Previous work suggested that former cultivation could be a major factor influencing nitrogen availability in forest soils.Using laboratory incubations, we compared the net production of ammonium and nitrate in soils from formerly manured lands planted with conifers 70–100 years ago with that in soils of surrounding ancient coniferous forests. Net nitrate production, available P content, and natural abundance of nitrogen 15, δ15N, were greater in soils from formerly manured plots than other land, whereas the C:N ratio of the soil was less. The difference in net nitrate production between previously manured sites and adjacent ancient forests was related to differences in δ15N values in the soil but not evidently to other soil properties. Because soil δ15N increases with the intensity of organic manuring, these results suggest that nitrification in forest soils depends on previous manurial practices under agriculture. In this context, the soil δ15N might be used as an indicator of both previous agricultural land use and potential nitrification. Because a significant proportion of West European forests grow on previously cultivated soils, past land use should be taken into account when evaluating the risks of nitrate leaching from forests.

Response of the natural abundance of15N in forest soils and foliage to high nitrate loss following clear-cutting

Export of microbially produced nitrate from an ecosystem is expected to increase δ15N in the remaining soil organic matter and NH4+. To test the hypothesis that nitrification and nitrate loss induced by clear-cutting cause an increase in soil and foliar δ15N, we measured δ15N in a clear-cut watershed at the Hubbard Brook Experimental Forest, New Hampshire. δ15N ranged from –0.02‰ in the Oie horizon to 7.7‰ in the Bs2 horizon prior to clear-cutting and increased significantly by 1.3‰ in the Oie horizon and 0.9‰ in the Oa horizon 3 years after clear-cutting. Fifteen years after clear-cutting, δ15N in both O horizons decreased to near-initial values. No significant temporal changes in the Bs2 and C horizons δ15N were observed. Foliar δ15N was highest (1.7‰) the first 2 years after clear-cutting and was significantly higher than in the reference watershed (mean δ15N = –1.2‰), decreasing to 0.0‰ 3–5 years after clear-cutting and to –1.3‰ 9–11 years after clear-cutting. Increased foliar δ15N coincided with increased stream-water nitrate concentration, suggesting that the increased nitrification responsible for elevated stream-water nitrate may also have caused an enrichment of the plant-available ammonium pool. The response observed in this catchment also suggests that sampling of soil or foliar δ15N may provide a practical alternative to long time series of stream chemistry for evaluating nitrogen saturation of forested ecosystems.

Alterations of nitrogen dynamics under elevated carbon dioxide in an intact Mojave Desert ecosystem: evidence from nitrogen-15 natural abundance.

We examined soil and vegetation N isotopic composition (δ15N) and soil inorganic N availability in an intact Mojave desert ecosystem to evaluate potential effects of elevated atmospheric CO2 on N cycling. Vegetation from the dominant perennial shrub Larrea tridentata under elevated CO2 was enriched in 15N. Over a 7-month sampling period, Larrea δ15N values increased from 5.7±0.1‰ to 9.0±1.1‰ with elevated CO2; under ambient conditions, δ15N values of shrubs increased from 4.9±0.3‰ to 6.6±0.7‰. No difference was found in soil δ15N under elevated and ambient CO2. Soil δ15N values under the drought deciduous shrubs Lycium spp. were greatest (7.2±0.3‰), and soil under the C4 perennial bunchgrass Pleuraphis rigida had the lowest values (4.5±0.2‰). Several mechanisms could explain the enrichment in 15N of vegetation with elevated CO2. Results suggest that microbial activity has increased with elevated CO2, enriching pools of plant-available N and decreasing N availability. This hypothesis is supported by a significant reduction of plant-available N under elevated CO2. This indicates that exposure to elevated CO2 has resulted in significant perturbations to the soil N cycle, and that plant δ15N may be a useful tool for interpreting changes in the N cycle in numerous ecosystems.

