Protozoan Response to the Addition of Bacterial Predators and Other Bacteria to Soil

Representatives of several categories of bacteria were added to soil to determine which of them might elicit responses from the soil protozoa. The various categories were nonobligate bacterial predators of bacteria, prey bacteria for these predators, indigenous bacteria that are normally present in high numbers in soil, and non-native bacteria that often find their way in large numbers into soil. The soil was incubated and the responses of the indigenous protozoa were determined by most-probable-number estimations of total numbers of protozoa. Although each soil was incubated with only one species of added bacteria, the protozoan response for the soil was evaluated by using most-probable-number estimations of several species of bacteria. The protozoa did not respond to incubation of the soil with either Cupriavidus necator, a potent bacterial predator, or one of its prey species, Micrococcus luteus. C. necator also had no effect on the protozoa. Therefore, in this case, bacterial and protozoan predators did not interact, except for possible competition for bacterial prey cells. The soil protozoa did not respond to the addition of Arthrobacter globiformis or Bacillus thuringiensis. Therefore, the autochthonous state of Arthrobacter species in soil and the survival of B. thuringiensis were possibly enhanced by the resistance of these species to protozoa. The addition of Bacillus mycoides and Escherichia coli cells caused specific responses by soil protozoa. The protozoa that responded to E. coli did not respond to B. mycoides or any other bacteria, and vice versa. Therefore, addition to soil of a nonsoil bacterium, such as E. coli, did not cause a general increase in numbers of protozoa or in protozoan control of the activities of other bacteria in the soil.     

Separation of Spores and Parasporal Crystals of Bacillus thuringiensis in Gradients of Certain X-Ray Contrasting Agents

Spores and parasporal crystals of Bacillus thuringiensis can be separated at moderate centrifugation speeds (10,000 to 12,000 rpm) in gradients of Renografin or sodium diatrizoate.     

两种热带木质藤本幼苗形态、生长和光合能力对光强和养分的响应

比较了两种不同攀援习性, 卷须缠绕种薄叶羊蹄甲(Bauhinia tenuiflora)和茎缠绕种刺果藤(Byttneria aspera), 木质藤本植物的形态、生长及光合特性对不同光强(4%、35%和全光照)和土壤养分(高和低)的响应。两种藤本植物大部分表型特征主要受光照的影响, 而受土壤养分的影响较小。弱光促进地上部分生长, 弱光下两种植物均具有较大的比叶面积(specific leaf area, SLA)、茎生物量比(stem mass ratio, SMR)和平均叶面积比(mean leaf area ratio, LARm)。高光强下, 两种植物的总生物量和投入到地下部分的比重增加, 具有更大的根生物量比(root mass ratio, RMR)、更多的分枝数、更高的光合能力( maximum photosynthetic rate, Pmax)和净同化速率(net assimilation rate, NAR), 综合表现为相对生长速率(relative growth rate, RGR)增加。两种藤本植物的Pmax与叶片含氮量的相关性均未达显著水平, 但刺果藤的Pmax与SLA之间呈显著的正相关, 而薄叶羊蹄甲的Pmax与SLA之间相关性不显著。在相同光照强度和土壤养分条件下, 卷须缠绕种薄叶羊蹄甲的RGR显著高于茎缠绕种刺果藤。薄叶羊蹄甲的RGR与NAR呈显著正相关, 其RGR与SLA、平均叶面积比(LARm)及Pmax之间相关性不显著。刺果藤的RGR与NAR呈显著的正相关, 而与SLA存在显著的负相关。上述结果表明, 与土壤养分相比, 光照强度可能是决定木质藤本分布更为重要的生态因子。卷须缠绕种薄叶羊蹄甲由于具有特化的攀援器官, 在形态上和生理上具有更大的可塑性, 这使得卷须缠绕种木质藤本在与其它植物的竞争中更具优势。     

