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121.
P. J. Goodman 《Plant and Soil》1988,112(2):247-254
The stable isotope15N is particularly valuable in the field for measuring N fixation by isotope dilution. At the same time other soil-plant processes can be studied, including15N recovery, and nitrogen transfer between clover and grass. Three contrasting sites and soils were used in the present work: a lowland soil, an upland soil, and an upland peat. Nitrogen fixation varied from 12 gm–2 on lowland soil to 2.7 gm–2 on upland peat. Most N transfer occurred on upland soil (4.2 gm–2) which, added to nitrogen fixed, made a total of 8.7 gm2 input during summer 1985.15N recovery for the whole experiment was small, around 25%.Measurement of dead and dying leaves, stubble and roots, suggests that plant organ death is the first stage in N transfer from white clover to ryegrass, through the decomposer cycle. Decomposition was fastest on lowland soils, slowest on peat. On lowland soil this decomposer nitrogen is apparently subverted before transfer, probably by soil microbes.Variations in natural abundance of15N in plants were found in the two species on the different soils. These might be used to measure nitrogen fixation without adding isotope, but the need for many replicates and repeat samples would limit throughput. 相似文献
122.
The response of temperate forest ecosystems to elevated atmospheric CO2 concentrations is important because these ecosystems represent a significant component of the global carbon cycle. Two important
but not well understood processes which elevated CO2 may substantially alter in these systems are regeneration and nitrogen cycling. If elevated CO2 leads to changes in species composition in regenerating forest communities then the structure and function of these ecosystems
may be affected. In most temperate forests, nitrogen appears to be a limiting nutrient. If elevated CO2 leads to reductions in nitrogen cycling through increased sequestration of nitrogen in plant biomass or reductions in mineralization
rates, long-term forest productivity may be constrained. To study these processes, we established mesocosms of regenerating
forest communities in controlled environments maintained at either ambient (375 ppm) or elevated (700 ppm) CO2 concentrations. Mesocosms were constructed from intact monoliths of organic forest soil. We maintained these mesocosms for
2 years without any external inputs of nitrogen and allowed the plants naturally present as seeds and rhizomes to regenerate.
We used 15N pool dilution techniques to quantify nitrogen fluxes within the mesocosms at the end of the 2 years. Elevated atmospheric
CO2 concentration significantly affected a number of plant and soil processes in the experimental regenerating forest mesocosms.
These changes included increases in total plant biomass production, plant C/N ratios, ectomycorrhizal colonization of tree
fine roots, changes in tree fine root architecture, and decreases in plant NH4
+ uptake rates, gross NH4
+ mineralization rates, and gross NH4
+ consumption rates. In addition, there was a shift in the relative biomass contribution of the two dominant regenerating tree
species; the proportion of total biomass contributed by white birch (Betula papyrifera) decreased and the proportion of total biomass contributed by yellow birch (B. alleghaniensis) increased. However, elevated CO2 had no significant effect on the total amount of nitrogen in plant and soil microbial biomass. In this study we observed
a suite of effects due to elevated CO2, some of which could lead to increases in potential long term growth responses to elevated CO2, other to decreases. The reduced plant NH4
+ uptake rates we observed are consistent with reduced NH4
+ availability due to reduced gross mineralization rates. Reduced NH4
+ mineralization rates are consistent with the increases in C/N ratios we observed for leaf and fine root material. Together,
these data suggest the positive increases in plant root architectural parameters and mycorrhizal colonization may not be as
important as the potential negative effects of reduced nitrogen availability through decreased decomposition rates in a future
atmosphere with elevated CO2.
Received: 10 January 1997 / Accepted: 25 July 1997 相似文献
123.
Field experiments were established to assess possible allelopathic suppression by Lantana camara L. of two indigenous tree species. The design allowed comparison of allelopathic effects with density-dependent resource competition effects. Fire and its role in competitive interactions was included as an experimental treatment. Allelopathic responses were measured in L. camara thickets by germinating and growing Alectryon subcinereus (A. Gray) Radlk. in dry rainforest ecotones (Macleay River) or Cryptocarya rigida (Meissner) in warm temperate rainforest and wet sclerophyll forest (Lake Macquarie) at 10, 20 and 30 seedlings m-2, where L. camara was either physically removed (LR), burnt (LB), or cut and left in place (LT). Germination for both species increased significantly by completely removing L. camara (LR) whereas burning (LB) was significant only for C. rigida. Seedling growth for both species was negatively related to increasing density when all L. camara was removed (LR) but was positively related in the other two treatments (LB and LT). C. rigida seedling biomass increased 47.4% (1.75%2.58 g) and 68.6% (1.98%2.95 g) with increasing seedling density for LT and LB respectively and decreased 23.2% (2.93–2.25 g) for LR. A. subcinereus seedling biomass increased 29.7% (1.95–2.53 g) and 34.7% (2.25–3.03 g) with increasing seedling density for LT and LB respectively and decreased 27.9% (3.30–2.38 g) for LR. Phytotoxin dilution effects were inferred in LT and LB rather than density-dependent intraspecific competition, whereas the reverse was true for LR. Seedling biomass for C. rigida resulting from potential phytotoxin dilution at high seedling density was not significantly different from the response of LR at low seedling density but, for A. subcinereus, the phytotoxin dilution response was significantly less than LR at low seedling density. Moderately intense fire (LB) was not significantly different from the LT treatment at both locations, emphasising that moderate to low intensity fires should not be used to control existing invasions of L. camara. Competitive strategies for invasive populations are identified that may modify succession following disturbance, thereby allowing thicket formation and long-term persistence to affect community dynamics. Such strategies need to be recognised in managing natural communities, particularly for biodiversity conservation.Nomenclature: Harden (1990). 相似文献
124.