Nitrogen deposition and forest expansion in the northern Great Plains

Summary Atmospheric nitrogen (N) deposition has become one of the most important agents of vegetation change in densely populated regions. It may also contribute to forest expansion into grasslands at the northern edge of the North American Great Plains. We measured N deposition and available soil N with ion‐exchange resin over 2 years in six national parks in areas varying in population density and industrialization. N deposition was significantly higher in four parks in densely populated regions than in two remote parks. Available soil N increased significantly with N deposition across all parks. We measured N mass and 15N abundance (δ15N) in vegetation and soil in two parks: Elk Island, receiving 22 kg N ha−1 year−1, and Jasper, receiving 8 kg N ha−1 year−1. Differences between parks in tissue N concentrations were small, but forest expansion over five decades resulted in the mass of N in vegetation increasing by 74% in Elk Island but by only 26% in Jasper. δ15N in forest vegetation was significantly lower in Elk Island than in Jasper, suggesting that anthropogenic sources contribute significantly to the high rates of N entering that ecosystem. We determined the rate of forest expansion within parks using six decades of aerial photographs. Parks in aspen parkland and boreal forest showed a strong positive relationship between forest expansion and N deposition. The relationships found between N deposition, available soil N and forest expansion suggest that even comparatively low rates of N deposition may accelerate the expansion of forest into temperate grasslands.

Carbon and Nitrogen Dynamics in Preferential Flow Paths and Matrix of a Forest Soil

Natural abundance (δ) of the stable isotopes 13C and 15N has gained acceptance for studying C and N cycling in forests. In most studies, bulk soil samples are collected to determine isotope abundance. Such sampling overlooks the potential impact of preferential flow on isotope distribution. The objective of this study was to investigate the effects of preferential flow on the distribution of soil organic carbon (SOC), total N, δ13C, and δ15N in a forest soil in Central Switzerland. Preferential flow paths in the soil were identified with a dye tracer, Brilliant Blue (Plüss‐Staufer AG, Oftringen, Switzerland), that was homogeneously applied to the soil surface. In the stained preferential flow paths, concentrations of SOC and total N were 15 to 75% higher than in the soil matrix. The total increase of SOC in preferential flow paths ranged from 740 to 960 g C m−2 in four individual soil plots. Values of δ13C and δ15N were lowest in tree leaves and in the forest floor, and increased with soil depth, thus with the degree of decomposition of SOC. In the mineral soil, preferential flow paths were significantly depleted in 13C by 0.15 to 0.4‰ as compared with the soil matrix. The δ15N values increased with soil depth from 0.9 to 4.7‰ in the preferential flow paths and from 0.5 to 6‰ in the soil matrix. Adding a highly enriched 15N‐tracer homogeneously to the soil surface showed a higher recovery of 15N in the soil and in the fine roots sampled from preferential flow paths than in those sampled from the soil matrix. Our results suggest that in preferential flow paths, SOC is younger and N cycling is more rapid than in the soil matrix.

Variability of δ15N in soil and plants at a New Zealand hill country site: correlations with soil chemistry and nutrient inputs

This study investigated 15N enrichment and nutrient cycling in hill country used for semi-extensive pastoral agriculture, at a site where pre-European seabird breeding occurred. Soil (0–15 cm) and plant samples were taken from 18 ridgeline and sideslope transects. Three stock camps (locations which grazing animals frequent) were identified within the study area, two on the ridgeline and one on the sideslope. Soil 15N enrichment was greatest at stock camps, and lowest where stock input was minimal. Soil natural abundance 15N (δ15N) was therefore an index of stock nutrient inputs. Soil δ15N increased with decreasing C: N ratio, consistent with N loss through volatilisation and/or nitrate leaching from net mineralisation. Plant δ15N from stock camps was lower than its associated soil, implying that 15N enrichment of plant-available N was lower than that of total soil N. However, the correlation between plant δ15N and soil δ15N varied between stock camps, indicating differences in N cycling. Olsen P was higher at stock camps, although again differences were found between stock camps. Total P and N were correlated neither with stock camps nor topography, but were higher than expected from parent material concentrations and literature results, respectively. It is postulated that significant contributions of both elements from former seabird breeding remain in the soil.

Natural abundance of stable carbon and nitrogen isotopes in Cannabis sativa reflects growth conditions

