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1.
Vertical Distribution and Activity of Soil Microbial Population in a Sandy Desert Ecosystem 总被引:1,自引:0,他引:1
The vertical distribution of a microbial community on a temporal and spatial (vertical) scale under the Tamarix aphylla salt-resistant plant was studied. The uniqueness of this plant is in the creation of “islands of salinity” beneath the plant,
due to salt excretion and leaf shedding at the plant base. Our working hypothesis was that the activity and biomass of the
soil microbial population and the ratio between bacterial biomass and fungal biomass will decrease toward the driest season.
Soil samples were collected beneath and between plants under the T. aphylla canopy down to the 0- to 50-cm depth at 10-cm intervals. The substrate-induced respiration method was used to estimate the
total soil microbial biomass (MB), CO2 evolution, metabolic quotient (qCO2) index, and MB of both the fungal and bacterial communities. A significant difference was observed (p < 0.05) in MB and qCO2 index between the different soil layers in the control samples, whereas there was no significant difference in the soil samples
in the vicinity of the plant (p > 0.05) between the depths, due to the plant rhizosphere effect. 相似文献
2.
Heterotrophic Soil Respiration in Relation to Environmental Factors and Microbial Biomass in Two Wet Tropical Forests 总被引:2,自引:0,他引:2
We examined the correlation between fungal and bacterial biomass, abiotic factors such as soil moisture, carbon in the light
soil fraction and soil nitrogen to a depth of 0–25 cm and heterotrophic soil respiration using a trenching technique – in
a secondary forest (Myrcia splendens, Miconia prasina and Casearia arborea) and a pine (Pinus caribeae) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil respiration was significantly reduced where roots were
excluded for 7 years in both the secondary forest and the pine plantation. Microbial biomass was also significantly reduced
in the root exclusion plots. In root exclusion treatment, total fungal biomass was on average 31 and 65% lower than the control
plots in the pine plantation and the secondary forest, respectively, but the total bacterial biomass was 24 and 8.3% lower
than the control plots in the pine plantation and the secondary forest, respectively. Heterotrophic soil respiration was positively
correlated with fungal biomass (R2=0.63, R2=0.39), bacterial biomass (R2=0.16, R2=0.45), soil moisture (R2=0.41, R2=0.56), carbon in light fraction (R2=0.45, R2=0.39) and total nitrogen (R2=0.69, R2=0.67) in the pine plantation and the secondary forest, respectively. The regression analysis suggested that fungal biomass
might have a greater influence on heterotrophic soil respiration in the pine plantation, while the bacterial biomass might
have a greater influence in the secondary forest. Heterotrophic soil respiration was more sensitive to total N than to carbon
in the light fraction, and soil moisture was a major factor influencing heterotrophic soil respiration in these forests where
temperature is high and relatively invariable. 相似文献
3.
Drying and rewetting is a frequent physiological stress for soil microbial communities; a stress that is predicted to grow
more influential with future climate change. We investigated the effect of repeated drying–rewetting cycles on bacterial (leucine
incorporation) and fungal (acetate in ergosterol incorporation) growth, on the biomass concentration and composition (PLFA),
and on the soil respiration. Using different plant material amendments, we generated soils with different initial fungal:bacterial
compositions that we exposed to 6–10 repetitions of a drying–rewetting cycle. Drying–rewetting decreased bacterial growth
while fungal growth remained unaffected, resulting in an elevated fungal:bacterial growth ratio. This effect was found irrespective
of the initial fungal:bacterial biomass ratio. Many drying–rewetting cycles did not, however, affect the fungal:bacterial
growth ratio compared to few cycles. The biomass response of the microbial community differed from the growth response, with
fungal and total biomass only being slightly negatively affected by the repeated drying–rewetting. The discrepancy between
growth- and biomass-based assessments underscores that microbial responses to perturbations might previously have been misrepresented
with biomass-based assessments. In light of this, many aspects of environmental microbial ecology may need to be revisited
with attention to what measure of the microbial community is relevant to study. 相似文献
4.
