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1.

Four meadows of turtle grass (Thalassia testudinum Banks ex Konig) in Sarasota Bay, Florida were sampled on a bimonthly basis from June 1992 to July 1993 to determine spatial and temporal variation in short shoot density, biomass, productivity, and epiphyte loads. Concurrent with the seagrass sampling, quarterly water-quality monitoring was undertaken at ≥3 sites in the vicinity of each studied seagrass meadow. Three months after termination of the seagrass sampling effort, a biweekly water-quality monitoring program was instituted at two of the seagrass sampling sites. In addition, a nitrogen loading model was calibrated for the various watersheds influencing the seagrass meadows. Substantial spatial and temporal differences in turtle grass parameters but smaller spatial variation in water quality parameters are indicated by data from both the concurrent quarterly monitoring program and the biweekly monitoring program instituted after termination of the seagrass study. Turtle grass biomass and productivity were negatively correlated with watershed nitrogen loads, while water quality parameters did not clearly reflect differences in watershed nutrient inputs. We suggest that traditional water-quality monitoring programs can fail to detect the onset or continuance of nutrient-induced declines in seagrass health. Consequently, seagrass meadows should be monitored directly as a part of any effort to determine status and/or trends in the health of estuarine environments. *** DIRECT SUPPORT *** A01BY074 00029

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2.
The effects of light reduction on community metabolism and sediment sulfate reduction rates (SRR) were assessed experimentally in a shallow (<2.0 m) seagrass (Thalassia testudinum) meadow along Florida's north-central Gulf coast. Nine experimental plots (1.5 m×1.5 m) were shaded differentially to achieve a 0–90% gradient in light reduction within the seagrass meadow. Gross primary production and net community production (NCP), estimated with in situ benthic chamber incubations, decreased with increasing light reduction. The compensation irradiance for community metabolism, i.e., the shading level at which NCP shifted from net autotrophic to net heterotrophic, was determined to be 52.5% of the incoming irradiance at canopy height in the seagrass bed (308.7 μE m−2 s−1 PAR at noon). Sediment SRR, determined with the use of a35S−SO4 2− radiotracer technique, increased quickly (within 5 d) and markedly with increased shade, i.e., simulated light reduction. SRR increased 50-fold when shading exceeded the light compensation point for the seagrass community, rendering the community net heterotrophic. Five days after restoring ambient light conditions, SRR had decreased sharply for all shading treatments. The observed decrease in NCP, coincident with the increase in the SRR with light reduction, suggests that light reduction has an indirect influence on sediment SRR mediated through its effect on seagrass metabolism.  相似文献   

3.
A year-long analysis of the characteristics of the seagrassSyringodium filiforme and the associated dynamics of the nutrient pool in the sediment pore water was done to assess co-variation. Changes in seagrass growth rate and standing stock throughout the year were accompanied by seasonal changes in the nutrient pools. The link between plant production and morphometrics and the sediment nutrient pool was found to be predominantly physiological, with the plant balancing the ability to photosynthesize with the nutrients needed for maintaining production. Measurements of whole plant growth for this seagrass, rather than the more typical leaf growth measurements, show that the production of new shoots and rhizome elongation for these plants represents as substantial amount of growth that usually goes unmeasured. Further, these whole plant growth measures demonstrate the rapid lateral rhizome spread of this species, exceeding one meter per plant per year. The primary cause of seasonal variation in the yearly seagrass cycle was investigated. Correlation analysis supported the hypothesis that the major factor controlling seasonal variation in this seagrass was light. During the peak growing season, however, growth was not regulated by light but by nitrogen. Depletion of the sediment ammonium pool and reduction in pore water ammonium relative to adsorbed ammonium, as well as changes in N content of seagrass leaves, support our hypothesis of peak growing season nitrogen limitation. Our results forSyringodium filiforme in terrigenous sediments are in contrast to our recent findings of phosphorus limitation in this same species occurring in carbonate sediments.  相似文献   

