Microbial activity and diversity during extreme freeze–thaw cycles in periglacial soils, 5400 m elevation, Cordillera Vilcanota, Perú

High-elevation periglacial soils are among the most extreme soil systems on Earth and may be good analogs for the polar regions of Mars where oligotrophic mineral soils abut with polar ice caps. Here we report on preliminary studies carried out during an expedition to an area where recent glacial retreat has exposed porous mineral soils to extreme, daily freeze–thaw cycles and high UV fluxes. We used in situ methods to show that inorganic nitrogen (NO₃ ⁻ and NH₄ ⁺) was being actively cycled even during a period when diurnal soil temperatures (5 cm depth) ranged from −12 to 27°C and when sub-zero, soil cooling rates reached 1.8°C h⁻¹ (the most rapid soil cooling rates recorded to date). Furthermore, phylogenetic analyses of microbial phylotypes present at our highest sites (5410 m above sea level) showed the presence of nitrifying bacteria of the genus Nitrospira and newly discovered nitrite-oxidizing Betaproteobacteria. These soils were overwhelmingly dominated (>70% of phylotypes) by photosynthetic bacteria that were related to novel cyanobacteria previously found almost exclusively in other plant-free, high-elevation soils. We also demonstrated that soils from our highest sites had higher potential for mineralizing glutamate and higher microbial biomass than lower elevation soils that had been more recently covered by ice. Overall, our findings indicate that a diverse and robustly functioning microbial ecosystem is present in these previously unstudied high-elevation soils.     

Adaptation of plants to salt stress: the role of the ion transporters

Adaptation to high salinity is achieved by cellular ion homeostasis which involves regulation of toxic sodium ion (Na⁺) and Chloride ion (Cl⁻) uptake, preventing the transport of these ions to the aerial parts of the plants and vacuolar sequestration of these toxic ions. Ion transporters have long been known to play roles in maintaining ion homeostasis. Na⁺ enters the cell through various voltage dependent selective and non-selective ion channels. High Na⁺ concentration in the plasma membrane is balanced either by uptake of potassium ion (K⁺) by various potassium importing channels, by salt exclusion mechanism or by sequestration of Na⁺ in the vacuoles. Therefore, the role of high-affinity potassium transporter, the salt overly sensitive pathway, the most well-defined Na⁺ exclusion pathway that exports Na⁺ from cell into xylem and tonoplast localized cation transporters that compartmentalizes Na⁺ in vacuoles need to be studied in detail and applied to make the plant adaptable to saline soil. Knowledge on the regulation of expression of these transporters by the hormones, microRNAs and other non-coding RNAs can be utilized to manipulate the ion transport. Here, we reviewed paradigm of the ion transporters in salt stress signalling pathways from the recent and past studies aiding transformation of basic knowledge into biotechnological applications to generate engineered salt stress tolerant crops.

     

Osmotic adaptation in halotolerant yeast, Debaryomyces nepalensis NCYC 3413: role of osmolytes and cation transport

Debaryomyces nepalensis NCYC 3413, a food spoiling yeast isolated from rotten apple, has been previously demonstrated as halotolerant yeast. In the present study, we assessed its growth, change in cell size, and measured the intracellular polyol and cations (Na⁺ or K⁺) accumulated during growth in the absence and presence of different concentrations of salts (NaCl and KCl). Cells could tolerate 2 M NaCl and KCl in defined medium. Scanning electron microscopic results showed linear decrease in mean cell diameter with increase in medium salinity. Cells accumulated high amounts of K⁺ during growth at high concentrations of KCl. However, it accumulated low amounts of Na⁺ and high amounts of K⁺ when grown in the presence of NaCl. Cells grown in the absence of salt showed rapid influx of Na⁺/K⁺ on incubation with high salt. On incubation with 2 M KCl, cells grown at 2 M NaCl showed an immediate efflux of Na⁺ and rapid uptake of K⁺ and vice versa. To withstand the salt stress, osmotic adjustment of intracellular cation was accompanied by intracellular accumulation of polyol (glycerol, arabitol, and sorbitol). Based on our result, we hypothesize that there exists a balanced efflux and synthesis of osmolytes when D. nepalensis was exposed to hypoosmotic and hyperosmotic stress conditions, respectively. Our findings suggest that D. nepalensis is an Na⁺ excluder yeast and it has an efficient transport system for sodium extrusion.     