Ecosystem properties and microbial community changes in primary succession on a glacier forefront 总被引:13,自引:0,他引:13
We studied microbial community composition in a primary successional chronosequence on the forefront of Lyman Glacier, Washington,
United States. We sampled microbial communities in soil from nonvegetated areas and under the canopies of mycorrhizal and
nonmycorrhizal plants from 20- to 80-year-old zones along the successional gradient. Three independent measures of microbial
biomass were used: substrate-induced respiration (SIR), phospholipid fatty acid (PLFA) analysis, and direct microscopic counts.
All methods indicated that biomass increased over successional time in the nonvegetated soil. PLFA analysis indicated that
the microbial biomass was greater under the plant canopies than in the nonvegetated soils; the microbial community composition
was clearly different between these two types of soils. Over the successional gradient, the microbial community shifted from
bacterial-dominated to fungal-dominated. Microbial respiration increased while specific activity (respiration per unit biomass)
decreased in nonvegetated soils over the successional gradient. We proposed and evaluated new parameters for estimating the
C use efficiency of the soil microbial community: “Max” indicates the maximal respiration rate and “Acc” the total C released
from the sample after a standard amount of substrate is added. These, as well as the corresponding specific activities (calculated
as Max and Acc per unit biomass), decreased sharply over the successional gradient. Our study suggests that during the early
stages of succession the microbial community cannot incorporate all the added substrate into its biomass, but rapidly increases
its respiration. The later-stage microbial community cannot reach as high a rate of respiration per unit biomass but remains
in an “energy-saving state,” accumulating C to its biomass.
Received: 4 June 1998 / Accepted: 11 January 1999 相似文献
5.
Yingxin Huang Xueyong Zhao Hongxuan Zhang Wisdom Japhet Xiaoan Zuo Yayong Luo Gang Huang 《Journal of Plant Biology》2009,52(3):210-219
We monitored the allometric effects for greenhouse-grown Agriophyllum squarrosum plants in response to variations in population density and the availability of soil nutrients and water. Biomass allocations
were size-dependent. The plasticity of roots, stems, leaves, and reproductive effort was “true” in response to changes in
nutrient content. At a low level of soil minerals, plants allocated more resources to the development of roots and reproductive
organs than to leaves, but data for stem allocations were consistent for tradeoffs between the effects of nutrients and plant
size. The plasticities of leaf allocation and reproductive effort were “true” whereas those of root and stem allocations were
“apparent” in response to fluctuations in soil water, being a function of plant size. Decreasing soil water content was associated
with higher leaf allocation and lower reproductive effort. Except for this “apparent” plasticity of leaf allocation, none
was detected with population density on biomass allocation. Architectural traits were determinants of the latter. For roots,
the determining trait was the ratio of plant height to total biomass; for stems and reproduction, plant height; and for leaves,
the ratio of branch numbers to plant height. 相似文献
6.
P. J. Bottomley R. R. Yarwood S. A. Kageyama K. E. Waterstripe M. A. Williams K. Cromack Jr D. D. Myrold 《Plant and Soil》2006,289(1-2):35-45
Little information exists on the responses of soil fungal and bacterial communities in high elevation coniferous forest/open meadow ecosystems of the northwest United States of America to treatments that impact vegetation and soil conditions. An experiment was conducted in which soil cores were reciprocally transplanted between immediately adjacent forests and meadows at two high elevation (∼1,600 m) sites (Carpenter and Lookout) in the H.J. Andrews Experimental Forest located in the Cascade Mountains of Oregon. Half of the cores were placed in PVC pipe (closed) to prevent new root colonization, whereas the other cores were placed in mesh bags (open) to allow recolonization by fine roots. A duplicate set of open and closed soil cores was not transferred between sites and was incubated in place. After 2 year, soil cores were removed and changes in fungal and bacterial biomasses determined using light microscopy, and changes in microbial community composition determined by PLFA analysis, and by length heterogeneity PCR of the internal transcribed spacer region of fungal ribosomal DNA. At both sites soil microbial community structures had responded to treatments after 2 year of incubation. At Carpenter, both fungal and bacterial community structures of forest soil changed significantly in response to transfer from forest to meadow, with the shift in fungal community structure being accompanied by a significant decrease in the PLFA biomarker of fungal biomass,18:2ω6,9. At Lookout, both fungal and bacterial community structures of forest soil changed significantly in response to open versus closed core treatments, with the shift in the fungal community being accompanied by a significant decrease in the 18:2ω6,9 content of closed cores, and the shift in the bacterial community structure being accompanied by a significant increase in bacterial biomass of closed cores. At both sites, fungal community structures of meadow soils changed differently between open and closed cores in response to transfer to forest, and were accompanied by increases in the18:2ω6,9 content of open cores. Although there were no significant treatment effects on the bacterial community structure of meadow soil at either site, bacterial biomass was significantly higher in closed versus open cores regardless of transfer. 相似文献
7.