4.
Phragmites australis (common reed), a clonal grass, has expanded from a minor component of the mid-Atlantic wetlands to a dominant species. It has been suggested that invasive populations ofPhragmites are an exotic haplotype responsible for the dramatic increase in the distribution of the species. We used field observations and measurements and a greenhouse assay to compare native (haplotype F) and exotic (haplotype M) populations, growing adjacent to one another in a brackish marsh near Odessa, Delaware. In the marsh, shoots of the exotic strain emerged from the rhizomes earlier than those of the native and by March there was an order of magnitude more new shoots of the exotic strain than the native. In August, the exotic strain was 30% taller than the native, had twice the leaf biomass, and twice the total biomass. Nine of ten morphological and biomass characteristics measured differed significantly between the native and exotic strains. A greenhouse assay was conducted by planting rhizomes collected in March in shallow trays and growing them for 70 d followed by shoot harvest (Harvest 1). Rhizomes were measured, replanted, and grown for 35 d after which time they were measured and shoots were harvested (Harvest 2). At Harvest 1, shoot height was approximately 80% greater in the exotic strain, shoot biomass was three times higher, aboveground to belowground biomass ratio was twice as high, and rhizome internode length was 50% greater in the exotic strain than the native. These traits, in addition to number of shoots, were also greater in the exotic strain at Harvest 2. The number of rhizome buds at Harvest 1 was three times greater in the native than in the exotic strain. The greater number of rhizome buds in the native would seem to be an advantage, but it did not result in more shoot production. Buds were maintained in an inactive state that does not allow this strain to compete well in a wetland environment inhabited by a more efficient spreader. The earlier emergence of new shoots from the rhizomes, the greater aboveground structure, the greater rhizome internode length, and the quick transition of rhizome buds to shoot or rhizome explain in part the exotic strain's advantage over the native and the mechanisms for its invasive nature.  相似文献   

5.
Studies of seagrass meadows have shown that the production of algal epiphytes attached to seagrass blades approaches 20% of the seagrass production and that epiphytes are more important as food for associated fauna than are the more refractory seagrass blades. Since epiphytes may compete with seagrasses for light and water column nutrients, excessive epiphytic fouling could have serious consequences for seagrass growth. We summarize much of the literature on epiphytegrazer relationships in seagrass meadows within the context of seagrass growth and production. We also provide insights from mathematical modeling simulations of these relationships for a Chesapeake BayZostera marina meadow. Finally we focus on future research needs for more completely understanding the influences that epiphyte grazers have on seagrass production.  相似文献   

6.
Bermuda grass (Cynodon dactylon) can endure long-term and deep submergence stress and cadmium toxicity, respectively. But we do not know whether they can endure cadmium plus waterlogging and what are the differences of ecophysiological responses and metal bio-accumulation of Bermuda grass subjected to normal condition, waterlogging, cadmium or cadmium-and-waterlogging treatments, respectively. Here, the four treatments on Bermuda grass are performed with four replications. Our objective is to determine the ecophysiological responses and endurance abilities, cadmium and zinc bio-concentration and translocation, and the metal uptake mechanisms of Bermuda grass subjected to the four treatments. Our results demonstrated different treatments showed different ecophysiological responses in Bermuda grass. The shoot weights of cadmium treatment were significantly lower than those of the other treatments when the values determined after treated 25 d. Cadmium had a certain promotion effect on the root growth at the early growth stage. SPAD values indicate the relative amounts of chlorophyll present in plant leaves. SPAD value of 35 d was higher than 25 d of cadmium treatment, indicating cadmium had negative effects on chlorophyll contents in the early stage, but the negative effects might be ameliorated as plants adapted to cadmium stress. Waterlogging induced the biomass decrease and accelerated cadmium translocation. The plant leaves had the highest cadmium contents exposed to cadmium-and-waterlogging treatment, while the root showed the highest cadmium contents suffered only cadmium stress. The bio-concentration factor of cadmium-and-waterlogging treatment was less than that of cadmium treatment, but the translocation factor of cadmium-and-waterlogging treatment was much greater than that of cadmium treatment, which indicated phytostabilization was the main phytoremediation process subjected to only cadmium stress, while phytoextraction was the main process when exposed to cadmium-and-waterlogging treatment. Bermuda grass was a cadmium hyperaccumulator and had strong adaptability to waterlogging, cadmium toxicity or both.  相似文献   