Olivier Jolliet Pierre Crettaz 《The International Journal of Life Cycle Assessment》1997,2(2):104-110
The inclusion of fate and exposure is a central issue in Life Cycle Impact Assessment (LCIA). According to the framework developed
by the Society of Environmental Toxicity and Chemistry (SETAC), fate and exposure route are included through a fate coefficient
which makes the link between an emission and the related increase in concentration.
In the Critical surface-time 95 methodology, fate factors of air pollutants are determined empirically at a world level as
the ratio of measured concentration to the total estimated emission flow. Based on a detailed study performed for seventeen
pollutants, a correlation is developed to predict fate factors from the residence time. Variation of a factor 10000 arc observed
for the fate coefficient. Empirical fate factors are compared to modelled fate factors and are found to have a similar order
of magnitude. 相似文献
125.
Habitat heterogeneity drives the host‐diversity‐begets‐parasite‐diversity relationship: evidence from experimental and field studies
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Pieter T. J. Johnson Chelsea L. Wood Maxwell B. Joseph Daniel L. Preston Sarah E. Haas Yuri P. Springer 《Ecology letters》2016,19(7):752-761
Despite a century of research into the factors that generate and maintain biodiversity, we know remarkably little about the drivers of parasite diversity. To identify the mechanisms governing parasite diversity, we combined surveys of 8100 amphibian hosts with an outdoor experiment that tested theory developed for free‐living species. Our analyses revealed that parasite diversity increased consistently with host diversity due to habitat (i.e. host) heterogeneity, with secondary contributions from parasite colonisation and host abundance. Results of the experiment, in which host diversity was manipulated while parasite colonisation and host abundance were fixed, further reinforced this conclusion. Finally, the coefficient of host diversity on parasite diversity increased with spatial grain, which was driven by differences in their species–area curves: while host richness quickly saturated, parasite richness continued to increase with neighbourhood size. These results offer mechanistic insights into drivers of parasite diversity and provide a hierarchical framework for multi‐scale disease research. 相似文献
126.
J. V. D. K. Kumar Rao J. A. Thompson P. V. S. S. Sastry K. E. Giller J. M. Day 《Plant and Soil》1987,101(1):107-113
Two experiments were carried out from 1981 to 1983 in Vertisol field at ICRISAT Center, Patancheru, India to measure N2-fixation of pigeonpea [Cajanus cajan (L.) Millsp.] using the15N isotope dilution technique. One experiment examined the effect of control of a nodule-eating insect on fixation while another
in vestigated the effect of intercroping with cereals on fixation and the residual effect of pigeonpea on a succeeding cereal
crop. Although both experiments indicated that at least 88% of the N in pigeonpea was fixed from the atmosphere, one result
is considered fortuitous in view of the differential rates of growth of the legume and the control, sorghum [Sorghum bicolor (L.) Moench]. The difference method of calculation in dieated negative fixation and the results emphasized the problem of
finding a suitable nonfixing control. In a second experiment, when all plants were confined to a known volume of soil to which15N fertilizer was added in the field, these problems were overcome, and isotope dilution and difference methods gave similar
results of N2-fixation of about 90%. In intercropped pigeonpea 96% of the total N was derived from the atmosphere. This estimate might
be an artifact. There was no evidence of benefit from N fixed by pigeonpea to intercropped sorghum plants. Plant tissue15N enrichments of cereal crops grown after pigeonpea indicated that the cereal derived some N fixed by the previous pigeonpea.
Thus residual benefits to cereals are not only an effect of ‘sparing’ of soil N. 相似文献
127.
128.