Temporal changes in soil microbial biomass carbon in an arable soil. Consequences for soil sampling 总被引:2,自引:0,他引:2
Sugar beet, winter wheat and winter barley were planted within a crop rotation on an arable soil with conventional soil management.
Soil samples were taken monthly from different depths of the whole plough layer (0–10, 10–20 and 20–30 cm) during a 56 month
period. The samples were analysed for microbial biomass carbon using the substrate-induced respiration technique.
Temporal changes in the amount of microbial biomass carbon were observed. Within a year, microbial biomass-C varied from low
values (−15% of total mean) in winter to high values (+15% of total mean) in summer. Relative deviations from the annual means
were calculated for each month in the year to demonstrate these fluctuations.
Temporal changes in microbial biomass-C depended on the sources of sample variation (5 years, 3 crops, 3 sampling depths).
The highest relative deviation from the annual mean microbial biomass-C was attributable to the factor “year”. Less variations
were caused by “crops” and “sampling depth”.
Soil microbial biomass-C remained constant during frost periods. From the observed temporal changes, recommendations for a
suitable date for soil sampling are given, which allows a representative estimation of the mean annual microbial biomass-C
content in arable soils. 相似文献
8.
Through the use of principal components analysis of the correlation matrix between excentricity quotients of intra- and inter-individual
transitions between pairs of the 22 most frequent behaviour elements of male and female gelada baboons observed in captivity,
the total variance could be described in terms of a low number of causal factors. In intra-individual sequences the five factors
requested of the analysis explain 74 per cent of the total variance. The following intra-individual groupings were found:
(I) “autostimulation”, (II) “intensive social” versus “groom”, (III) “male sexual”, (IV) “attack” versus “sexual presentation”,
and (V) “greeting” versus “threat”. In inter-individual transitions, the five factors explain 68 per cent of the variance.
The following communicative sets were found: (1) “female-sexual releasing”, (2) “greeting and approaching releasing” versus
“groom releasing”, (3) “austostimulation releasing” versus “male-sexual releasing”, (4) “fight releasing”, and (5) “conflict”.
Mimetic induction of same acts or acts within one particular intra-individual set was frequent. 相似文献
9.
南亚热带3种人工林土壤微生物生物量和微生物群落结构特征 总被引:7,自引:0,他引:7
以我国南亚热带格木、红椎和马尾松人工林为对象,采用氯仿熏蒸浸提法和磷脂脂肪酸法(PLFA)分析了林地土壤微生物生物量和微生物群落结构组成.结果表明: 林分和季节因素均显著影响土壤微生物生物量、总PLFAs量、细菌PLFAs量和真菌PLFAs量,且干季林分下的土壤微生物生物量、总PLFAs量、单个PLFA量均大于雨季.红椎人工林土壤微生物生物量碳(MBC)和总PLFAs量最高,而格木人工林土壤微生物生物量氮(MBN)最高.土壤pH值对土壤丛枝菌根真菌(16:1ω5c)的影响达到极显著正相关水平.土壤总PLFAs量、革兰氏阳性菌(G+)以及腐生真菌(18:2ω6,9c)、革兰氏阳性菌/革兰氏阴性菌(G+/G-)与土壤有机碳、全氮和全磷显著相关,表明土壤有机碳、全氮、全磷含量是影响该地区土壤微生物数量和种类的重要因素.外生菌根真菌(18:1ω9c)和丛枝菌根真菌与土壤碳氮比值呈极显著相关. 相似文献
10.