两种热带木质藤本幼苗形态、生长和光合能力对光强和养分的响应

比较了两种不同攀援习性,卷须缠绕种薄叶羊蹄甲（Bauhinia tenuiflora）和茎缠绕种刺果藤（Byttneria aspera）,木质藤本植物的形态、生长及光合特性对不同光强（4％、35％和全光照）和土壤养分（高和低）的响应。两种藤本植物大部分表型特征主要受光照的影响,而受土壤养分的影响较小。弱光促进地上部分生长,弱光下两种植物均具有较大的比叶面积（specific leaf area,SLA）、茎生物量比（stem mass ratio,SMR）和平均叶面积比（mean leaf area ratio,LARm）。高光强下,两种植物的总生物量和投入到地下部分的比重增加,具有更大的根生物量比（root mass ratio,RMR）、更多的分枝数、更高的光合能力（maximum photosynthetic rate,Pmax）和净同化速率（net assimilation rate,NAR）,综合表现为相对生长速率（relative growth rate,RGR）增加。两种藤本植物的Pmax与叶片含氮量的相关性均未达显著水平,但刺果藤的Pmax与SU志间呈显著的正相关,而薄叶羊蹄甲的Pmax与SLA之间相关性不显著。在相同光照强度和土壤养分条件下,卷须缠绕种薄叶羊蹄甲的RGR显著高于茎缠绕种刺果藤。薄叶羊蹄甲的RGR与NAR呈显著正相关,其RGR与SLA、平均叶面积比（EARm）及Pmax之间相关性不显著。刺果藤的RGR与NAR呈显著的正相关,而与SLA存在显著的负相关。上述结果表明,与土壤养分相比,光照强度可能是决定木质藤本分布更为重要的生态因子。卷须缠绕种薄叶羊蹄甲由于具有特化的攀援器官,在形态上和生理上具有更大的可塑性,这使得卷须缠绕种木质藤本在与其它植物的竞争中更具优势。     

Factors Affecting Variability in Time between Addition of Nutrient Germinants and Rapid Dipicolinic Acid Release during Germination of Spores of Bacillus Species

