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991.
Mutualisms may play an important role in the establishment and invasion success of introduced species, but their influence is little studied. To test whether a lack of root nodule symbionts may limit the performance of invasive legumes, seedlings of Cytisus scoparius were introduced to an old-field habitat and then either inoculated with Bradyrhizobium strains from existing C. scoparius populations, or left uninoculated. In two separate years, inoculation more than doubled average plant biomass. For uninoculated transplants, nodule formation was positively correlated with proximity to plants of the native legume Desmodium canadense , but not related to distance from a second legume species, Apios americana. Polymerase chain reaction assays and DNA sequencing confirmed that bacteria isolated from uninoculated C. scoparius plants were indistinguishable from Bradyrhizobium strains in root nodules of D. canadense . By contrast, bacterial strains associated with A. americana were never found in C. scoparius nodules. Transplants in seven other habitats across a 160 km region also showed a highly significant, fivefold biomass increase in response to inoculation. Thus, colonizing legumes can suffer from a scarcity of nodule symbionts. However, certain indigenous legumes may create favourable microhabitats for invasion, by increasing symbiont availability in their vicinity. 相似文献
992.
The term “keystone species” is used to describe organisms that exert a disproportionately important influence on the ecosystems
in which they live. Analogous concepts such as “keystone mutualism” and “mobile links” illustrate how, in many cases, the
interactions of two or more species produce an effect greater than that of any one species individually. Because of their
role in transporting nutrients from the ocean to river and riparian ecosystems, Pacific salmon (Oncorhynchus spp.) and brown bear (Ursus arctos) have been described as keystone species and mobile links, although few data are available to quantify the importance of
this interaction relative to other nutrient vectors. Application of a mass balance model to data from a southwestern Alaskan
stream suggests that nitrogen (N) influx to the riparian forest is significantly increased in the presence of both salmon
and bear, but not by either species individually. The interactions of salmon and bear may provide up to 24% of riparian N
budgets, but this percentage varies in time and space according to variations in salmon escapement, channel morphology and
watershed vegetation characteristics, suggesting interdependence and functional redundancy among N sources. These findings
illustrate the complexity of interspecific interactions, the importance of linkages across ecosystem boundaries and the necessity
of examining the processes and interactions that shape ecological communities, rather than their specific component parts. 相似文献
993.
Calcium Additions and Microbial Nitrogen Cycle Processes in a Northern Hardwood Forest 总被引:1,自引:0,他引:1
Peter M. Groffman Melany C. Fisk Charles T. Driscoll Gene E. Likens Timothy J. Fahey Christopher Eagar Linda H. Pardo 《Ecosystems》2006,9(8):1289-1305
Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an
important topic in the northeastern United States. We added 850 kg Ca ha−1 as wollastonite (CaSiO3) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA,
in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial
biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil
solution and stream chemistry, soil:atmosphere trace gas (CO2, N2O, CH4) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before
and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment.
By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in
the Oa soil horizon. However, there were declines in the N content of the microbial biomass, potential net and gross N mineralization
rates, and soil inorganic N pools in the Oie horizon of the treated watershed. Stream, soil solution, and foliar concentrations
of N showed no response to treatment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be
stimulated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus,
or Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed.
Possible fates for the approximately 10 kg N ha−1 decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example,
seedlings, understory shrubs), increased fluxes of N2 and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and
activity in northern hardwood forests that should be considered in the context of proposals to mitigate the depletion of nutrient
cations in soil. 相似文献
994.