7.
Tidally driven flows, waves, and suspended sediment concentrations were monitored seasonally within a Zostera marina seagrass (eelgrass) meadow located in a shallow (1–2 m depth) coastal bay. Eelgrass meadows were found to reduce velocities approximately 60 % in the summer and 40 % in the winter compared to an adjacent unvegetated site. Additionally, the seagrass meadow served to dampen wave heights for all seasons except during winter when seagrass meadow development was at a minimum. Although wave heights were attenuated across the meadow, orbital motions caused by waves were able to effectively penetrate through the canopy, inducing wave-enhanced bottom shear stress (τ b ). Within the seagrass meadow, τ b was greater than the critical stress threshold (=0.04 Pa) necessary to induce sediment suspension 80–85 % of the sampling period in the winter and spring, but only 55 % of the time in the summer. At the unvegetated site, τ b was above the critical threshold greater than 90 % of the time across all seasons. During low seagrass coverage in the winter, near-bed turbulence levels were enhanced, likely caused by stem–wake interaction with the sparse canopy. Reduction in τ b within the seagrass meadow during the summer correlated to a 60 % reduction in suspended sediment concentrations but in winter, suspended sediment was enhanced compared to the unvegetated site. With minimal seagrass coverage, τ b and wave statistics were similar to unvegetated regions; however, during high seagrass coverage, sediment stabilization increased light availability for photosynthesis and created a positive feedback for seagrass growth.  相似文献   

8.
The dominant plant in Humboldt Bay salt marshes in Spartina densiflora, a species of cordgrass apparently introduced from South America. At several salt marshes and restoration sites around Humboldt Bay, distribution of this plant has increased significantly. We investigated the relative contributions of vegetative tiller production and seed germination to the establishment and expansion of S. densiflora. Lateral spread of plants surrounded by competitors were compared to areas without competing plant species. Plants growing in areas without competitors had significantly higher rates of vegetative expansion (p<0.0001). Viable seed production, germination rates, seedling survivorship, and growth of adult plants were measured in six salinity treatments. Approximately 1,977±80 viable seeds are produced per plant (0.25–0.5 m2). The number of germinating seeds was inversely related to increases in salinity. Salinity treatments between 19‰ and 35‰ produced significantly lower germination rates than salinities of 0–18‰ (p<0.0001). Seedling survivorship was 50% at ≤4‰ and 8–14% at ≥11‰. Lateral expansion of adult, greenhouse-grown plants occurred in all salinity treatments, with modest decreases in the highest salinity treatments (p<0.05). Our findings indicate that S. densiflora expands primarily by vegetative expansion, and lateral tillers are produced by throughout the year. Spartina densiflora produces prolific amounts of seed, but recruitment in mature salt marshes may be limited by competitors and higher salinities. At restoration sites, planting of native species such as Salicornia virginica, Distichlis spicata, or Jaumea carnosa may prevent monospecific stands of S. densiflora from developing.  相似文献   

9.
Vegetation growth on spoil placed on aSpartina patens andS. alterniflora marsh in Barnegat Bay, New Jersey, was examined for two years following spoil deposition. In areas where spoil was leveled to match the elevation of the marsh, the percent cover by the end of the first growing season ranged from 60 to 90%. By the second year, cover was 100% in all study plots. Even during the first year, there was no difference in species diversity or vegetation species in the experimental and control areas. The percentage of shrubs did not increase on the perturbed (spoil) areas compared to the control plots. During the first year, but not the second, live and dead biomass was greater in the perturbed areas compared to the control plots. In an area where the spoil was thicker succession was exceedingly slow and there was only about 5% grass cover by the end of the first growing season. BecauseSpartina colonizes primarily by rhizome growth, the comparatively slow recovery was attributed to the inability of the grass to penetrate the thick spoil layer.  相似文献   

10.
Terrestrial ecosystems near breeding/roosting colonies of piscivorous seabirds can receive a large amount of marine-derived N in the form of bird feces. It has been well demonstrated that N input from seabirds strongly affects plant communities in forests or coastal grasslands. The effects of nutrient input on plant communities in agricultural ecosystems near seabird colonies, however, have rarely been evaluated. This relationship was examined in rice-paddy fields irrigated by a pond system located near a colony of the Great Cormorant Phalacrocorax carbo in Aichi, central Japan. In the present study, spatial variations in N content (N %) and N stable isotope composition (δ15N) of soils and wild grass species together with the growth height of plants in paddy fields in early spring (fallow period) were examined. Soils had a higher N % and δ15N values in fields associated with an irrigation pond that had N input from cormorants. The δ15N values tended to be higher around the inlet of irrigation waters, relative to the outlet. These results indicate that cormorant-derived N was input into the paddy fields via the irrigation systems. Plants growing in soil with higher δ15N had higher δ15N in the above-ground part of the plants and had luxurious growth. A positive correlation in plant height and δ15N of NO3–N was observed in soil plough horizons.  相似文献   

