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1.
The effects of immediately adjacent agricultural fertilization on nitrogen (N) at upland forest edges have not been previously
studied. Our objective was to determine whether N from fertilized agriculture enters northern Idaho forest edges and significantly
impacts their N status. We stratified 27 forest edge sampling sites by the N fertilization history of the adjacent land: current,
historical, and never. We measured N stable isotopes (δ15N), N concentration (%N), and carbon-to-nitrogen (C/N) ratios of conifer tree and deciduous shrub foliage, shrub roots, and
bulk soil, as well as soil available N. Conifer foliage δ15N and %N, shrub root δ15N, and bulk soil N were greater and soil C/N ratios lower (P < 0.05) at forest edges than interiors, regardless of adjacent fertilization history. For shrub foliage and bulk soil δ15N, shrub root %N and C/N ratios, and soil nitrate, significant edge–interior differences were limited to forests bordering
lands that had been fertilized currently or historically. Foliage and soil δ15N were most enriched at forest edges bordering currently fertilized agriculture, suggesting that these forests are receiving
N fertilizer inputs. Shrub root %N was greater at forest edges bordering currently fertilized agriculture than at those bordering
grasslands that had never been fertilized (P = 0.01). Elevated N at forest edges may increase vegetation growth, as well as susceptibility to disease and insects. The
higher N we found at forest edges bordering agriculture may also be found elsewhere, given similar agricultural practices
in other regions and the prevalence of forest fragmentation. 相似文献
2.
δ13C values of C3 plants are indicators of plant carbon–water relations that integrate plant responses to environmental conditions. However,
few studies have quantified spatial variation in plant δ13C at the landscape scale. We determined variation in leaf δ13C, leaf nitrogen per leaf area (Narea), and specific leaf area (SLA) in April and August 2005 for all individuals of three common woody species within a 308 × 12-m
belt transect spanning an upland–lowland topoedaphic gradient in a subtropical savanna in southern Texas. Clay content, available
soil moisture, and soil total N were all negatively correlated with elevation. The δ13C values of Prosopis glandulosa (deciduous N2-fixing tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) leaves increased 1–4‰ with decreasing elevation, with the δ13C value of P. glandulosa leaves being 1–3‰ higher than those of the two shrub species. Contrary to theory and results from previous studies, δ13C values were highest where soil water was most available, suggesting that some other variable was overriding or interacting
with water availability. Leaf Narea was positively correlated with leaf δ13C of all species (p < 0.01) and appeared to exert the strongest control over δ13C along this topoedaphic gradient. Since leaf Narea is positively related to photosynthetic capacity, plants with high leaf Narea are likely to have low p
I/p
a ratios and therefore higher δ13C values, assuming stomatal conductance is constant. Specific leaf area was not correlated significantly with leaf δ13C. Following a progressive growing season drought in July/August, leaf δ13C decreased. The lower δ13C in August may reflect the accumulation of 13C-depleted epicuticular leaf wax. We suggest control of leaf δ13C along this topoedaphic gradient is mediated by leaf Narea rather than by stomatal conductance limitations associated with water availability. 相似文献
3.
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.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
4.
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.
Received: 22 January 1999 / Accepted: 22 July 1999 相似文献
5.
Ungulate stimulation of nitrogen cycling and retention in Yellowstone Park grasslands 总被引:4,自引:0,他引:4
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.
Received: 5 May 1999 / Accepted: 1 November 1999 相似文献
6.
Nutrient dynamics on a precipitation gradient in Hawai'i 总被引:10,自引:0,他引:10
We evaluated soil and foliar nutrients in five native forests in Hawai'i with annual rainfall ranging from 500 mm to 5500 mm.
All of the sites were at the same elevation and of the same substrate age; all were native-dominated forests containing Metrosiderospolymorpha Gaud. Soil concentrations of extractable NO3-N and PO4-P, as well as major cations (Ca, Mg, and K), decreased with increasing annual precipitation, and δ15N values became more depleted in both soils and vegetation. For M.polymorpha leaves, leaf mass per area (LMA) and lignin concentrations increased significantly, while δ13C values became more depleted with increasing precipitation. Foliar phosphorus, and major cation (Ca, Mg, and K) concentrations
for M.polymorpha all decreased significantly with increasing precipitation. For other native forest species, patterns of LMA, δ13C, and δ15N generally mirrored the pattern observed for M.
polymorpha. Decreasing concentrations of available rock-derived nutrients in soil suggest that the effect of increased rainfall on leaching
outweighs the effect of increasing precipitation on weathering. The pattern of decreased foliar nutrient concentrations per
unit leaf area and of increased lignin indicates a shift from relatively high nutrient availability to relatively high carbon
gain by producers as annual precipitation increases. For nitrogen cycling, the pattern of higher inorganic soil nitrogen concentrations
in the drier sites, together with the progressively depleted δ15N signature in both soils and vegetation, suggests that nitrogen cycling is more open at the drier sites, with smaller losses
relative to turnover as annual precipitation increases.