A bacterial phylogenetic survey of three environmentally distinct Antarctic Dry Valley soil biotopes showed a high proportion of so-called “uncultured” phylotypes, with a relatively low diversity of identifiable phylotypes. Cyanobacterial phylotypic signals were restricted to the high-altitude sample, whereas many of the identifiable phylotypes, such as the members of the Actinobacteria, were found at all sample sites. Although the presence of Cyanobacteria and Actinobacteria is consistent with previous culture-dependent studies of microbial diversity in Antarctic Dry Valley mineral soils, many phylotypes identified by 16S rDNA analysis were of groups that have not hitherto been cultured from Antarctic soils. The general belief that such “extreme” environments harbor a relatively low species diversity was supported by the calculation of diversity indices. The detection of a substantial number of uncultured bacterial phylotypes showing low BLAST identities (<95%) suggests that Antarctic Dry Valley mineral soils harbor a pool of novel psychrotrophic taxa. 相似文献
11.
Atmospheric culturable bacteria associated with meteorological conditions at a summer-time site in the mid-Willamette Valley,Oregon 总被引:1,自引:0,他引:1
Bruce Lighthart Brenda T. Shaffer A. Shelby Frisch Dorothea Paterno 《Aerobiologia》2009,25(4):285-295
A set of simultaneously collected quantitative measurements of 12 meteorological and 6 culturable atmospheric bacterial (CAB)
variables was made over a grass seed field during several early, mid, and late summer days. The observation site was located
between the Cascade and Coastal Mountain Ranges near Corvallis in the Willamette Valley, Oregon. Principal component analysis
identified those meteorological variables that had the highest loadings in six eigenvectors that account for 95.9% of variation
in the data factors, i.e., temperature @ 6.3 m above ground level (AGL), wind velocity @ 10.0 m AGL, wind velocity difference
@ 1.7–10.0 m, barometric pressure, wind direction standard deviation, and wind direction. When these variables were used in
a cluster analysis, they formed three statistically distinct meteorological variable clusters with means at ca. “midnight”,
ca. “midday”, and ca. “evening.” The highest mean density of CAB (i.e., 153.4 ± 162.5 CFU/m3) was associated with the “midday” meteorological Cluster-1 that had warm, dry “gentle breezes” from the southeast, in the
relatively bacteria loaded Willamette Valley air. The lowest mean density of CAB (i.e., 35.5 ± 24.1 CFU/m3) was associated with meteorological Cluster-3 in the late afternoon and “evening” occurring during the hottest and driest
part of the day with “fresh breezes” coming from the north northwest in air off the Pacific Ocean. Finally, the last cluster,
Cluster-2 occurred about midnight and had an intermediate density of CAB (74.2 ± 76.2 CFU/m3) in “light air” coming from the northwest, perhaps off the Pacific Ocean. The CAB associated with each of the three meteorological
clusters was only partially statistically distinct. Partially because the CAB in both the Pacific Ocean derived air masses
of the “evening” Cluster-3 and “midnight” Cluster-2 were not statically separable, though both were statistically separable
from the midday, Willamette Valley derived Cluster-1. Further indicating their common source, both Pacific Ocean derived air
masses had very similar percentages of pigmented bacteria, which were dissimilar to the pigmented bacterial population density
in the Willamette Valley air masses. In short, it is speculated that “midnight” atmosphere may simply contain the settling
concentrated residual bacterial particles in the abated fresh Pacific Ocean breezes after sundown. It is clearly shown that
with the methods employed, it is possible to associate the uniqueness of the quantity, and to a lesser extent the quality,
of the CAB population with the atmospheric conditions reported herein. From this project comes speculation that the strategies
relating the quasi-conservative bacterial populations associated with distinct but nonconservative air mass properties can
help to better understand more of the bacterial dynamics found in such situations. And to a further extent, molecular biological
methods could be applied to identify bacterial taxa in specific air masses. 相似文献
12.