The simultaneous nutrient germination of hundreds of individual wild-type spores of three Bacillus species and a number of Bacillus subtilis strains has been measured by two new methods, and rates of release of the great majority of the large pool of dipicolinic acid (DPA) from individual spores of B. subtilis strains has been measured by Raman spectroscopy with laser tweezers. The results from these analyses and published data have allowed a number of significant conclusions about the germination of spores of Bacillus species as follows. (i) The time needed for release of the great majority of a Bacillus spore''s DPA once rapid DPA release had begun (ΔT_release) during nutrient germination was independent of the concentration of nutrient germinant used, the level of the germinant receptors (GRs) that recognize nutrient germinants used and heat activation prior to germination. Values for ΔT_release were generally 0.5 to 3 min at 25 to 37°C for individual wild-type spores. (ii) Despite the conclusion above, germination of individual spores in populations was very heterogeneous, with some spores in wild-type populations completing germination ≥15-fold slower than others. (iii) The major factor in the heterogeneity in germination of individual spores in populations was the highly variable lag time, T_lag, between mixing spores with nutrient germinants and the beginning of ΔT_release. (iv) A number of factors decrease spores'' T_lag values including heat activation, increased levels of GRs/spore, and higher levels of nutrient germinants. These latter factors appear to affect the level of activated GRs/spore during nutrient germination. (v) The conclusions above lead to the simple prediction that a major factor causing heterogeneity in Bacillus spore germination is the number of functional GRs in individual spores, a number that presumably varies significantly between spores in populations.Spores of various Bacillus species are metabolically dormant and can survive for years in this state (30). However, spores constantly sense their environment, and if appropriate small molecules termed germinants are present, spores can rapidly return to life in the process of germination followed by outgrowth (25, 29, 30). The germinants that most likely trigger spore germination in the environment are low-molecular-weight nutrient molecules, the identities of which are strain and species specific, including amino acids, sugars, and purine nucleosides. Metabolism of these nutrient germinants is not needed for the triggering of spore germination. Rather, these germinants are recognized by germinant receptors (GRs) located in the spore''s inner membrane that recognize their cognate germinants in a stereospecific manner (17, 24, 25, 29). Spores have a number of such GRs, with three functional GRs in Bacillus subtilis spores and even more in Bacillus anthracis, Bacillus cereus, and Bacillus megaterium spores (6, 29, 30). Binding of nutrient germinants to some single GRs is sufficient to trigger spore germination, for example the triggering of B. subtilis spore germination by binding of l-alanine or l-valine to the GerA GR. However, many GRs cooperate such that binding of germinants by ≥2 different GRs is needed to trigger germination (2, 29): for example, the triggering of B. subtilis spore germination by the binding of components of a mixture of l-asparagine, d-glucose, d-fructose, and K⁺ ions (AGFK) to the GerB and GerK GRs. The binding of nutrient germinants to GRs triggers subsequent events in germination, although how this is accomplished is not known.The first readily measured biochemical event after addition of nutrient germinants to Bacillus spores is the rapid release of the spore''s large depot (∼10% of spore dry weight) of pyridine-2,6-dicarboxylic acid (dipicolinic acid [DPA]) plus its chelated divalent cations, predominantly Ca²⁺ (Ca-DPA), from the spore core (25, 29). Ca-DPA release then results in the activation of two redundant cortex-lytic enzymes (CLEs), CwlJ and SleB, which hydrolyze the spore''s peptidoglycan cortex layer (16, 22, 27, 29). CwlJ is activated by Ca-DPA as it is released from the spore while SleB is activated only after most DPA is released (17, 20, 22, 26, 27). Cortex hydrolysis ultimately allows the spore core to expand and take up more water, raising the core water content from the 35 to 45% of wet weight in the dormant spore to the 80% of wet weight characteristic of growing cells. Full hydration of the spore core then allows enzyme action, metabolism, and macromolecular synthesis to resume in the now fully germinated spore.Germination of spores in populations is very heterogeneous, with some spores germinating rapidly and some extremely slowly (4, 5, 9, 11, 13-15, 19, 26, 31, 32). Where it has been studied, the reason for this heterogeneity has been suggested to be due to a variable lag period (T_lag) between the time of mixing spores with a germinant and the time at which rapid DPA release begins, since once rapid DPA release begins, the time required for release of almost all DPA as well as for subsequent cortex hydrolysis is generally rather short compared to T_lag values in individual spores (5, 11, 13-15, 19, 26, 31, 32). The times required for DPA release and cortex hydrolysis are also similar in wild-type spores with both very short and long T_lag values (5, 15, 19, 27). The reasons for the variability in T_lag times between individual spores in populations are not known, although there are reports that both activation of spores for germination by a sublethal heat treatment (heat activation) as well as increasing concentrations of nutrient germinants can shorten T_lag values (12, 14, 15, 18, 32). However, there has been no detailed study of the causes of the variability in T_lag values between very large numbers of individual spores in populations.In order to study the heterogeneity in spore germination thoroughly, methods are needed to follow the germination of hundreds of individual spores over several hours. Initial studies of the germination of individual spores examined a single spore in a phase-contrast microscope and followed the germination of this spore by changes in the core''s refractive index due to DPA release and core swelling (14, 15, 32, 34). However, this method is labor-intensive for gathering data with hundreds of individual spores. More recently, confocal microscopy and then surface adsorption and optical tweezers have been used to capture single spores, and germination events have been followed by methods such as Raman spectroscopy to directly measure DPA release, as well as phase-contrast microscopy and elastic light scattering (3, 5, 9, 10, 19, 26). While the latter recent advances have allowed accumulation of much information about germination, collection of this type of data for large numbers of individual spores is still labor-intensive, although use of dual optical traps (35) and perhaps multiple traps in the future may alleviate this problem. However, phase-contrast microscopy plus appropriate computer software has recently allowed the monitoring of many hundreds of individual spores for several hours, with automated assessment of various changes in the cells during the period of observation (19). In the present work, we have used both phase-contrast and differential interference contrast (DIC) microscopy to monitor the germination of many hundreds of individual spores of three Bacillus species adhered on either an agarose pad or a glass coverslip for 1 to 2 h. This work, as well as examination of times needed for release of most DPA once rapid DPA release has begun during germination of individual spores under a variety of conditions, has allowed detailed examination of the effects of heat activation, nutrient germinant concentration, GR numbers per spore, and individual CLEs on spore germination heterogeneity and on values of T_lag for individual spores.     