Received: 24 March 1997 / Accepted: 19 September 1997 相似文献
7.
Casper BB Goldman R Lkhagva A Helliker BR Plante AF Spence LA Liancourt P Boldgiv B Petraitis PS 《Oecologia》2012,169(1):85-94
Topography should create spatial variation in water and nutrients and play an especially important role in the ecology of
water-limited systems. We use stable isotopes to discern how plants respond both to ecological gradients associated with elevation
and to neighboring legumes on a south-facing slope in the semi-arid, historically grazed steppe of northern Mongolia. Out
of three target species, Potentilla acaulis, Potentilla sericea, and Festuca lenensis, when >30 cm from a legume, all showed a decrease in leaf δ15N with increasing elevation. This, together with measures of soil δ15N, suggests greater N processing at the moister, more productive, lower elevation, and more N fixation at the upper elevation,
where cover of legumes and lichens and plant-available nitrate were greater. Total soil N was greater at the lower elevation,
but not lichen biomass or root colonization by AMF. Leaf δ13C values for P. acaulis and F. lenensis are consistent with increasing water stress with elevation; δ13C values indicated the greatest intrinsic water use efficiency for P. sericea, which is more abundant at the upper elevation. Nearby legumes (<10 cm) moderate the effect of elevation on leaf δ15N, confirming legumes’ meaningful input of N, and affect leaf δ13C for two species, suggesting an influence on the efficiency of carbon fixation. Variation in leaf %N and %C as a function
of elevation and proximity to a legume differs among species. Apparently, most N input is at upper elevations, pointing to
the possible importance of grazers, in addition to hydrological processes, as transporters of N throughout this landscape. 相似文献
8.
The projected recession of forests in the forest–steppe ecotone under projected climate drying would restrict the carbon sink
function of terrestrial ecosystems. Previous studies have shown that the forest–steppe ecotone in the southeastern Inner Mongolia
Plateau originally resulted from climate drying and vegetation shifts during the mid- to late-Holocene, but the interrelated
processes of changing soil carbon storage and vegetation and soil shifts remain unclear. A total of 44 forest soil profiles
and 40 steppe soil profiles were excavated to determine soil carbon storage in deciduous broadleaf forests (DBF), coniferous
forests (CF) and steppe (ST) in this area. Carbon density was estimated to be 106.51 t/hm2 (DBF), 73.20 t/hm2 (CF), and 28.14 t/hm2 (ST) for these ecosystems. Soil organic carbon (SOC) content was negatively correlated with sand content (R = −0.879, P < 0.01, n = 42), and positively correlated with silt (R = 0.881, P < 0.01, n = 42) and clay (R = 0.858, P < 0.01, n = 42) content. Consistent trends between fractions of coarse sand and a proxy index of relative aridity in sediment sequences
from two palaeo-lakes further imply that climate drying reduced SOC through coarsening of the soil texture in the forest–steppe
ecotone. Changes in carbon storage caused by climate drying can be divided into two stages: (1) carbon storage of the ecosystem
was reduced to 68.7%, mostly by soil coarsening when DBF were replaced by CF at ~5,900 14C years before present (BP); and (2) carbon storage was reduced to 26.4%, mostly by vegetation shifts when CF were replaced
by ST at ~2,900 14C years BP. 相似文献
9.