盐渍化土壤根际微生物群落及土壤因子对AM真菌的影响

为探明盐渍化土壤影响下AM真菌与根际土壤间的关系,试验选取宁夏碱化龟裂土、草甸盐土、盐化灌淤土3种类型4个样地上典型植被群落,测定了植物根际土壤养分含量、微生物群落结构、AM真菌侵染率以及孢子密度。结果显示:盐渍化土壤根际微生物碳源利用类型显著不同,对芳香类化合物的代谢能力整体较弱;红寺堡草甸盐土上微生物优势群落为氨基酸代谢类群,惠农盐化灌淤土为多聚化合物代谢群,西大滩碱化龟裂土为碳水化合物代谢群。AM真菌孢子密度与微生物碳源代谢群间的关系比较复杂。其中,惠农样点根际土壤孢子密度与多聚化合物微生物代谢群呈显著正相关,西大滩地区孢子密度与碳水化合物微生物代谢群呈显著正相关。土壤有机质、全盐、全氮、碱解氮等土壤肥力因子及土壤中的HCO-3、Na+、Cl-等盐基离子含量能解释AM真菌孢子密度与土壤环境因子之间相互关系的大部分信息。较高的HCO-3浓度促进了AM真菌侵染率的提高,但高盐浓度下Na+和Cl-降低了菌根侵染率。土壤对AM真菌孢子密度、侵染率的影响因土壤盐分组成类型的不同而异。研究结果为深入了解AM真菌多样性,促进宁夏盐碱地的合理开发与利用提供了理论依据。     

Nitrate‐dependent shoot sodium accumulation and osmotic functions of sodium in Arabidopsis under saline conditions

Improving crop plants to be productive in saline soils or under irrigation with saline water would be an important technological advance in overcoming the food and freshwater crises that threaten the world population. However, even if the transformation of a glycophyte into a plant that thrives under seawater irrigation was biologically feasible, current knowledge about Na⁺ effects would be insufficient to support this technical advance. Intriguingly, crucial details about Na⁺ uptake and its function in the plant have not yet been well established. We here propose that under saline conditions two nitrate‐dependent transport systems in series that take up and load Na⁺ into the xylem constitute the major pathway for the accumulation of Na⁺ in Arabidopsis shoots; this pathway can also function with chloride at high concentrations. In nrt1.1 nitrate transport mutants, plant Na⁺ accumulation was partially defective, which suggests that NRT1.1 either partially mediates or modulates the nitrate‐dependent Na⁺ transport. Arabidopsis plants exposed to an osmotic potential of ?1.0 MPa (400 mOsm) for 24 h showed high water loss and wilting in sorbitol or Na/MES, where Na⁺ could not be accumulated. In contrast, in NaCl the plants that accumulated Na⁺ lost a low amount of water, and only suffered transitory wilting. We discuss that in Arabidopsis plants exposed to high NaCl concentrations, root Na⁺ uptake and tissue accumulation fulfil the primary function of osmotic adjustment, even if these processes lead to long‐term toxicity.     

Li/Na exchange and Li active transport in human lymphoid cells U937 cultured in lithium media

Lithium transport across the cell membrane is interesting in the light of general cell physiology and because of its alteration during numerous human diseases. The mechanism of Li⁺ transfer has been studied mainly in erythrocytes with a slow kinetics of ion exchange and therefore under the unbalanced ion distribution. Proliferating cultured cells with a rapid ion exchange have not been used practically in study of Li⁺ transport. In the present paper, the kinetics of Li⁺ uptake and exit, as well as its balanced distribution across the plasma membrane of U937 cells, were studied at minimal external Li⁺ concentrations and after the whole replacement of external Na⁺ for Li⁺. It is found that a balanced Li⁺ distribution attained at a high rate similar to that for Na⁺ and Cl^? and that Li⁺/Na⁺ discrimination under balanced ion distribution at 1–10 mM external Li⁺ stays on 3 and drops to 1 following Na, K-ATPase pump blocking by ouabain. About 80% of the total Li⁺ flux across the plasma membrane under the balanced Li⁺ distribution at 5 mM external Li⁺ accounts for the equivalent Li⁺/Li⁺ exchange. The majority of the Li⁺ flux into the cell down the electrochemical gradient is a flux through channels and its small part may account for the NC and NKCC cotransport influxes. The downhill Li⁺ influxes are balanced by the uphill Li⁺ efflux involved in Li⁺/Na⁺ exchange. The Na⁺ flux involved in the countertransport with the Li⁺ accounts for about 0.5% of the total Na⁺ flux across the plasma membrane. The study of Li⁺ transport is an important approach to understanding the mechanism of the equivalent Li⁺/Li⁺/Na⁺/Na⁺ exchange, because no blockers of this mode of ion transfer are known and it cannot be revealed by electrophysiological methods. Cells cultured in the medium where Na⁺ is replaced for Li⁺ are recommended as an object for studying cells without the Na,K-ATPase pump and with very low intracellular Na⁺ and K⁺ concentration.     