Salt-marsh Crustaceans, Gammarus duebeni and Palaemonetes varians as Predators of Mosquito Larvae and Their Reaction to Bacillus thuringiensis subsp. israelensis

The predatory potential of two onmnivorous crustaceans, Gammarus duebeni and Palaemonetes varians, has been examined to investigate their effect as mosquito larval predators. Mature gammarids ate 4-8 Aedes detritus larvae/24 h and palaemonids, 22-30/1 h, often killing larvae when not hungry. The crustaceans were exposed to Bacillus thuringiensis subsp. israelensis (Bti) and to mosquito larvae killed by Bti with no adverse effects, endorsing the safety of this microbial pesticide. Crustacean faecal pellets, collected post-feeding and placed in fresh marsh water, were toxic to mosquito larvae the following day. After placing the crustaceans in fresh salt-marsh water for 6 days, the fresh faecal pellets failed to kill mosquitoes apart from pellets obtained from crustaceans originally fed on the highest concentration of Bti, where there was a 50% kill after 3 days of incubation. Mosquito larvae on salt-marshes are not easy to control with ecologically undesirable toxic chemicals. Encouraging the breeding of predators such as crustaceans, or even their release, could prove to be a useful method of mosquito control to supplement the periodic inundative use of the ecologically acceptable Bti.     

Activity and Regulation of Various Forms of CwlJ,SleB, and YpeB Proteins in Degrading Cortex Peptidoglycan of Spores of Bacillus Species In Vitro and during Spore Germination

Germination of Bacillus spores requires degradation of a modified layer of peptidoglycan (PG) termed the spore cortex by two redundant cortex-lytic enzymes (CLEs), CwlJ and SleB, plus SleB''s partner protein, YpeB. In this study, in vitro and in vivo analyses have been used to clarify the roles of individual SleB and YpeB domains in PG degradation. Purified mature Bacillus cereus SleB without its signal sequence (SleB^M) and the SleB C-terminal catalytic domain (SleB^C) efficiently triggered germination of decoated Bacillus megaterium and Bacillus subtilis spores lacking endogenous CLEs; previously, SleB''s N-terminal domain (SleB^N) was shown to bind PG but have no enzymatic activity. YpeB lacking its putative membrane anchoring sequence (YpeB^M) or its N- and C-terminal domains (YpeB^N and YpeB^C) alone did not exhibit degradative activity, but YpeB^N inhibited SleB^M and SleB^C activity in vitro. The severe germination defect of B. subtilis cwlJ sleB or cwlJ sleB ypeB spores was complemented by ectopic expression of full-length sleB [sleB(FL)] and ypeB [ypeB(FL)], but normal levels of SleB^FL in spores required normal spore levels of YpeB^FL and vice versa. sleB(FL) or ypeB(FL) alone, sleB(FL) plus ypeB(C) or ypeB(N), and sleB(C) or sleB(N) plus ypeB(FL) did not complement the cortex degradation defect in cwlJ sleB ypeB spores. In addition, ectopic expression of sleB(FL) or cwlJ(FL) with a Glu-to-Gln mutation in a predicted active-site residue failed to restore the germination of cwlJ sleB spores, supporting the role of this invariant glutamate as the key catalytic residue in SleB and CwlJ.     

Case Study of the Distribution of Mucosa-Associated Bifidobacterium Species, Lactobacillus Species, and Other Lactic Acid Bacteria in the Human Colon

The distribution of mucosa-associated bacteria, bifidobacteria and lactobacilli and closely related lactic acid bacteria, in biopsy samples from the ascending, transverse, and descending parts of the colon from four individuals was investigated by denaturing gradient gel electrophoresis (DGGE). Bifidobacterial genus-specific, Lactobacillus group-specific, and universal bacterial primers were used in a nested PCR approach to amplify a fragment of the 16S rRNA gene. DGGE profiles of the bifidobacterial community were relatively simple, with one or two amplicons detected at most sampling sites in the colon. DGGE profiles obtained with Lactobacillus group-specific primers were complex and varied with host and sampling site in the colon. The overall bacterial community varied with host but not sampling site.     