11.
Innovative flume experiments were conducted in a recirculating straight flume. Zostera noltei meadows were sampled in their natural bed sediments in the field at contrasting stages of their seasonal growth. The aims of this study were: (i) to quantify the combined effects of leaf flexibility and development characteristics of Zostera noltei canopies on their interaction with hydrodynamics; and (ii) to quantify the role of Zostera noltei meadows in suspended sediment trapping and bed sediment resuspension related with changes in hydrodynamic forcing caused by the seasonal development of seagrasses. Velocity within the canopy was significantly damped. The attenuation in velocity ranged from 34 to 87% compared with bare sediments and was associated with a density threshold resulting from the flow‐induced canopy reconfiguration. The reduction in flow was higher in dense canopies at higher velocities than in less dense canopies, in which the reduction in flow was greater at low velocities. These contrasted results can be explained by competition between a rough‐wall boundary layer caused by the bed and a shear layer caused by the canopy. The velocity attenuation was associated with a two to three‐fold increase in bottom shear stress compared with unvegetated sediment. Despite the increase in near‐bed turbulence, protection of the sediment against erosion increased under a fully developed meadow, while sediment properties were found to be the main factor controlling erosion in a less developed meadow. Deposition fluxes were higher on the vegetated bed than on bare sediments, and these fluxes increased with leaf density. Fewer freshly deposited sediments were resuspended in vegetated beds, resulting in an increase in net sediment deposition with meadow growth. However, in the case of a very high leaf area index, sediment was mostly deposited on leaves, which facilitated subsequent resuspension and resulted in less efficient sediment trapping than in the less developed meadow.  相似文献   

12.
Cellulose and silica phytoliths were extracted from the leaves and stems of Calamovilfa longifolia, a C4 grass, grown under varying climatic conditions across the North American prairies. The oxygen-isotope compositions of both cellulose and silica record a complex signal of the isotopic composition of the soil water that feeds the plants and the relative humidity conditions that influence transpiration rates, stomatal conductance, and ultimately the 18O-enrichment of leaf water. As the initial stages of cellulose formation occur in the leaves, cellulose in both the leaves and stems forms primarily from leaf water and does not differ greatly in its oxygen-isotope composition between these locations. In contrast, the δ18O values of leaf phytoliths are significantly enriched in 18O relative to stem phytoliths, reflecting the varying isotopic composition of the water in these tissues. The oxygen-isotope compositions of leaf cellulose may be used as a proxy for the isotopic composition of water involved in leaf phytolith formation, while the δ18O values of stem phytoliths can be used to determine the δ18O values of stem water involved in partial exchange reactions during the transport of carbohydrates through the plant. A comparison of the isotopic compositions of phytoliths with cellulose allows for the deduction of soil and leaf water δ18O values as well as temperature and relative humidity conditions during plant growth. This approach has application in paleoclimate studies that traditionally have required estimations of one or more of these variables because direct measurements were unavailable.  相似文献   

13.
青藏高原两种草甸类型人工降雨截留特征分析   总被引：3，自引：0，他引：3       下载免费PDF全文
选取青藏高原典型多年冻土区风火山小流域高寒草甸和沼泽化草甸典型样地进行人工降雨截留试验,综合运用统计分析、模型回归分析等方法,对两种草甸类型截留特征及其影响因子进行分析。结果表明:两种草甸类型对降雨的截留能力是不同的,高寒草甸最大截留量0.61mm,沼泽化草甸为0.18mm;高寒草甸的最大截留率为12.4%,沼泽化草甸为3.8%。通过分析各因子对截留的影响,得出高寒草甸截留量与降雨量呈对数函数关系,沼泽化草甸截留量与降雨量呈二次多项式关系;两种草甸截留量与降雨强度都呈幂函数关系,与植被盖度均呈线性相关关系。  相似文献   