A laser ablation technique maps differences in elemental composition in roots of two barley cultivars subjected to salinity stress

In saline soils, high levels of sodium (Na⁺) and chloride (Cl^?) ions reduce root growth by inhibiting cell division and elongation, thereby impacting on crop yield. Soil salinity can lead to Na⁺ toxicity of plant cells, influencing the uptake and retention of other important ions [i.e. potassium (K⁺)] required for growth. However, measuring and quantifying soluble ions in their native, cellular environment is inherently difficult. Technologies that allow in situ profiling of plant tissues are fundamental for our understanding of abiotic stress responses and the development of tolerant crops. Here, we employ laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) to quantify Na, K and other elements [calcium (Ca), magnesium (Mg), sulphur (S), phosphorus (P), iron (Fe)] at high spatial resolution in the root growth zone of two genotypes of barley (Hordeum vulgare) that differ in salt‐tolerance, cv. Clipper (tolerant) and Sahara (sensitive). The data show that Na⁺ was excluded from the meristem and cell division zone, indicating that Na⁺ toxicity is not directly reducing cell division in the salt‐sensitive genotype, Sahara. Interestingly, in both genotypes, K⁺ was strongly correlated with Na⁺ concentration, in response to salt stress. In addition, we also show important genetic differences and salt‐specific changes in elemental composition in the root growth zone. These results show that LA‐ICP‐MS can be used for fine mapping of soluble ions (i.e. Na⁺ and K⁺) in plant tissues, providing insight into the link between Na⁺ toxicity and root growth responses to salt stress.     

Bacterial chitin utilisation at extremely haloalkaline conditions

Chitin is produced in large amounts in hypersaline habitats with neutral pH due to the high biomass production of brine shrimp Artemia. Recently, a high abundance of Artemia was also noticed in hypersaline soda lakes in the Kulunda Steppe (Altai, Russia), which prompted us to survey the possibility of microbial chitin utilization at extremely haloalkaline conditions in soda brines. Most active chitin utilisation-supporting microbial growth was found at anaerobic conditions at pH 10 and up to 3.5?M total Na⁺. At aerobic conditions, the degradation of chitin was slower, mostly incomplete and active at <2?M total Na⁺, although very slow partial degradation was possible up to 4?M Na⁺. Anaerobic enrichments at pH 10 yielded two different groups of obligately haloalkaliphilic fermentative anaerobes, exclusively specialized to utilise insoluble chitin as the only growth substrate. One group was represented by a single strain growing at moderate salinity, and another comprised multiple isolates growing up to 3.5?M Na⁺. These groups represent two novel bacterial phyla not closely related to any other cultured bacteria. Aerobic enrichments from the lake sediments were dominated by several obligately haloalkaliphilic members of the genus Marinimicrobium in the Gammaproteobacteria. They were less specialised than the anaerobes and grew with chitin and its monomer and oligomers at a pH of 10 up to 2.5?M Na⁺. Furthermore, several strains of haloalkaliphilic Gram-positive chitinolytics belonging to bacilli and actinobacteria were isolated from soda lake sediments and surrounding soda soils. In general, the results indicate the presence of an active and diverse haloalkaliphilic chitinolytic microbial community in hypersaline soda habitats.     

Physiology and microbial community structure in soil at extreme water content

A sandy loam soil was brought to 6 water contents (13-100% WHC) to study the effects of extreme soil moistures on the physiological status of microbiota (represented by biomass characteristics, specific respiration, bacterial growth, and phospholipid fatty acid, PLFA, stress indicators) and microbial community structure (assessed using PLFA fingerprints). In dry soils, microbial biomass and activity declined as a consequence of water and/or nutrient deficiency (indicated by PLFA stress indicators). These microbial communities were dominated by G+ bacteria and actinomycetes. Oxygen deficits in water-saturated soils did not eliminate microbial activity but the enormous accumulation of poly-3-hydroxybutyrate by bacteria showed the unbalanced growth in excess carbon conditions. High soil water content favored G bacteria.     