Shrinkage of Attached Roots of Opuntia ficus-indica in Response to Lowered Water Potentials--Predicted Consequences for Water Uptake or Loss to Soil

Attached 2-month-old roots of the succulent plant, Opuntia ficus-indica,shrank 0.4% radially during periods of maximal transpirationunder wet conditions. In contrast, reversible decreases in diameterof nearly 20% occurred for these roots as their ambient waterpotential () in the vapour phase decreased from –0.01to –10 MPa over 8 d, the changes being slightly more rapidat 40 °C than at 10 °C. Such substantial diameter changesbecame progressively less with root age, from a 43% decreasein diameter at 3 weeks to a 6% decrease at 12 months Root shrinkagewas slight when was decreased from –0.01 to –0.3MPa, the latter being similar to the root water potential.As was further decreased from –0.3 to –10 MPa,water movement out of cortical cells caused considerable rootshrinkage. The root hydraulic conductivity (L_p) decreased only30 to 60% for a change in from –0.01 to –10 MPacompared with a decrease of over 10⁶-fold for the soil hydraulicconductivity over this range. The overall conductivity of thesoil, the root-soil air gap, and the root was predicted to bedominated by L_p for _soil above –0.3 MPa. As simulated_soil decreased below –0.3 MPa, the root-soil air gap initiallybecame the primary limiter of water loss from the roots. Below–5 MPa for 1-month-old roots and below –2 MPa for12-month-old roots, the soil became the main limiter of waterloss. Thus, water uptake from wet soils apparently was mainlycontrolled by root properties Water loss to drying soils wascontrolled by the development of a root-soil air gap aroundshrinking roots during the initial phase of soil drying andby the reduction of the soil hydraulic conductivity at evenlower _soil. Root diameter, root hydraulic conductivity, root-soil air gap, soil hydraulic conductivity     

Tolerance to individual and joint effects of arsenic and Bacillus thuringiensis subsp,israelensis or Lysinibacillus sphaericus in Culex mosquitoes

Arsenic contamination of global water supplies has come to the forefront in policy decisions in recent decades. However, the effects of arsenic on lower trophic levels of insects inhabiting contaminated ecosystems are not well understood. One approach to document both acute and sublethal effects of toxicants like arsenic is to assay them in combination with microbial pathogens to evaluate shifts in survival curves of the test organisms. Larvae of Culex quinquefasciatus and Culex tarsalis were reared in water containing 0 or 1 000μg/L of arsenate or arsenite. Fourth instars were then exposed to a range of doses of Bacillus thuringiensis subsp, israelensis （Bti） or Lysinibacillus sphaericus （Ls）, with shifts in lethal concentrations determined. Arsenic accumulation in 4th instars was also quantified, and a relative growth index （RGI） calculated for the treatments and compared to controls. Larvae of both species accumulated between 4 447 ± 169 ng As/g and 6 983 4- 367 ng As/g, though RGI values indicated accumulation did not affect growth and development. In all cases, the LC50＇s and LC90＇s of Cx. quinquefasciatus exposed jointly with arsenic and Bti/Ls were higher than Cx. tarsafis. Cx. tarsafis reared in arsenite showed a significant reduction in their Bti LC90 values compared to the control, indicating a sublethal effect of Bti. When exposed jointly with Ls, arsenite was more toxic than arsenate in Cx. tarsalis. Overall, these results indicate tolerance of these Culex species to arsenic exposures, and why this may occur is discussed.     