We examined the effects of root and litter exclusion on the rate of soil CO2 efflux and microbial biomass using trenching and tent separation techniques in a secondary forest (SF) and a pine (Pinus caribaea Morelet) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil surface CO2 efflux was measured using the alkali trap method at 12 randomly-distributed locations in each treatment (control, root exclusion, litter exclusion, and both root and litter exclusion) in the plantation and the SF, respectively. We measured soil CO2 efflux every two months and collected soil samples at each sampling location in different seasons to determine microbial biomass from August 1996 to July 1997. We found that soil CO2 efflux was significantly reduced in the litter and root exclusion plots (7-year litter and/or root exclusion) in both the secondary forest and the pine plantation compared with the control. The reduction of soil CO2 efflux was 35.6% greater in the root exclusion plots than in the litter exclusion plots in the plantation, whereas a reversed pattern was found in the secondary forest. Microbial biomass was also reduced during the litter and root exclusion period. In the root exclusion plots, total fungal biomass averaged 31.4% and 65.2% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 24% and 8.3% lower than the control plots in the plantation and the secondary forest, respectively. In the litter exclusion treatment, total fungal biomass averaged 69.2% and 69.7% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 48% and 50.1% lower than the control plots in the plantation and the secondary forest, respectively. Soil CO2 efflux was positively correlated with both fungal and bacterial biomass in both the plantation the secondary forest. The correlation between soil CO2 efflux and active fungal biomass was significantly higher in the plantation than in the secondary forest. However, the correlation between the soil CO2 efflux and both the active and total bacterial biomass was significantly higher in the secondary forest than in the plantation in the day season. In addition, we found soil CO2 efflux was highly related to the strong interactions among root, fungal and bacterial biomass by multiple regression analysis (R2 > 0.61, P < 0.05). Our results suggest that carbon input from aboveground litterfall and roots (root litter and exudates) is critical to the soil microbial community and ecosystem carbon cycling in the wet tropical forests. 相似文献
13.
Successive soil treatment with captan or oxytetracycline affects non-target microorganisms 总被引:1,自引:0,他引:1
Z. Piotrowska-Seget R. Engel E. Nowak Jacek Kozdrój 《World journal of microbiology & biotechnology》2008,24(12):2843-2848
Changes in microbial biomass and activity were determined in a sandy-loam soil treated with successive dosages of oxytetracycline
(a bactericide) or captan (a fungicide) throughout 98 days of incubation under laboratory conditions. The numbers of culturable
bacteria and fungi, total bacterial and fungal biomass (as amounts of phospholipid fatty acids, PLFA), the fungal/bacterial
ratio, activities of acid and alkaline phosphatases and urease as well as concentrations of N-NH4
+ and N-NO3
− were assessed. Both oxytetracycline and captan significantly decreased numbers of culturable bacteria whereas total bacterial
biomass (bactPLFA) was not affected. Oxytetracycline did not effect on the fungal biomass, however their numbers were reduced
after the first and second time of soil amendment with the bactericide. Conversely, fungal numbers and biomass (PLFA 18:2ω6,9)
significantly decreased in response to soil treatment with the fungicide. Compared to oxytetracycline, captan significantly
decreased activities of acid and alkaline phosphatases. For urease activity, the decreased activity was only observed in the
soil after the third dosage of captan. Both biocides significantly increased concentrations of N-NH4
+ and decreased concentrations of N-NO3
− after the soil treatments. The results of this study indicate that successive soil treatment with oxytetracycline or captan
dosages may negatively affect non-target soil microorganisms and their activities. 相似文献
14.
Bacteria and nutrients were determined in upper soil samples collected underneath and between canopies of the dominant perennial
in each of three sites along a steep precipitation gradient ranging from the Negev desert in the south of Israel to a Mediterranean
forest in the north. Bacterial abundance, monitored by phospholipid fatty acid analysis, was significantly higher under the
shrub canopy (compared to barren soils) in the arid and semi-arid sites but not in the Mediterranean soils. Bacterial community
composition, determined using terminal restriction fragment length polymorphism and clone libraries, differed according to
the sample’s origin. Closer examination revealed that in the arid and semi-arid sites, α-Proteobacteria are more abundant
under the shrub canopy, while barren soils are characterized by a higher abundance of Actinobacteria. The bacterial communities
in the Mediterranean soils were similar in both patch types. These results correspond to the hypothesis of “resource islands”,
suggesting that shrub canopies provide a resource haven in low-resource landscapes. Yet, a survey of the physicochemical parameters
of inter- and under-shrub soils could not attribute the changes in bacterial diversity to soil moisture, organic matter, or
essential macronutrients. We suggest that in the nutrient-poor soils of the arid and semi-arid sites, bacteria occupying the
soil under the shrub canopy may have longer growth periods under favorable conditions, resulting in their increased biomass
and altered community composition. 相似文献
15.