Stable carbon and nitrogen isotope signatures (δ13C and δ15N) of Cannabis sativa were assessed for their usefulness to trace seized Cannabis leaves to the country of origin and to source crops by determining how isotope signatures relate to plant growth conditions. The isotopic composition of Cannabis examined here covered nearly the entire range of values reported for terrestrial C3 plants. The δ13C values of Cannabis from Australia, Papua New Guinea and Thailand ranged from –36 to –25‰, and δ 15N values ranged from –1.0 to 15.8‰. The stable isotope content did not allow differentiation between Cannabis originating from the three countries, but δ13C values of plantation-grown Cannabis differed between well-watered plants (average δ13C of–30.0‰) and plants that had received little irrigation (average δ13C of –26.4‰). Cannabis grown under controlled conditions had δ13C values of –32.6 and –30.6‰ with high and low water supply, respectively. These results indicate that water availability determines leaf 13C in plants grown under similar conditions of light, temperature and air humidity. The δ13C values also distinguished between indoor- and outdoor-grown Cannabis; indoor-grown plants had overall more negative δ13C values (average –31.8‰) than outdoor-grown plants (average –27.9‰). Contributing to the strong 13C-depletion of indoor-grown plants may be high relative humidity, poor ventilation and recycling of 13C-depleted respired CO2. Mineral fertilizers had mostly lower δ15N values (–0.2 to 2.2‰) than manure-based fertilizers (7.6 to 22.7‰). It was possible to link δ15N values of fertilizers associated with a crop site to soil and plant δ15N values. The strong relationship between soil, fertilizer, and plant δ15N suggests that Cannabis δ15N is determined by the isotopic composition of the nitrogen source. The distinct d15N values measured in Cannabis crops make δ15N an excellent tool for matching seized Cannabiswith a source crop. A case study is presented that demonstrates how δ13C and δ15N values can be used as a forensic tool.

Temporal Stability of Spatial Patterns of Nitrous Oxide Fluxes from Sloping Grassland

AbstractInsight into the temporal and spatial variability of nitrous oxide (N2O) fluxes from soils is required to set up efficient sampling protocols of N2O fluxes and to set up strategies to reduce N2O fluxes. The aim of the present study was to assess the temporal stability of the spatial pattern of N2O fluxes along a transect (400 m) on a fertilized sloping grassland soil. Geostatistical analyses of flux chamber measurements over 4 d showed a clear spatial dependency of N2O fluxes. The spatial variability of N2O fluxes was much higher than the temporal variability during the 4 d. Despite the decreasing magnitude of the fluxes in time, the spatial pattern persisted during 4 d, that is, fluxes were always highest at the steepest part of the transect. The 15N natural abundance (δ15N) in soil N was significantly correlated with N2O fluxes, indicating a long‐term effect of N loss as N2O on the 15N abundance in the soil. It was suggested that topography played a role in the observed spatial patterns of N2O fluxes and Δ15N in soil N. The study showed that despite the persistent spatial pattern of N2O fluxes, flux magnitude may strongly fluctuate in time. Therefore, it is often not possible to combine N2O flux measurements made at different times in order to obtain enough data for geostatistical analysis.

ORIGIN AND DISTRIBUTION OF SOIL ORGANIC MATTER IN TAYLOR VALLEY, ANTARCTICA

Soil organic matter in the dry valleys of the McMurdo Sound Region sustains a unique terrestrial microbial community in one of the most extreme climates on earth. The origin of this organic material has long been in question, given the absence of higher plants in this arid ecosystem. We examined the sources and distribution of organic matter from six elevational transects extending along a 30-km section of Taylor Valley (163° E, 77.35° S), from the head of the valley to the Ross Sea. The δ13C and δ15N of soil were determined both to identify potential sources of soil organic matter (i.e., marine, lacustrine, and/or terrestrial) and to map C and N source distribution in the valley. Results suggest that the primary source for soil organic matter is not the wind transport of organic material from modern aquatic systems (a long-held hypothesis). Instead, our data indicate that modern distributional patterns of low-elevation (<150 m above sea level [a.s.l.]) soil organic matter (SOM) in Taylor Dry Valley are s...

Origin and Distribution of Soil Organic Matter in Taylor Valley, Antarctica

Soil organic matter in the dry valleys of the McMurdo Sound Region sustains a unique terrestrial microbial community in one of the most extreme climates on earth. The origin of this organic material has long been in question, given the absence of higher plants in this arid ecosystem. We examined the sources and distribution of organic matter from six elevational transects extending along a 30-km section of Taylor Valley (163° E, 77.35° S), from the head of the valley to the Ross Sea. The δ13C and δ15N of soil were determined both to identify potential sources of soil organic matter (i.e., marine, lacustrine, and/or terrestrial) and to map C and N source distribution in the valley. Results suggest that the primary source for soil organic matter is not the wind transport of organic material from modern aquatic systems (a long-held hypothesis). Instead, our data indicate that modern distributional patterns of low-elevation (<150 m above sea level [a.s.l.]) soil organic matter (SOM) in Taylor Dry Valley are strongly influenced by the climatic history of the region, with the isotopic signatures of SOM along the valley floor corresponding to the spatial distribution of ancient glacial tills and lacustrine systems. At many sites, higher elevation soils (>150 m a.s.l.) have isotopic signatures similar to those of autotrophs currently inhabiting nearby cryptoendolithic communities, suggesting that similar autotrophic activity may be occurring in these soils and that long-term primary productivity in dry valley soils contributes to organic matter accumulations. Taken together, these findings support a new “legacy” model for soil organic matter origins and fluxes in the dry valley ecosystem.