Roles of Small,Acid-Soluble Spore Proteins and Core Water Content in Survival of Bacillus subtilis Spores Exposed to Environmental Solar UV Radiation

Spores of Bacillus subtilis contain a number of small, acid-soluble spore proteins (SASP) which comprise up to 20% of total spore core protein. The multiple α/β-type SASP have been shown to confer resistance to UV radiation, heat, peroxides, and other sporicidal treatments. In this study, SASP-defective mutants of B. subtilis and spores deficient in dacB, a mutation leading to an increased core water content, were used to study the relative contributions of SASP and increased core water content to spore resistance to germicidal 254-nm and simulated environmental UV exposure (280 to 400 nm, 290 to 400 nm, and 320 to 400 nm). Spores of strains carrying mutations in sspA, sspB, and both sspA and sspB (lacking the major SASP-α and/or SASP-β) were significantly more sensitive to 254-nm and all polychromatic UV exposures, whereas the UV resistance of spores of the sspE strain (lacking SASP-γ) was essentially identical to that of the wild type. Spores of the dacB-defective strain were as resistant to 254-nm UV-C radiation as wild-type spores. However, spores of the dacB strain were significantly more sensitive than wild-type spores to environmental UV treatments of >280 nm. Air-dried spores of the dacB mutant strain had a significantly higher water content than air-dried wild-type spores. Our results indicate that α/β-type SASP and decreased spore core water content play an essential role in spore resistance to environmentally relevant UV wavelengths whereas SASP-γ does not.Spores of Bacillus spp. are highly resistant to inactivation by different physical stresses, such as toxic chemicals and biocidal agents, desiccation, pressure and temperature extremes, and high fluences of UV or ionizing radiation (reviewed in references 33, 34, and 48). Under stressful environmental conditions, cells of Bacillus spp. produce endospores that can stay dormant for extended periods. The reason for the high resistance of bacterial spores to environmental extremes lies in the structure of the spore. Spores possess thick layers of highly cross-linked coat proteins, a modified peptidoglycan spore cortex, a low core water content, and abundant intracellular constituents, such as the calcium chelate of dipicolinic acid and α/β-type small, acid-soluble spore proteins (α/β-type SASP), the last two of which protect spore DNA (6, 42, 46, 48, 52). DNA damage accumulated during spore dormancy is also efficiently repaired during spore germination (33, 47, 48). UV-induced DNA photoproducts are repaired by spore photoproduct lyase and nucleotide excision repair, DNA double-strand breaks (DSB) by nonhomologous end joining, and oxidative stress-induced apurinic/apyrimidinic (AP) sites by AP endonucleases and base excision repair (15, 26-29, 34, 43, 53, 57).Monochromatic 254-nm UV radiation has been used as an efficient and cost-effective means of disinfecting surfaces, building air, and drinking water supplies (31). Commonly used test organisms for inactivation studies are bacterial spores, usually spores of Bacillus subtilis, due to their high degree of resistance to various sporicidal treatments, reproducible inactivation response, and safety (1, 8, 19, 31, 48). Depending on the Bacillus species analyzed, spores are 10 to 50 times more resistant than growing cells to 254-nm UV radiation. In addition, most of the laboratory studies of spore inactivation and radiation biology have been performed using monochromatic 254-nm UV radiation (33, 34). Although 254-nm UV-C radiation is a convenient germicidal treatment and relevant to disinfection procedures, results obtained by using 254-nm UV-C are not truly representative of results obtained using UV wavelengths that endospores encounter in their natural environments (34, 42, 50, 51, 59). However, sunlight reaching the Earth''s surface is not monochromatic 254-nm radiation but a mixture of UV, visible, and infrared radiation, with the UV portion spanning approximately 290 to 400 nm (33, 34, 36). Thus, our knowledge of spore UV resistance has been constructed largely using a wavelength of UV radiation not normally reaching the Earth''s surface, even though ample evidence exists that both DNA photochemistry and microbial responses to UV are strongly wavelength dependent (2, 30, 33, 36).Of recent interest in our laboratories has been the exploration of factors that confer on B. subtilis spores resistance to environmentally relevant extreme conditions, particularly solar UV radiation and extreme desiccation (23, 28, 30, 34 36, 48, 52). It has been reported that α/β-type SASP but not SASP-γ play a major role in spore resistance to 254-nm UV-C radiation (20, 21) and to wet heat, dry heat, and oxidizing agents (48). In contrast, increased spore water content was reported to affect B. subtilis spore resistance to moist heat and hydrogen peroxide but not to 254-nm UV-C (12, 40, 48). However, the possible roles of SASP-α, -β, and -γ and core water content in spore resistance to environmentally relevant solar UV wavelengths have not been explored. Therefore, in this study, we have used B. subtilis strains carrying mutations in the sspA, sspB, sspE, sspA and sspB, or dacB gene to investigate the contributions of SASP and increased core water content to the resistance of B. subtilis spores to 254-nm UV-C and environmentally relevant polychromatic UV radiation encountered on Earth''s surface.     