Abstract We used differences in soil carbon δ13C values between forested sites and grasslands dominated by the C4 grass Schizachyrium scoparium (little bluestem) to detect the presence of former grasslands in the historical landscape of the coastal sand plain of Martha's Vineyard, Massachusetts, U.S.A. Soil δ13C was measured at (1) sites with long‐term forest or grassland vegetation and (2) sites with known histories where forest vegetation invaded grassland and where forest converted to grassland. The δ13C of soil under long‐term grassland was –24.1‰ at 0 to 2 cm depth and –23.4‰ at 2 to 10 cm and was enriched by 3.4‰ and 2.8‰ compared with soil under long‐term forest. In forests that invaded grasslands dominated by S. scoparium, soil δ13C decreased as C derived from trees replaced C from S. scoparium. This decline occurred faster in surface soils and in the light soil organic matter fraction than in the mineral soil. In forests that converted to grasslands, soil δ13C increased and the rate of increase was similar in surface and mineral soil and in the different soil organic matter fractions. Rates of change indicated that soil δ13C could be used to detect changes in vegetation involving the presence or absence of S. scoparium during the last 150 years. Application of this model to a potential grassland restoration site on Martha's Vineyard where the landscape history was not known indicated that the site was previously unoccupied by S. scoparium during this time. The δ13C of surface mineral soil can be useful for detecting the presence of historic S. scoparium grasslands but only in the period well after European settlement of these coastal sand plain landscapes. 相似文献
10.
11.
In abandoned or extensively managed grasslands, the mechanisms involved in pioneer tree species success are not fully explained. Resource competition among plants and microclimate modifications have been emphasised as possible mechanisms to explain variation of survivorship and growth. In this study, we evaluated a number of mechanisms that may lead to successful survival and growth of seedlings of a pioneer tree species (Pinus sylvestris) in a grass-dominated grassland. Three-year-old Scots pines were planted in an extensively managed grassland of the French Massif Central and for 2 years were either maintained in bare soil or subjected to aerial and below-ground interactions induced by grass vegetation. Soil temperatures were slightly higher in bare soil than under the grass vegetation, but not to an extent explaining pine growth differences. The tall grass canopy reduced light transmission by 77% at ground level and by 20% in the upper part of Scots pine seedlings. Grass vegetation presence also significantly decreased soil volumetric water content (Hv) and soil nitrate in spring and in summer. In these conditions, the average tree height was reduced by 5% compared to trees grown in bare soil, and plant biomass was reduced by 85%. Scots pine intrinsic water-use efficiency (A/g), measured by leaf gas-exchange, increased when Hv decreased owing to a rapid decline of stomatal conductance (g). This result was also confirmed by δ
13C analyses of needles. A summer 15N labelling of seedlings and grass vegetation confirmed the higher NO3 capture capacity of grass vegetation in comparison with Scots pine seedlings. Our results provide evidence that the seedlings' success was linked to tolerance of below-ground resource depletion (particularly water) induced by grass vegetation based on morphological and physiological plasticity as well as to resource conservation. 相似文献
12.
Because nitrogen and phosphorus are primary resources for plant, algal, and microbial production, increases in nutrient inputs
can markedly alter aquatic ecosystems. Coastal wetland plots at Belle W. Baruch Marine Field Laboratory (South Carolina, USA)
have been amended with nitrogen and phosphorus for ~20 years to determine the effects of nutrient loading on coastal wetlands.
We conducted a survey of δ15N and δ13C natural abundance in coastal wetland organic pools (sediment, vegetation) with long-term nutrient amendments (control, no
addition; nitrogen; phosphorus; and nitrogen + phosphorus additions). Additionally, we conducted laboratory assays to quantify
pore water nutrient availability and nitrification rates. Marsh vegetation (Spartina alterniflora) had enriched δ13C values (mean −14‰) relative to bulk sediment samples (mean −18‰). Nitrogen-amended plots (alone and in combination with
phosphorus) had enriched δ13C values in the surface sediment (0–5 cm; mean −16.1‰) relative to control (mean −16.5‰) and phosphorus-amended plots (mean
−16.8‰). Nitrogen-amended plots also had depleted δ15N in S. alterniflora leaf tissues (−3.3‰) and surface sediment samples (mean 2.1‰) relative to leaf tissues (mean 2.1‰) or sediment samples (mean
5.8‰) from control or phosphorus-only amended plots. Nitrate availability (as increased pore water concentration) was higher
in N-amended plots although ammonium availability did not differ. Phosphorus availability was higher only in phosphorus-only
amended plots. Overall, we found that long-term nutrient amendments to coastal wetlands significantly altered nutrient availability
and uptake rates as well as natural abundance of δ13C and δ15N in multiple organic matter sources. 相似文献
13.