Microbial diversity of topographical gradient profiles in Fushan forest soils of Taiwan

To evaluate the microbial diversity of Fushan forest soils, the variation of soil properties, microbial populations, and soil DNA with soil depth in three sites of different altitude were analyzed. Microbial population, moisture content, total organic carbon (C_org), and total nitrogen (N_tot) decreased with increasing soil depth. The valley site had the lowest microbial populations among the three tested sites due to the low organic matter content. Bacterial population was the highest among the microbial populations. The ratios of cellulolytic microbes to the total bacteria in organic layers were high, implying their roles in the carbon cycle. The microbial biomass carbon (C_mic) and nitrogen (N_mic) contents ranged from 130.5 to 564.1 μg g⁻¹ and from 16.7 to 95.4 μg g⁻¹, respectively. The valley had the lowest C_mic and N_mic. The organic layer had the highest C_mic and N_mic and decreased with soil depth. Analysis using denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplicons of 16S rDNA showed that the bacterial diversity of the three sites were very similar to each other in the major bands, and the variation was in the minor bands. However, the patterns in PCR-DGGE profile through gradient horizons were different, indicating the prevalence of specific microbes at different horizons. These results suggest that the microbial diversity in the deeper horizons is not simply the diluted analogs of the surface soils and that some microbes dominate only in the deeper horizons. Topography influenced the quantity and diversity of microbial populations.     

Desodification from calcareous saline sodic soil through phytoremediation with Phragmites australis (Cav.) Trin. ex Steud. and gypsum

The reclamation of saline sodic soils requires sodium removal and the phytoremediation is one of the proven low-cost, low-risk technologies for reclaiming such soils. However, the role of Phragmites australis in reclaiming saline sodic soils has not been evaluated extensively. The comparative reclaiming role of P. australis and gypsum was evaluated in a column experiment on a sandy clay saline sodic soil with EC_e 74.7 dS m^?1, sodium adsorption ratio (SAR) 63.2, Na⁺ 361 g kg^?1, and pH 8.46. The gypsum at 100% soil requirement, planting common reed (P. australis) alone, P. australis + gypsum at 50% soil gypsum requirements, and leaching (control without plant and gypsum) were four treatments applied. After 11 weeks of incubation, the results showed that all treatments including the control significantly reduced pH, EC, exchangeable Na⁺, and SAR from the initial values, the control being with least results. The gypsum and P. australis + gypsum were highly effective in salinity (EC_e) reduction, while sodicity (SAR) and Na⁺ reductions were significantly higher in P. australis + gypsum treatment. The reclamation efficiency in terms of Na⁺ (83.4%) and SAR (86.8%) reduction was the highest in P. australis + gypsum. It is concluded that phytoremediation is an effective tool to reclaim saline sodic soil.     

历史因素对土壤微生物群落与外来细菌入侵间关系的影响

群落的组成和结构如何影响其可入侵性一直是入侵生态学的研究热点。然而关于群落可入侵性和群落特征间关系的认知却很不统一。采用交叉互换的试验方法,首先将野外采集的两种长期不同施肥土壤(有机肥和化肥)进行灭菌并回接已方和对方的土壤悬液,研究土壤环境(历史非生物因素)和土壤微生物群落(历史生物因素)对重建土壤微生物群落特征的相对贡献。随后将用红色荧光蛋白标记的青枯菌作为外来种接入重建的土壤中,探究不同土壤微生物群落特征对外来细菌存活数量(前期入侵潜力)和存活时间(后期入侵潜力)的影响。结果表明,历史生物因素对重建土壤的原生动物数量、革兰氏阴性与阳性细菌比等群落特征和外来细菌的存活数量有影响;历史非生物因素对土壤微生物活性、细菌物种多样性和功能多样性等群落特征以及外来细菌入侵土壤后总的存活时间有影响;外来细菌入侵前期状况仅与原生动物数量、革兰氏阴性与阳性细菌比相关,而入侵后期的状况则仅与微生物活性、细菌物种多样性和功能多样性相关。总之,外来细菌在土壤中各时期的入侵潜力和土著微生物群落特征的相关性主要取决于二者是否由同种历史影响因素控制。本研究对于阐明生物群落结构与群落可入侵性之间关系,及指导土壤外来病原菌的防控均具有重要意义。     