模拟氮沉降对典型阔叶红松林土壤有机碳和养分的影响简

为探讨氮沉降对典型阔叶红松（Pinus koraiensis）林的影响,从2008年6月~2010年8月进行了人工模拟氮沉降实验,实验分为对照、低N、中N、高N4个处理,每个处理3个重复。所施氮肥为CO(NH₂)₂,以溶液的形式喷施,4个处理浓度分别为0、30、60、120 kg·hm^-2·a^-1。在氮沉降进行1年后,采集各处理0～20、20～40和40～60 cm的土壤样品,测定其土壤有机C、全N、碱解N和速效P、速效K。结果表明：相同处理下,有机C和全N含量随土层的加深均逐渐减少。总体上低、中N处理显著增加了土壤有机C、碱解N和速效K含量,中、高N处理显著降低了土壤速效P含量（P<0.05）,而对全N含量影响不显著（P>0.05）。土壤有机C与土壤全N、碱解N、速效P、速效K之间存在极显著正相关关系（P<0.001）。有机C和土壤养分对氮沉降的响应说明氮沉降在短期内可能影响阔叶红松林土壤碳库积累和土壤肥力水平。     

Root Hydraulic Conductivity of Two Cactus Species in Relation to Root Age, Temperature, and Soil Water Status

The effects of root age, temperature, and soil water statuson root hydraulic conductivity (L_P) were investigated for twocactus species, Ferocactus acanthodes and Opuntia ficus-indica.The volumetric flux density of water was measured for excisedroot segments, either using negative hydrostatic pressures appliedto the proximal end or using reverse flow of water from theroot to the soil. For both species, L_P at 20 ?C increased withroot age, average values reaching a maximum of 3.9 ? 10^–7m s^–1 MPa^–1 for F. acanthodes and 5.2 ? 10^–7m s^–1 MPa^–1 for O.ficus-indica at 11 to 17 weeksof age; L_P subsequently declined with increasing root age forboth species. L_P was maximal at a temperature of about 10 ?Cfor the youngest roots (1–3 weeks), this optimum shiftingto 40 ?C for 8-week-old roots of both species. For older roots(up to 1.5-years-old), L_P increased with temperature from 0?C to 50 ?C, with a Q₁₀ of 1.3 between 20 ?C and 30 ?C. At asoil water potential (_soil) of –0.016 MPa, root L_P wasindependent of the direction of water flow for both species.Depending on root age, L_P declined 45- to 500-fold for F. acanthodesand 90- to 800-fold for O.ficus-indica as _soil was reduced from–0.016 to –1.06 MPa, consistent with a rectifier-likebehaviour with respect to water movement between soil and roots.Incorporation of such responses into water uptake models shouldlead to a better understanding of root function. Key words: Ferocactus acanthodes, Opuntia ficus-indica, water potential, tension, reverse flow     

Long-Term Nitrogen Addition Leads to Loss of Species Richness Due to Litter Accumulation and Soil Acidification in a Temperate Steppe

Background

Although community structure and species richness are known to respond to nitrogen fertilization dramatically, little is known about the mechanisms underlying specific species replacement and richness loss. In an experiment in semiarid temperate steppe of China, manipulative N addition with five treatments was conducted to evaluate the effect of N addition on the community structure and species richness.