Reconciling Carbon-cycle Concepts, Terminology, and Methods 总被引:5,自引:1,他引:4
F. S. Chapin III G. M. Woodwell J. T. Randerson E. B. Rastetter G. M. Lovett D. D. Baldocchi D. A. Clark M. E. Harmon D. S. Schimel R. Valentini C. Wirth J. D. Aber J. J. Cole M. L. Goulden J. W. Harden M. Heimann R. W. Howarth P. A. Matson A. D. McGuire J. M. Melillo H. A. Mooney J. C. Neff R. A. Houghton M. L. Pace M. G. Ryan S. W. Running O. E. Sala W. H. Schlesinger E.-D. Schulze 《Ecosystems》2006,9(7):1041-1050
Recent projections of climatic change have focused a great deal of scientific and public attention on patterns of carbon (C)
cycling as well as its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric
carbon dioxide (CO2). Net ecosystem production (NEP), a central concept in C-cycling research, has been used by scientists to represent two different
concepts. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary
production (GPP) and ecosystem respiration (ER). We further propose that a new term—net ecosystem carbon balance (NECB)—be
applied to the net rate of C accumulation in (or loss from [negative sign]) ecosystems. Net ecosystem carbon balance differs
from NEP when C fluxes other than C fixation and respiration occur, or when inorganic C enters or leaves in dissolved form.
These fluxes include the leaching loss or lateral transfer of C from the ecosystem; the emission of volatile organic C, methane,
and carbon monoxide; and the release of soot and CO2 from fire. Carbon fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However,
even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological
advances have led to a diversity of approaches to the measurement of C fluxes at different temporal and spatial scales. These
approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully
specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components
of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we can provide a less ambiguous framework
for understanding and communicating recent changes in the global C cycle. 相似文献
995.
Ellison AM 《Plant biology (Stuttgart, Germany)》2006,8(6):740-747
The cost-benefit model for the evolution of carnivorous plants posits a trade-off between photosynthetic costs associated with carnivorous structures and photosynthetic benefits accrued through additional nutrient acquisition. The model predicts that carnivory is expected to evolve if its marginal benefits exceed its marginal costs. Further, the model predicts that when nutrients are scarce but neither light nor water is limiting, carnivorous plants should have an energetic advantage in competition with non-carnivorous plants. Since the publication of the cost-benefit model over 20 years ago, marginal photosynthetic costs of carnivory have been demonstrated but marginal photosynthetic benefits have not. A review of published data and results of ongoing research show that nitrogen, phosphorus, and potassium often (co-)limit growth of carnivorous plants and that photosynthetic nutrient use efficiency is 20 - 50 % of that of non-carnivorous plants. Assessments of stoichiometric relationships among limiting nutrients, scaling of leaf mass with photosynthesis and nutrient content, and photosynthetic nutrient use efficiency all suggest that carnivorous plants are at an energetic disadvantage relative to non-carnivorous plants in similar habitats. Overall, current data support some of the predictions of the cost-benefit model, fail to support others, and still others remain untested and merit future research. Rather than being an optimal solution to an adaptive problem, botanical carnivory may represent a set of limited responses constrained by both phylogenetic history and environmental stress. 相似文献
996.
Previous research showed that nano-TiO2 could significantly promote photosynthesis and greatly improve growth of spinach, but, we also speculated that an increase
of spinach growth by nano-TiO2 treatment might be closely related to the change of nitrogen metabolism. The effects of nano-anatase TiO2 on the nitrogen metabolism of growing spinach were studied by treating them with nano-anatase TiO2. The results showed that, nano-anatase TiO2 treatment could obviously increase the activities of nitrate reductase, glutamate dehydrogenase, glutamine synthase, and
glutamic-pyruvic transaminase during the growing stage. Nano-anatase TiO2 treatment could also promote spinach to absorb nitrate, accelerate, inorganic nitrogen (such as NO
3
t-
−N and NH
4
+
−N) to be translated into organic nitrogen (such as protein and chlorophyll), and enhance the fresh weight and dry weights. 相似文献
997.