Sediment samples collected during the expedition “Arctic Ocean `96” with the Swedish ice-breaker ODEN were investigated to
estimate for the first time heterotrophic activity and total microbial biomass (size range from bacteria to small metazoans)
from the perennially ice-covered central Arctic Ocean. Benthic activities and biomass were evaluated analysing a series of
biogenic sediment compounds (i.e. bacterial exoenzymes, total adenylates, DNA, phospholipids, particulate proteins). In contrast
to the very time-consuming sorting, enumeration and weight determination, analyses of biochemical sediment parameters may
represent a useful method for obtaining rapid information on the ecological situation in a given benthic system. Bacterial
cell numbers and biomass were estimated for comparison with biochemically determined biomass data, to evaluate the contribution
of the bacterial biomass to the total microbial biomass. It appeared that bacterial biomass made up only 8–31% (average of
all stations = 20%) of the total microbial biomass, suggesting a large fraction of other small infaunal organisms within the
sediment samples (most probably fungi, yeasts, protozoans such as flagellates, ciliates or amoebae, as well as a fraction
of small metazoans). Activity and biomass values determined within this study were generally extremely low, and often even
slightly lower than those given for other deep oceanic regions, thus characterizing the seafloor of the central Arctic Ocean
as a “benthic desert”. Nevertheless, some clear trends in the data could be found, e.g. generally sharply decreasing values
within the sediment column, a vague tendency for declining values with increasing water depth of sampling stations, and also
differences between various Arctic deep-sea regions.
Received: 16 May 1997 / Accepted: 28 August 1997 相似文献
16.
The rhodophytes Hypnea cervicornis and Cryptonemia crenulata are abundant along the Brazilian coastline and are rich in nutrients. They may therefore be used as a source of protein in
shrimp diets. The aim of the present study was to test this hypothesis. The experiment was conducted in a laboratory, where
10-day-old post-larvae aged underwent 7 days of acclimation in a 1,000 L tank. They were then kept in plastic aquariums, each
containing 10 L, and 20 larvae were fed daily (10% of biomass) in four equal portions with one of four diets (five repetitions
of each) for a period of 45 days. All diets contained 30% crude protein (isoprotein) and 300 kcal 100 g−1 (isocaloric), with different percentages of seaweed powder: Diet “A” 39%; Diet “B” 26%, Diet “C” 13%, and Diet “D” without
seaweed (control diet). Algae were collected, rinsed, dried and ground up for the feed formulations. Weight of the animals
was measured at the beginning of the experiment and at 15-day intervals to assess their growth. The physico-chemical variables
of the water were measured every 2 days. Final biomass, biomass gain and specific growth rate (SGR) exhibited no significant
differences between treatments (P > 0.05). Survival rate was equal under the four experimental conditions, being consistent within four decimal places 95.2%
to 97.00% (P > 0.05). Diets “A” and “B”, with a greater content of algae, exhibited better feed conversion (1.79:1 and 1.82:1) than Diets
“C” and “D” (2.04:1 and 2.08:1) (P < 0.05). The physical-chemical variables of the water showed no significant variation and remained within the standards necessary
for the wellbeing of the animals. If sufficient biomass of beached algae can be practically and economically collected, it
may be used as a component in the making of shrimp feed. 相似文献
17.
Two successive hydroponic experiments were carried out to identify barley varieties tolerant to Cd toxicity via examining
Soil–Plant Analyses Development (SPAD) value, plant height, leaves and tillers per plant, root number and volume, and biomass
accumulation. The results showed that SPAD values (chlorophyll meter readings), plant height, leaf number, root number and
volume, and biomass accumulation of shoot/root were significantly reduced in the plants grown in 20 μM Cd nutrient solution
compared with control, and the uptake and translocation of Zn, Mn, and Cu was also strictly hindered. Furthermore, there was
a highly significant difference in the reduction in these growth parameters among varieties, and varieties “Weisuobuzhi” and
“Jipi 1” showed the least reduction both in the two experiments, suggesting their high tolerance to Cd toxicity, while “Dong
17” and “Suyinmai 2” with the greatest reduction and the toxicity symptoms appeared rapidly and severely, denoting as Cd-sensitive
varieties. Significant variety difference in Cd concentration was also found, with Weisuobuzhi containing the highest and
Jipi 1 the lowest Cd concentration in shoots. 相似文献
18.