Understanding what governs patterns of soil δ15N and δ13C is limited by the absence of these data assembled throughout the development of individual ecosystems. These patterns are important because stable isotopes of soil organic N and C are integrative indicators of biogeochemical processing of soil organic matter. We examined δ15N of soil organic matter (δ15NSOM) and δ13CSOM of archived soil samples across four decades from four depths of an aggrading forest in southeastern USA. The site supports an old-field pine forest in which the N cycle is affected by former agricultural fertilization, massive accumulation of soil N by aggrading trees over four decades, and small to insignificant fluxes of N via NH3 volatilization, nitrification, and denitrification. We examine isotopic data and the N and C dynamics of this ecosystem to evaluate mechanisms driving isotopic shifts over time. With forest development, δ13CSOM became depth-dependent. This trend resulted from a decline of ~2‰ in the surficial 15 cm of mineral soil to −26.0‰, due to organic matter inputs from forest vegetation. Deeper layers exhibited relatively little trend in δ13CSOM with time. In contrast, δ15NSOM was most dynamic in deeper layers. During the four decades of forest development, the deepest layer (35–60 cm) reached a maximum δ15N value of 9.1‰, increasing by 7.6‰. The transfer of >800 kg ha−1 of soil organic N into aggrading vegetation and the forest floor and the apparent large proportion of ectomycorrhizal (ECM) fungi in these soils suggest that fractionation via microbial transformations must be the major process changing δ15N in these soils. Accretion of isotopically enriched compounds derived from microbial cells (i.e., ECM fungi) likely promote isotopic enrichment of soils over time. The work indicates the rapid rate at which ecosystem development can impart δ15NSOM and δ13CSOM signatures associated with undisturbed soil profiles. 相似文献
14.
Tropospheric O3 and deposition of reactive N threaten the composition and function of natural and semi-natural vegetation even in remote
regions. However, little is known about effects of these pollutants individually or in combination on plant species in alpine
habitats. We analyzed 11 frequent plant species of a subalpine Geo-Montani-Nardetum pasture exposed at 2,000 m a.s.l. in the Swiss Alps during 3 years using a factorial free-air exposure system with three
concentrations of O3 and five rates of N application. The aim was to detect subtle effects on leaf chlorophyll and N concentrations, leaf weight,
specific leaf area (SLA), and δ18O and δ13C as proxies for gas exchange. We expected that the species’ responsiveness to O3 and N would be related to their functional traits and that N-induced changes in these traits would modify the species’ response
to O3 via increased growth and higher leaf conductance (g
s). Most species reacted to N supply with the accumulation of N and chlorophyll, but with no change in SLA, g
s, and growth, except Carex sempervirens which showed increased water use efficiency and leaf weight. Elevated O3 reduced g
s
in most species, but this was not related to a reduction in leaf weight, which was recorded in half of the species. Contrary to our expectation, the magnitude of the response to both O3 and N was not related to species-specific traits such as SLA or g
s. No pronounced O3 × N interactions were observed. In conclusion, since for most species neither N nor gas exchange limited growth, their short-term
response to O3 and N and to their combination was small. O3 × N interactive effects are expected to be more pronounced in habitats where species are more responsive to N due to favorable
growth conditions in terms of nutrient availability and temperature.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
15.
Sílvia Fernanda Mardegan Gabriela Bielefeld Nardoto Niro Higuchi Marcelo Zacharias Moreira Luiz Antonio Martinelli 《Trees - Structure and Function》2009,23(3):479-488
Addressing spatial variability in nitrogen (N) availability in the Central Brazilian Amazon, we hypothesized that N availability
varies among white-sand vegetation types (campina and campinarana) and lowland tropical forests (dense terra-firme forests) in the Central Brazilian Amazon, under the same climate conditions.
Accordingly, we measured soil and foliar N concentration and N isotope ratios (δ15N) throughout the campina-campinarana transect and compared to published dense terra-firme forest results. There were no differences between white-sand vegetation
types in regard to soil N concentration, C:N ratio and δ15N across the transect. Both white-sand vegetation types showed very low foliar N concentrations and elevated foliar C:N ratios,
and no significant difference between site types was observed. Foliar δ15N was depleted, varying from −9.6 to 1.6‰ in the white-sand vegetations. The legume Aldina heterophylla had the highest average δ15N values (−1.5‰) as well as the highest foliar N concentration (2.1%) while the non-legume species had more depleted δ15N values and the average foliar N concentrations varied from 0.9 to 1.5% among them. Despite the high variation in foliar
δ15N among plants, a significant and gradual 15N-enrichment in foliar isotopic signatures throughout the campina–campinarana transect was observed. Individual plants growing in the campinarana were significantly enriched in 15N compared to those in campina. In the white-sand N-limited ecosystems, the differentiation of N use seems to be a major cause of variations observed in
foliar δ15N values throughout the campina–campinarana transect. 相似文献
16.