Peter J. Davies Roger Horgan James K. Heald Brian A. McGaw 《Plant Growth Regulation》1986,4(4):311-323
In G2 peas senescence only takes place in long days. In order to determine the role of cytokinins in this process the endogenous cytokinins from vegetative shoots of G2 peas were characterized using gas chromatography-mass spectroscopy following purification by HPLC. Cytokinins were extracted and purified with and without the addition of 15N labelled internal standards of several cytokinins to estimate cytokin content by isotope dilution in the mass spectra. Samples without internal standards were bioassayed after HPLC. Bioassays showed the presence of zeatin, zeatin riboside and zeatin-0-glucoside. The presence of zeatin was confirmed by its mass spectrum of its permethylated derivative. Tentative identification of zeatin riboside, zeatin-0-glucoside, dihydrozeatin, and dihydrozeatin-0-glucoside was obtained by the coincidence of the major ion for the permethylated natural and 15N labelled internal standards on GC-MS, and the similar coincidence of ions for permethylated zeatin riboside-0-glucoside by direct probe MS. There was no indication of the presence of significant quantities of zeatin-7-glucoside or zeatin-9-glucoside. The amounts in the tissue ranged from 200–1000 ng/kg fresh weight for each cytokinin and about 2–4 g/kg fresh weight for total cytokinins. There was no apparent difference in the levels in mature but pre-senescent shoots grown in long days and short days indicating that apical senesecence in G2 peas does not appear to be induced by a decline in cytokinin level in the shoots.Cytokinin abbreviations CK
Cytokinin
- Z
trans zeatin
- [9R]Z
t-zeatin riboside
- [9R-5P] Z
t-zeatin riboside-5-monophosphate
- (OG)Z
t-zeatin-0-glucoside
- (OG)[9R]Z
t-zeatin riboside-0-glucoside
- [7Z]G
t-zeatin-7-glucoside
- [9G]Z
t-zeatin-9-glucoside
- (diH)Z
dihydrozeatin
- (diH)[9R]Z
dihydrozeatin riboside
- iP
N6(2-isopentenyl) adenine
- [9R]iP
N6(2-isopentenyl) adenosine
Work performed while PJD was on leave at the University College of Wales at Aberystwyth. 相似文献
129.
Hybridoma I.13.17 was grown in semicontinuous culture in an attempt to investigate the steady-state concentrations of key components of monoclonal antibody (MAb) synthesis (e.g., intracellular MAb, IgG messenger RNAs) at different dilution rates between 0.008 and 0.055 h(-1). There was a general trend of increasing steady-state levels of total cytoplasmic RNA, total cell-associated MAb or cytoplasmic MAb, DNA synthesis rate, cellular metabolic activity, heavy (H-) and light (L-) chain IgG mRNAs with the increase in dilution rates. Increase in the half-lives of H- and L-chain mRNAs with increase in dilution rates may be sufficient to account for their increasing levels found under the same conditions. The specific growth rate was profoundly affected by the dilution rate, particularly near the lower end of the dilution rate range. Linear relationships were observed between the steady-state amounts of total cell-associated MAb and the relative levels of H- and L-chain mRNAs. Material balances on intracellular MAb demonstrated an increasing percentage of antibody not released into the growth medium (e.g., stored within the cell or anchored to the cell membrane) with increasing dilution rate. The MAb production rate per cell decreased significantly with the increase in dilution rates. No correlation was found between the relative levels of H- or L-chain mRNAs and the specific MAb production rate. Possible implications of rate-limiting steps in MAb synthesis and secretion are discussed. 相似文献
130.
Summary Like many polyphagous herbivores, individuals of Sarasinula plebeia (Fischer) (Soleolifera: Veronicellidae) consume a variety of plant species that may differ in nutritional content. In this study we determined the ability of these slugs to compensate for such variation in diet composition. Dilution with water of an agar-based diet containing commercial guinea pig food or carrot root to obtain dry weights (dw) of 90, 70, 40 and 10% of diet fresh weight (fw), caused immature slugs to consume increasingly more fresh weight of food [as much as 4.7-(guinea pig) to 6.1-fold (carrot) more]. Dry weight consumption and body mass-relative dry weight consumption rate also increased at intermediate dilutions, buth with further dilution, dry weight intake declined despite the greater fresh weight consumption. At each dilution level, slugs fed the guinea pig diet consumed from ca. 5-to 6.4-fold more fresh weight than the carrotfed slugs. The former grew substantially, with their final biomass and body mass-relative growth rate varying curvilinearly with diet % dw. If these slugs had not fed more but instead maintained the same fresh weight consumption as slugs in the 90% dw tretments, without altering food utilization efficiencies, then their biomass gain in the 70, 40 and 10% dw treatments would have been only about 62, 43, and 21%, respectively, of the values actually attained. In contrast, carrot-fed slugs did not grow and were only able to maintain their initial biomass. For each diet, slug tissue water (% fw) was highest in the most diluted treatment but did not differ significantly among the other dilution levels. Approximate digestibility of the carrot diet was highest at intermediate dilution levels (ca. 75% of ingested food was digested and absorbed); for the guinea pig diet, this efficiency declined linearly from about 66% to 59% with increased dilution. For slugs that grew (i.e., those fed the guinea pig diet), effeciences of converting digested (29–52%) and ingested (18–33%) food to dry biomass were both curvilinearly related to diet % dw. Thus, S. plebeia, like many other herbivores, has the capacity to increase food consumption substantially inresponse to reduced dietary nutrient level, allowing the slugs to cope with variable nutrient content in their food plants. 相似文献