Little is known of how bacterial diversity in soils varies with elevation. One previous study found a decline with elevation,
whereas another found no trend. We chose Mount Fuji of Japan as a geologically and topographically simple mountain system.
Samples were taken at elevational intervals, between the base of the mountain at 1,000 m and its summit at 3,700 m. Polymerase
chain reaction-amplified soil DNA for the bacterial 16S gene targeting V1–V3 region was pyrosequenced using the 454 Roche
machine, and taxonomically classified with reference to a bioinformatic database. There was a significant “peak” in total
bacterial diversity at around 2,500 m above the tree line with a decline towards the highest elevations around 3,700 m near
the summit. Individual bacterial phyla show distinct trends—increase, decrease, or a mid-elevational “bulge” in diversity.
Bacterial diversity does not parallel woody plant or herbaceous plant diversity. We suggest that beyond the tree and vegetation
line, the more extreme temperature fluctuations, stronger UV, lack of nutrients, and more frequent disturbance of the loose
substrate of these slopes allows less competition and greater bacterial species diversity due to “lottery” recruitment. However,
at the highest elevations, the physiological challenges are so extreme that fewer bacterial species are capable of surviving. 相似文献
19.
The effect of fire on nutrients in a pine forest soil 总被引:7,自引:0,他引:7
The effect of a hot summer fire on soil nutrient contents in the upper 2 cm of Aleppo pine forest with a dense woody understory
was studied from September 1985 to May 1986.
In comparison with the adjacent unburned forest, total nitrogen decreased by 25% but available forms of nitrogen were much
higher. In burned and unburned soils there was a similar trend to increase and decrease in NH
4
+
−N, However, while (NO
2
−
+NO
3
−
−N decreased in the unburned soil it rose rapidly in the burned ash soil. Total phosphorus increased by 300% after the fire
but decreased again 2 months later. Also water-soluble P increased up to November and then decreased to the levels of the
unburned soils. The same was true for electrical conductivity and pH, increasing immediately after the fire and then leveling
off again.
This increase in nutrient levels in the “ash soil” was reflected in the striking increase in shoot and root biomass and in
the content of N, P, Mg, K, Ca, Zn and Fe in wheat and clover plants grown in pots in these soils. These nutrient levels were
much higher in the wheat plants, which also produced 12 times more seeds in the “ash soil.”
It seems that fire in these pine forests causes a short-term flush of the mineral elements in the upper “ash soil” layer which
is reverted gradually via the herbaceous post-fire to the ecosystem. 相似文献
20.
Parasites often confront conflicting demands when evaluating and distributing themselves among host individuals, in order to attain maximum reproductive success. We tested two alternative hypotheses about host preference by fleas in relation to the age of their rodent host. The first hypothesis suggests that fleas select adult over juvenile rodents because the latter represent a better nutritional resource (the “well-fed host” hypothesis), whereas the second hypothesis suggests that fleas prefer the weaker and less resistant juveniles because they are easier to colonise and exploit (“poorly fed host” hypothesis). We sampled fleas (Synosternus cleopatrae) on the gerbil (Gerbillus andersoni) in 23 different plots in the Negev desert and found an unequal distribution of fleas between adult and juvenile hosts. Furthermore, flea distribution changed as a function of flea density—from juvenile-biased flea parasitism (the “poorly fed host” hypothesis) at low densities to adult-biased flea parasitism (the “well-fed host” hypothesis) at high densities. Other factors that influenced flea preference were soil temperature and the presence of ticks. These results suggest that host selection is not an explicit alternative choice between adults and juveniles (“well-fed host” versus “poorly fed host” hypotheses), but rather a continuum where the distribution between adults and juveniles depends on host, parasite, and environmentally related factors. 相似文献