Site history affects soil and plant 15N natural abundances (δ15N) in forests of northern Vancouver Island, British Columbia

Abstract1. About 10 years after establishment, plantations of Western Redcedar (Thuja plicata Donn ex D. Don) on northern Vancouver Island, British Columbia become nutrient deficient and chlorotic, grow slowly, and are susceptible to invasion by the ericaceous shrub Salal (Gaultheria shallon Pursh.).2. To test the hypothesis that δ15N can be related to site histories (site disturbance, soil N dynamics and plant development), we measured soil and foliar δ15N in the summer of 1992 in 3‐year‐old (nutrient‐sufficient) and 10‐year‐old (nutrient‐deficient) plantations and in old‐growth stands. The foliar and soil δ15N values of the plantations and old‐growth forests were different and closely reflected site histories. Salal invasion and nutrient deficiency interacted to depress the growth of Redcedar in 10‐year‐old plantations.3. Site preparation destroyed the top soil organic layers (fresh and decaying litter) and forced Salal (ecto‐ and ericoid mycorrhizal) into the humus layer, where it was in direct competition with Redcedar, thereby disadvantaging arbuscular mycorrhizal/non‐mycorrhizal Redcedar in its nutrient acquisition during a period when N and P are severely limited.4. There was a large seasonal range of foliar δ15N (5·5 and 4·3‰ for 10‐year‐old Redcedar and Salal, respectively), and there was no relationship between foliar δ15N and measured rooting depth, demonstrating that rooting depths cannot be used to explain foliar δ15N variation among coexisting woody taxa.5. Foliar and soil δ15N declined with site age and with a presumed change from ‘open’ to ‘closed’ N cycling; the 15N‐depleting effects of mycorrhizal N transformations contributed to the observed δ15N decline.

Ungulate stimulation of nitrogen cycling and retention in Yellowstone Park grasslands.

We studied how ungulates and a large variation in site conditions influenced grassland nitrogen (N) dynamics in Yellowstone National Park. In contrast to most grassland N studies that have examined one or two soil N processes, we investigated four rates, net N mineralization, nitrification, denitrification, and inorganic N leaching, at seven paired sites inside and outside long-term (33+ year) exclosures. Our focus was how N fluxes were related to one another among highly variable grasslands and how grazers influenced those relationships. In addition, we examined variation in soil δ15N among grasslands and the relationships between soil 15N abundance and N processes. Previously, ungulates were reported to facilitate net N mineralization across variable Yellowstone grasslands and denitrification at mesic sites. In this study, we found that herbivores also promoted nitrification among diverse grasslands. Furthermore, net N mineralization, nitrification, and denitrification (kg N ha-1 year-1, each variable) were postively and linearly related to one another among all grasslands (grazed and fenced), and grazers reduced the nitrification/net N mineralization and denitrification/net N mineralization ratios, indicating that ungulates inhibited the proportion of available NH4+ that was nitrified and denitrified. There was no relationship between net N mineralization or nitrification with leaching (indexed by inorganic N adsorbed to resin buried at the bottom of rooting zones) and leaching was unaffected by grazers. Soil δ15N was positively and linearly related to in situ net N mineralization and nitrification in ungrazed grasslands; however, there was no relationship between isotopic composition of N and those rates among grazed grasslands. The results suggested that grazers simultaneously increased N availability (stimulated net N mineralization and nitrification per unit area) and N conservation (reduced N loss from the soil per unit net N mineralization) in Yellowstone grasslands. Grazers promoted N retention by stimulating microbial productivity, probably caused by herbivores promoting labile soil C. Process-level evidence for N retention by grazers was supported by soil δ15N data. Grazed grassland with high rates of N cycling had substantially lower soil δ15N relative to values expected for ungrazed grassland with comparable net N mineralization and nitrification rates. These soil 15N results suggest that ungulates inhibited N loss at those sites. Such documented evidence for consumer control of N availability to plants, microbial productivity, and N retention in Yellowstone Park is further testimony for the widespread regulation of grassland processes by large herbivores.

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions.