The natural relationship13C/12C determined in three soil profiles under grass vegetation indicated a depletion in organic13C at depth: theδ
13C was between −18‰ and −15‰ in the A horizons and ranged from −18 to −22‰ at depth. Previous work showed that in forest soils,
whereδ
13C was near −28‰ in the upper horizon, there was, on the contrary, a relative enrichment of the lower strata. This meant thatδ
13C, initially different in the various topsoils, became more equal at depth. Comparison between dark, deep horizons (sombric
horizons), which are certainly of illuvial origine, would confirm this:δ
13C of grassland and a forest sombric horizon were almost equal at around −22‰. These results might mean that, in natural ecosystems,
the isotopic carbon composition of the soil underlying humus would be independent of the vegetation type. This would have
practical implications for the use of13C as a tracer for soil organic matter studies. 相似文献
17.
J. M. Guehl A. M. Domenach M. Bereau T. S. Barigah H. Casabianca A. Ferhi J. Garbaye 《Oecologia》1998,116(3):316-330
Functional aspects of biodiversity were investigated in a lowland tropical rainforest in French Guyana (5°2′N, annual precipitation
2200 mm). We assessed leaf δ15N as a presumptive indicator of symbiotic N2 fixation, and leaf and wood cellulose δ13C as an indicator of leaf intrinsic water-use efficiency (CO2 assimilation rate/leaf conductance for water vapour) in dominant trees of 21 species selected for their representativeness
in the forest cover, their ecological strategy (pioneers or late successional stage species, shade tolerance) or their potential
ability for N2 fixation. Similar measurements were made in trees of native species growing in a nearby plantation after severe perturbation
(clear cutting, mechanical soil disturbance). Bulk soil δ15N was spatially quite uniform in the forest (range 3–5‰), whereas average leaf δ15N ranged from −0.3‰ to 3.5‰ in the different species. Three species only, Diplotropis purpurea, Recordoxylon speciosum (Fabaceae), and Sclerolobium melinonii (Caesalpiniaceae), had root bacterial nodules, which was also associated with leaf N concentrations higher than 20 mg g−1. Although nodulated trees displayed significantly lower leaf δ15N values than non-nodulated trees, leaf δ15N did not prove a straightforward indicator of symbiotic fixation, since there was a clear overlap of δ15N values for nodulated and non-nodulated species at the lower end of the δ15N range. Perturbation did not markedly affect the difference δ15Nsoil − δ15Nleaf, and thus the isotopic data provide no evidence of an alteration in the different N acquisition patterns. Extremely large
interspecific differences in sunlit leaf δ13C were observed in the forest (average values from −31.4 to −26.7‰), corresponding to intrinsic water-use efficiencies (ratio
CO2 assimilation rate/leaf conductance for water vapour) varying over a threefold range. Wood cellulose δ13C was positively related to total leaf δ13C, the former values being 2–3‰ higher than the latter ones. Leaf δ13C was not related to leaf δ15N at either intraspecific or interspecific levels. δ13C of sunlit leaves was highest in shade hemitolerant emergent species and was lower in heliophilic, but also in shade-tolerant
species. For a given species, leaf δ13C did not differ between the pristine forest and the disturbed plantation conditions. Our results are not in accord with the
concept of existence of functional types of species characterized by common suites of traits underlying niche differentiation;
rather, they support the hypothesis that each trait leads to a separate grouping of species.
Received: 18 August 1997 / Accepted: 14 April 1998 相似文献
18.
Adrian Ares Constance A. Harrington Thomas A. Terry Joseph M. Kraft 《Trees - Structure and Function》2008,22(5):603-609
The stable carbon (C) composition of tree rings expressed as δ13C, is a measure of intrinsic water-use efficiency and can indicate the occurrence of past water shortages for tree growth.