Methodology/Principal Findings

Species richness and biomass of community in each plot were investigated in a randomly selected quadrat. Root element, available and total phosphorus (AP, TP) in rhizospheric soil, and soil moisture, pH, AP, TP and inorganic N in the soil were measured. The relationship between species richness and the measured factors was analyzed using bivariate correlations and stepwise multiple linear regressions. The two dominant species, a shrub Artemisia frigida and a grass Stipa krylovii, responded differently to N addition such that the former was gradually replaced by the latter. S. krylovii and A. frigida had highly-branched fibrous and un-branched tap root systems, respectively. S. krylovii had higher height than A. frigida in both control and N added plots. These differences may contribute to the observed species replacement. In addition, the analysis on root element and AP contents in rhizospheric soil suggests that different calcium acquisition strategies, and phosphorus and sodium responses of the two species may account for the replacement. Species richness was significantly reduced along the five N addition levels. Our results revealed a significant relationship between species richness and soil pH, litter amount, soil moisture, AP concentration and inorganic N concentration.

Conclusions/Significance

Our results indicate that litter accumulation and soil acidification accounted for 52.3% and 43.3% of the variation in species richness, respectively. These findings would advance our knowledge on the changes in species richness in semiarid temperate steppe of northern China under N deposition scenario.     

Identification of Bacteria Synthesizing Ribosomal RNA in Response to Uranium Addition During Biostimulation at the Rifle,CO Integrated Field Research Site

Understanding which organisms are capable of reducing uranium at historically contaminated sites provides crucial information needed to evaluate treatment options and outcomes. One approach is determination of the bacteria which directly respond to uranium addition. In this study, uranium amendments were made to groundwater samples from a site of ongoing biostimulation with acetate. The active microbes in the planktonic phase were deduced by monitoring ribosomes production via RT-PCR. The results indicated several microorganisms were synthesizing ribosomes in proportion with uranium amendment up to 2 μM. Concentrations of U (VI) >2 μM were generally found to inhibit ribosome synthesis. Two active bacteria responding to uranium addition in the field were close relatives of Desulfobacter postgateii and Geobacter bemidjiensis. Since RNA content often increases with growth rate, our findings suggest it is possible to rapidly elucidate active bacteria responding to the addition of uranium in field samples and provides a more targeted approach to stimulate specific populations to enhance radionuclide reduction in contaminated sites.     

Variation of Microbial Rhizosphere Communities in Response to Crop Species, Soil Origin, and Inoculation with Sinorhizobium meliloti L33

Abstract A greenhouse study with soil–plant microcosms was conducted in order to compare the effect of crop species, soil origin, and a bacterial inoculant on the establishment of microbial communities colonizing plant roots. Two crop species, alfalfa (Medicago sativa) and rye (Secale cereale), were grown separately in two soils collected from agricultural fields at different locations and with differing histories of leguminous crop rotation. A subset of microcosms was inoculated at 10⁶ cfu g^-1 soil with the luciferase marker gene-tagged Sinorhizobium meliloti strain L33, a symbiotic partner of M. sativa. Microbial consortia were collected from the rhizospheres of alfalfa after 10 weeks of incubation and from rye after 11 weeks. S. meliloti L33 populations were one to two orders of magnitude higher in the rhizospheres of alfalfa than of rye. In soil with previous alfalfa cultivation, 80% of the alfalfa nodules were colonized by indigenous bacteria, while in the other soil alfalfa was colonized almost exclusively (>90%) with S. meliloti L33. Three community-level targeting approaches were used to characterize the variation of the extracted microbial rhizosphere consortia: (1) Community level physiological profiles (CLPP), (2) fatty acid methyl ester analysis (FAME), and (3) diversity of PCR amplified 16S rRNA target sequences from directly extracted ribosomes, determined by temperature gradient gel electrophoresis (TGGE). All approaches identified the crop species as the major determinant of microbial community characteristics. Consistently, the influence of soil was of minor importance, while a modification of the alfalfa-associated microbial community structure after inoculation with S. meliloti L33 was only consistently observed by using TGGE. Received: 20 October 1999; Accepted: 15 January 2000; Online Publication: 18 July 2000