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ15N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ15N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ15N varied widely, between 0 and -2‰ for nitrogen-fixing species, between 0 and -7‰ for deciduous non-fixing species, and between 0 and -11‰ for coniferous species. Relatively constant δ15N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ15N differences among species at individual sites. In fact, the largest variation among plant δ15N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ15N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ15N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ15N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ15N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T r.

Effects of land use on 15N natural abundance of soils in Ethiopian highlands

We used the natural abundance of 15N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in 13C natural abundance, patterns of δ15N and δ13C values were compared to determine if shifts of 15N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ15N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ15N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ13C of the surface horizon and the high δ13C of the lower soil horizons is clear evidence of a long phase of C4 grass dominance or cultivation of C4 crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ13C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ15N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ15N values of surface mineral soils. The high δ15N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation.

δ15N of forest soil and understorey vegetation reflect the former agricultural land use.

Since the middle of the 19th century, the area covered by forests in France has doubled. These new forests grow on previous agricultural lands. We have studied the influence of this agricultural history on the 15N abundance of present-day forests planted on farmlands in the Vosges mountains (north-eastern France) between 1898 and 1930. Different types of land use were identified from old cadastres (1814-1836) of 16 farms. Ancient forests adjacent to farmlands were used as controls. Former pastures, meadows, croplands, gardens and ancient forests were compared for soil δ15N (fraction <50 µm and total soil), C/N, P and N content and fern (Dryopteris carthusiana) δ15N. The mean δ15N of soil increased in the order ancient forests (+0.0‰)<pastures (+1.4‰)<croplands (+1.6‰)<meadows (+2.5‰)<gardens (+3.8‰). This increase in soil δ15N with the intensity of former land use was related to the former input of 15N-enriched manure, and to an activation of soil nitrification leading to 15N-depleted nitrate export on previously manured parcels. Fern δ15N increased in the same order as soil δ15N in relation to past land use. The mean δ15N of fern in ancient forests (-4.4‰) and former pastures (-3.4‰) was 5‰ lower than soil δ15N and the two variables were strongly correlated. The δ15N of fern in formerly manured parcels varied little (cropland: -2.7‰, meadows: -2.6‰ and gardens: -2.2‰) and independently of soil δ15N, suggesting that the soil sources of fern N differed between unmanured and manured parcels. Understorey plant δ15N and soil δ15N appear to be excellent tracers of previous land use in forests, and could be used in historical studies. The persistence of high isotopic ratios in previously manured parcels, almost a century after afforestation, suggests a long-term influence of former land use on the N cycle in forest soils.

Interpretation of nitrogen isotope signatures using the NIFTE model.

Nitrogen cycling in forest soils has been intensively studied for many years because nitrogen is often the limiting nutrient for forest growth. Complex interactions between soil, microbes, and plants and the consequent inability to correlate δ15N changes with biologic processes have limited the use of natural abundances of nitrogen isotopes to study nitrogen (N) dynamics. During an investigation of N dynamics along the 250-year-old successional sequence in Glacier Bay, Alaska, United States, we observed several puzzling isotopic patterns, including a consistent decline in δ15N of the late successional dominant Picea at older sites, a lack of agreement between mineral N δ15N and foliar δ15N, and high isotopic signatures for mycorrhizal fungi. In order to understand the mechanisms creating these patterns, we developed a model of N dynamics and N isotopes (Nitrogen Isotope Fluxes in Terrestrial Ecosystems, NIFTE), which simulated the major transformations of the N cycle and predicted isotopic signatures of different plant species and soil pools. Comparisons with field data from five sites along the successional sequence indicated that NIFTE can duplicate observed patterns in δ15N of soil, foliage, and mineral N over time. Different scenarios that could account for the observed isotopic patterns were tested in model simulations. Possible mechanisms included increased isotopic fractionation on mineralization, fractionation during the transfer of nitrogen from mycorrhizal fungi to plants, variable fractionation on uptake by mycorrhizal fungi compared to plants, no fractionation on mycorrhizal transfer, and elimination of mycorrhizal fungi as a pool in the model. The model results suggest that fractionation during mineralization must be small (˜2‰), and that no fractionation occurs during plant or mycorrhizal uptake. A net fractionation during mycorrhizal transfer of nitrogen to vegetation provided the best fit to isotopic data on mineral N, plants, soils, and mycorrhizal fungi. The model and field results indicate that the importance of mycorrhizal fungi to N uptake is probably less under conditions of high N availability. Use of this model should encourage a more rigorous assessment of isotopic signatures in ecosystem studies and provide insights into the biologic transformations which affect those signatures. This should lead to an enhanced understanding of some of the fundamental controls on nitrogen dynamics.