We examined δ13C in 3- to 5-year-old rings of Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) trees to elucidate if decreased water supply or uptake was a critical factor in the observed growth reduction
of trees competing with understory herb and shrub vegetation compared to those growing without competition. We hypothesized
that there would be no differences in δ13C of earlywood in trees growing in plots with competing vegetation and those in plots receiving complete vegetation control
during 5 years because earlywood formed early in the growing season when soil water was ample. We also hypothesized that δ13C in latewood which was formed during the later half of the growing season when precipitation was low, would be greater (less
negative) in trees in plots without vegetation control. We then separated early and latewood from rings for three consecutive
years and analyzed their δ13C composition. No significant differences in earlywood δ13C in years 3–5 were observed for trees in the two vegetation control treatments. δ13C of untreated latewood separated from wood cores was greater in 4- and 5-year-old rings of trees growing with competing vegetation
compared to trees growing without vegetation competition (i.e., −25.5 vs. −26.3‰ for year 4, and −26.1 vs. −26.8‰ for year
5). Results suggest that water shortages occurred in Douglas-fir trees on this coastal Washington site in the latewood-forming
portion of the growing season of years 4 and 5 in the no-vegetation control treatment. We also compared δ13C from untreated wood, crude cellulose extracted with the Diglyme–HCl method, and holocellulose extracted with toluene–ethanol
to see if the extraction method would increase the sensitivity of the analysis. δ13C values from the two extraction methods were highly correlated with those from untreated samples (r
2 = 0.97, 0.98, respectively). Therefore, using untreated wood would be as effective as using crude cellulose or holocellulose
to investigate δ13C patterns in young Douglas-fir. 相似文献
19.
Benthic biofilms have been identified using stable isotope analysis (SIA) as an important resource supporting many freshwater
food webs. However, biofilm δ13C signatures are highly variable in freshwaters, which may hamper our understanding of energy flow through food webs in these
systems. There has been little consideration of the influence that substratum may have on biofilm δ13C signature variability and energy flows to primary consumers. We investigated the effect of organic and inorganic substrata
on biofilm dynamics by examining: (1) temporal variability of biofilm stable isotope (δ13C, δ15N) signatures on allochthonous leaf-litter (Eucalyptus camaldulensis) and cobble substrata over 12 months in a lowland river in south-eastern Australia; and (2) the effect of substrata on biofilm
energy flows to a grazer snail, Physa acuta (Gastropoda: Physidae), using SIA and ecological stoichiometry in a laboratory experiment. The temporal study indicated that
cobble biofilm varied significantly in δ13C signature during the 12 months (up to 11‰), whereas the δ13C signature of leaf biofilm was less variable (less than 2‰). In contrast, biofilm δ15N signatures varied temporally on both cobble (2.6‰) and leaf (1‰) substrata. This suggests that leaf biofilm was more reliant
on leaf tissue for carbon and therefore less limited by carbon supply than cobble biofilm whereas for nitrogen biofilm on
both substrata was reliant on external sources. In the laboratory experiment, snails fed leaf biofilm reflected more of an
allochthonous δ13C signature than cobble biofilm fed snails, suggesting assimilation of leaf carbon via the heterotrophic microbial community
within the biofilm. Snails grew largest on cobble biofilm, which had lower C:N ratios than leaf biofilm. Our results demonstrate
that the type of substratum can influence the temporal variability of biofilm δ13C signatures and energy flow to primary consumers. 相似文献
20.
Indirect Effects of Nitrogen Amendments on Organic Substrate Quality Increase Enzymatic Activity Driving Decomposition in a Mesic Grassland 总被引:2,自引:0,他引:2
The fate of soil organic carbon (SOC) is determined, in part, by complex interactions between the quality of plant litter
inputs, nutrient availability, and the microbial communities that control decomposition rates. This study explores these interactions
in a mesic grassland where C and nitrogen (N) availability and plant litter quality have been manipulated using both fertilization
and haying for 7 years. We measured a suite of soil parameters including inorganic N, extractable organic C and N (EOC and
EON), soil moisture, extracellular enzyme activity (EEA), and the isotopic composition of C and N in the microbial biomass
and substrate sources. We use these data to determine how the activity of microbial decomposers was influenced by varying
levels of substrate C and N quality and quantity and to explore potential mechanisms explaining the fate of enhanced plant
biomass inputs with fertilization. Oxidative EEA targeting relatively recalcitrant C pools was not affected by fertilization.
EEA linked to the breakdown of relatively labile C rich substrates exhibited no relationship with inorganic N availability
but was significantly greater with fertilization and associated increases in substrate quality. These increases in EEA were
not related to an increase in microbial biomass C. The ratio of hydrolytic C:N acquisition enzymes and δ13C and δ15N values of microbial biomass relative to bulk soil C and N, or EOC and EON suggest that microbial communities in fertilized
plots were relatively C limited, a feature likely driving enhanced microbial efforts to acquire C from labile sources. These
data suggest that in mesic grasslands, enhancements in biomass inputs and quality with fertilization can prompt an increase
in EEA within the mineral soil profile with no significant increases in microbial biomass. Our work helps elucidate the microbially
mediated fate of enhanced biomass inputs that are greater in magnitude than the associated increases in mineral soil organic
matter. 相似文献