14.
Seagrasses are indicators of ecosystem state because they are sensitive to variations in water composition and clarity resulting from watershed-level impacts. A simulation model designed to studyZostera marina (eelgrass) habitat dynamics in a variable littoral zone environment was used to address the potential ecological responses to eutrophication in lower Chesapeake Bay. The adjacent channel boundary environment is a source of dissolved and particulate materials to the littoral zone. In the simulations, concentrations of key water quality variables in the adjacent estuarine channel boundary were either halved or doubled relative to the base case to investigate light versus nitrogen effects. The role of the seagrass meadow in littoral zone carbon and nitrogen dynamics was evaluated when meadow size was changed in the model. Particulate and dissolved organic carbon accounted for 83% of the submarine light attenuation in the seagrass meadow. In all model runs, the water column concentrations of chlorophylla and dissolved inorganic nitrogen (DIN) were below the habitat criteria proposed as critical to seagrass survival. Eelgrass community production was carefully regulated by the interactive effects of light, nitrogen, and grazing on epiphyte growth. Increased eelgrass coverage in the littoral zone led to a simulated doubling of ecosystem primary production but reduced the fraction of production by planktonic and sediment microalgae. The simulation model presented here demonstrated the importance of material input from the channel in littoral zone biogeochemical dynamics. Submarine ligh regulated primary production more strongly than inorganic nitrogen concentrations in the model. External DIN concentrations influenced seagrass survival indirectly: enrichment stimulated growth of epiphytes and phytoplankton and promoted shading of the seagras leaf. The model was based upon a unimpacted ecosystem and deteriorated water quality negatively influenced primary production greater than the increases triggered by improved condition. Increased material loading to the littoral zone reduced submarine light availability, increased phytoplankton production, lowered ecosystem production, and reduced subtidal vegetated habitat. This simulation model of the estuarine littoral zone model combines hydrodynamics, biogeochemical sources and sinks, and living resources in order to better understand structure, function, and change in aquatic ecosystems.  相似文献   

15.
The influence of temperature, salinity, body size, and sex on the bioenergetics and life-history parameters of grass shrimp (Palaemonetes pugio Holthuis) was determined under laboratory conditions. Sex and/or size show a highly significant effect only on energy allocation for metabolism. Temperature, salinity and their interaction are the main effects on most energetic parameters, with temperature showing the strongest influence on both life-history parameters and energetics. The adult grass shrimp's energy budget is different from that of larval or juvenile stages, in that most of the energy ingested by adults is allocated to reproduction. Under optimal conditions (25°C and 28‰), allocation of ingested energy in the adult grass shrimp is 51.7% for reproduction, 25.4% for respiration, 14.1% for somatic growth, 4.8% for exoskeletons, and 1.8% for excretion.  相似文献   

16.
Experiments were performed seasonally to estimate grass shrimp,Palaemonetes pugio, grazing on the epiphytic microalgae of cordgrass,Spartina alterniflora, and to determine if grass shrimp have the potential to regulate epiphyte abundance. Grass shrimp were given access to live culms with low and high epiphytic abundance and standing dead culms collected from the streamside levee of a Louisiana salt marsh. Plexiglas frames were used to hold culms upright in aquaria and to restrict grass shrimp access to one half of each culm. We compared epiphyte biomass on the sides of culms exposed to shrimp with the corresponding unexposed sides. Epiphytes were removed from the lowest 10 cm of culms on days 0, 3, and 10, and chlorophylla (chla) measurements on each culm half were made by fluorometry. Chla biomass on culm halves not exposed to grass shrimp significantly increased over time. Percent reductions in chla on culm halves exposed to grass shrimp (calculated by subtraction from the corresponding half not exposed to shrimp) significantly increased over time for at least one culm type in all seasons. Grass shrimp caused an average 30% reduction of epiphyte biomass over 3 d and a 40% reduction over 10 d, suggesting that grass shrimp have the capability of consuming a substantial proportion of the daily production of epiphytes. Epiphytes from standing-dead culms may be more important than those from live culms at the marsh edge to the diet of grass shrimp because chla biomass was, on average, high, and standing-dead culms were seasonally abundant. Diagnostic photosynthetic pigments from selected culms, grass shrimp gut contents, and fecal pellets were identified by high performance liquid chromatography and were used to quantify the taxonomic groups of epiphytic microalgae. Results suggested that diatoms, brown algae, green algae, red algae, and cyanobacteria were present on all culms. Similarities in the pigment content of grass shrimp gut contents and fecal pellets suggested that all algal groups were ingested. Pigment data analysis could not detect a change in the composition of the microalgal assemblage associated with grass shrimp grazing. Assuming that the reduction in chla was due exclusively to grazing, grass shrimp consumed an average of 0.5–1.5 g epiphyte carbon shrimp?1 d?1, suggesting that grass shrimp benefit significantly from the consumption ofS. alterniflora epiphytic algae.  相似文献   