Hydrologic pathways through soil affect element leaching by determining the relative importance of biogeochemical processes such as sorption and decomposition. We used stable hydrogen isotopes of water (δD) to examine the influence of flowpaths on soil solution chemistry in a mature spruce–hemlock forest in coastal Oregon, USA. Soil solutions (50 cm depth, n = 13) were collected monthly for 1 year and analyzed for δD, major ions and dissolved organic carbon (DOC) and nitrogen (DON). We propose that the variability of δD can be used as an index of flowpath length and contact time. Throughfall variability in δD was much greater than soil solution variability, illustrating that soil solution integrates the variation in inputs. Lysimeters with greater variation in δD presumably have a greater proportion of flow through rapid flowpaths such as macropores. The variation in soil solution δD for individual lysimeters explained up to 53% of the variation in soil solution chemistry, and suggests that flowpaths influence leaching of some constituents. Soil solutions from lysimeters with greater δD variation had higher DOC and DON (r
2 = 0.51 and 0.37, respectively), perhaps because transport via macropores reduces interaction of DOM with the soil matrix. In contrast, nitrate concentrations were highest in lysimeters with a small variation in δD, where long contact time and low DOC concentrations may yield higher net nitrification. Our results demonstrate the utility of stable isotopes to link flowpaths and soil solution chemistry, and illustrate how the spatial complexity of soils can influence ecosystem-level nutrient losses. 相似文献
998.
During 1999–2001 the chemical composition and fluxes were measured in rainfall, throughfall, soil solution and stream water
in a remote forested site in the Italian Alps. The analysis of temporal patterns revealed the differential behaviour of nitrogen
and sulphur and suggested that different mechanisms controlled their flux. No important changes in sulphate concentration
and fluxes emerged as the solution passed through the various components of the forest ecosystem, and temporal variations
of SO4 in the soil solution and stream were likely driven by the physical process of dilution. The availability of nitrate and ammonia,
by contrast, was drastically reduced as throughfall water entered the soil and passed through the mineral layers, irrespective
of season. The calculated hydrochemical budget based on throughfall and soil solution N fluxes revealed that ~80% N retention
in the forest soil, corresponding to 12 kg ha−1 yr−1, despite a relatively high N deposition loading (15 kg ha−1 yr−1). Most of the leached nitrogen (90%) was in the organic form. Indicators of the N status of this ecosystem, such as C/N ratio
in solid and solution phase of the soil and N foliage content as well as land use history were examined. Despite the strong
N retention in the forested part of the catchment, the stream water N–NO3 levels were consistently above 10 μg l−1 suggesting that the Val Masino catchment as a whole was less efficient in processing atmospheric N inputs. This contrasting
N behaviour illustrates the role of landscape features, such as the soil cover and vegetation type, that is characteristic
of an alpine catchment. 相似文献
999.
Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO3) from agricultural runoff through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N2O) emissions from riparian forest soils exposed to prolonged nutrient runoff from plant nurseries and compares these to similar forest soils not exposed to nutrient runoff. Nursery runoff also contains high levels of phosphate (PO4). Since there are conflicting reports on the impact of PO4 on the activity of denitrifying microbes, the impact of PO4 on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N2O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 μg NO3-N g−1 soil, respectively. Net N2O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 μg NO3-N g−1 soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 μg NO3-N g−1 in soil slurries. The addition of PO4 (5 μg PO4-P g−1) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N2O emissions compared to emission rates from non-exposed forest soils. 相似文献
1000.
van der Heijden MG Bakker R Verwaal J Scheublin TR Rutten M van Logtestijn R Staehelin C 《FEMS microbiology ecology》2006,56(2):178-187
Symbiotic interactions are thought to play a key role in ecosystems. Empirical evidence for the impact of symbiotic bacteria on plant communities is, however, extremely scarce because of experimental constraints. Here, in three complementary experiments, we show that nitrogen-fixing rhizobia bacteria act as a determinant of plant community structure and diversity. Grassland microcosms inoculated with a mixture of rhizobia had a higher above-ground plant productivity (+35%), contained more nitrogen (+85%) and had significant higher community evenness (+34%) than control microcosms without rhizobia. Moreover, three of the four studied legume species required rhizobia to successfully coexist with other plant species. In contrast, the growth and survival of three grass and five forb species were not affected by the presence or absence of rhizobia. Finally, our results also showed that the legume species largely relied on symbiotically fixed nitrogen, both in the field and in the microcosms. This indicates that results in the microcosms are indicative for processes occurring in the field. It is concluded that symbiotic interactions between plants and prokaryotes can contribute to plant productivity, plant community structure and acquisition of limiting resources in legume-rich grassland communities. 相似文献