17.
西北荒漠草甸植被光谱反射特征研究   总被引：2，自引：0，他引：2  
选取位于西北干旱内陆地区的安西荒漠草场为观测试验区,分别对盐生低地草甸、极旱荒漠草甸和荒漠灌丛三类草地进行了地面反射光谱测定,并分析了主要荒漠草甸植被光谱反射的一般特征和红边参数特征,进而探讨了形成草地光谱特征差异的主要内在原因和影响因素。结果表明:在近红外波段,由于荒漠植被类别间叶片内部结构变化大,因此冠层光谱反射率差异较大;同一植被冠层光谱反射率的大小主要受植被长势的影响;受沙地背景的影响,在近红外波段,植被冠层的光谱反射率要明显小于叶片的光谱反射率;荒漠植被冠层光谱的红边也具有“双峰”现象,红边特征的参数表现为:沙地植被>绿洲植被>沙漠植被。对安西荒漠植被光谱特征的分析研究,对于研究干旱区荒漠植被的理化性能、遥感反演、植被分类、植被调查等都具有重要的意义。  相似文献   

18.
Aboveground and belowground biomass, density, blade length, and chlorophyll content of seagrass growing directly under docks were compared with adjacent unshaded sites. The amount of light reduction due to dock shading was measured using quantum spherical irradiance meters. Seagrasses were present under docks; they were shaded such that light levels were 19% and 16% of surface irradiance, at shallow and deep sites, respectively. Shoot density was 40–47% lower in shaded plots than in unshaded plots; total biomass was 30–33% lower in shaded plots than in unshaded plots. In contrast, blade length and chlorophyll content were increased at shaded plots relative to unshaded plots. Seagrasses were not found under docks at light levels less than 14% of surface irradiance. These results are in agreement with published in situ compensation irradiance estimates of 15–18% for Halodule wrightii in Texas coastal waters. Shading effects were most apparent between the hours of 1000 and 1500; therefore, the light received during the early morning and late afternoon hours may be critical for plants growing under docks. The north-south orientation of the docks in this study is likely an important factor contributing to the continued survival and growth of the seagrasses.  相似文献   

19.
The hypothesis thatTypha domingensis (cattail) can invade tidal marshes only after soil salinities are substantially reduced was tested experimentally by comparing the salt tolerance of seeds, seedlings, and plants reared from rhizomes. Germination rates for four southern California populations reached 100% in fresh water, decreasing to 2% at 20‰. The salt tolerance of seeds from three coastal populations was lower than that of the Salton Sea population. Salt tolerance of plants grown in the lab did not increase with age for seedlings up to 8 weeks old. Rhizome-bearing plants had greatly decreased growth at 10‰ and no growth at 25‰ However, rhizomes of about 5% of the plants survived 9 months at 45‰. The seeds and seedlings are salt sensitive, which explains why invasion into tidal marshes is restricted to prolonged periods of low soil salinity. The older, rhizome-bearing plants are salt tolerant, which explains how invading plants persist persist under hypersaline conditions.  相似文献   

20.
Two higher plant species (rye grass and clover) were cultivated under laboratory conditions on two substrates (solution, phlogopite) in order to constrain the corresponding Mg isotope fractionations during plant growth and Mg uptake. We show that bulk plants are systematically enriched in heavy isotopes relative to their nutrient source. The Δ26Mgplant-source range from 0.72‰ to 0.26‰ for rye grass and from 1.05‰ to 0.41‰ for clover. Plants grown on phlogopite display Mg isotope signatures (relative to the Mg source) ∼0.3‰ lower than hydroponic plants. For a given substrate, rye grass display lower δ26Mg (by ∼0.3‰) relative to clover. Magnesium desorbed from rye grass roots display a δ26Mg greater than the nutrient solution. Adsorption experiments on dead and living rye grass roots also indicate a significant enrichment in heavy isotopes of the Mg adsorbed on the root surface. Our results indicate that the key processes responsible for heavy isotope enrichment in plants are located at the root level. Both species also exhibit an enrichment in light isotopes from roots to shoots (Δ26Mgleaf-root = −0.65‰ and −0.34‰ for rye grass and clover grown on phlogopite respectively, and Δ26Mgleaf-root of −0.06‰ and −0.22‰ for the same species grown hydroponically). This heavy isotope depletion in leaves can be explained by biological processes that affect leaves and roots differently: (1) organo-Mg complex (including chlorophyll) formation, and (2) Mg transport within plant. For both species, a positive correlation between δ26Mg and K/Mg was observed among the various organs. This correlation is consistent with the link between K and Mg internal cycles, as well as with formation of organo-magnesium compounds associated with enrichment in heavy isotopes. Considering our results together with the published range for δ26Mg of natural plants and rivers, we estimate that a significant change in continental vegetation would induce a change of the mean river δ26Mg that is comparable to analytical uncertainties.  相似文献   

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