滩涂围垦和土地利用对土壤微生物群落的影响

土壤微生物在生态系统营养物质循环过程,特别是碳、氮循环过程中扮演着重要的角色。上海市崇明岛位于长江入海口,因其土壤发育时间较短、土地利用历史背景清晰、土壤本底均一,不同土壤围垦年代的土壤,代表了土壤发育年代的不同时期。以空间变化代替时间变化,对崇明岛稻田和旱地6个不同围垦年代土壤的磷酸脂肪酸(PLFA)指纹图谱研究表明,湿地滩涂围垦16a后土壤微生物总PLFA、细菌PLFA、革兰氏阳性菌(G+)PLFA和革兰氏阴性菌(G-)PLFA含量显著降低。随着围垦时间的逐步增加,PLFA含量逐步上升。经过长时间的农业种植,G+PLFA在围垦120a和300a稻田和旱地土壤中没有显著性差异;而总PLFA、细菌和G-PLFA在围垦75、120a和300a的土壤中含量趋于稳定且没有显著性差异。围垦16a和40a稻田土壤中总PLFA和G+PLFA显著高于旱地土壤;围垦40a稻田土壤中细菌和G-PLFA显著高于旱地土壤。不同围垦年代土壤总PLFA、细菌PLFA与土壤总氮、粘土含量成显著的正相关关系。河口湿地围垦后微生物数量的变化与土壤营养含量存在强烈相关关系,提示土壤围垦及演替过程中微生物与土壤肥力之间的紧密关系,对探讨土壤演替过程中微生物群落的变化具有重要意义。     

Microbial dynamics and carbon and nitrogen cycling following re-wetting of soils beneath two semi-arid plant species

Sporadic summer rainfall in semi-arid ecosystems can provide enough soil moisture to drastically increase CO₂ efflux and rates of soil N cycling. The magnitudes of C and N pulses are highly variable, however, and the factors regulating these pulses are poorly understood. We examined changes in soil respiration, bacterial, fungal and microfaunal populations, and gross rates of N mineralization, nitrification, and NH₄⁺ and NO₃^– immobilization during the 10 days following wetting of dry soils collected from stands of big sagebrush (Artemisia tridentata) and cheatgrass (Bromus tectorum) in central Utah. Soil CO₂ production increased more than tenfold during the 17 h immediately following wetting. The labile organic C pool released by wetting was almost completely respired within 2–3 days, and was nearly three times as large in sagebrush soil as in cheatgrass. In spite of larger labile C pools beneath sagebrush, microbial and microfaunal populations were nearly equal in the two soils. Bacterial and fungal growth coincided with depletion of labile C, and populations peaked in both soils 2 days after wetting. Protozoan populations, whose biomass was nearly 3,000-fold lower than bacteria and fungi, peaked after 2–4 days. Gross N mineralization and nitrification rates were both faster in cheatgrass soil than in sagebrush, and caused greater nitrate accumulation in cheatgrass soil. Grazing of bacteria and fungi by protozoans and nematodes could explain neither temporal trends in N mineralization rates nor differences between soil types. However, a mass balance model indicated that the initial N pulse was associated with degradation of microbial substrates that were rich in N (C:N <8.3), and that microbes had shifted to substrates with lower N contents (C:N =15–25) by day 7 of the incubation. The model also suggested that the labile organic matter in cheatgrass soil had a lower C:N ratio than in sagebrush, and this promoted faster N cycling rates and greater N availability. This study provides evidence that the high N availability often associated with wetting of cheatgrass soils is a result of cheatgrass supplying substrates to microbes that are of high decomposability and N content.     

Severe salt stress in Vaccinium myrtillus (L.) in response to Na+ ion toxicity

NaCl is spread on the northern roads as a winter de-icing agent. Na⁺ stress may occur in roadside forests especially during growing season, because snowmelt increases Na⁺ concentrations in the roadside forest soil. To simulate summertime Na⁺ stress and its effect on viability and anthocyanin concentrations, bilberry (Vaccinium myrtillus L.) plants were subjected to NaCl (0, 6, 30 and 60 g m⁻²) in a boreal mesic heath forest in northern Finland (65°N). It was hypothesised that Na⁺ stress decreases tissue water content (TWC) in below- and aboveground stems, where resulting water stress increases anthocyanin level for osmotic regulation. Uptake of Na⁺ from the soil to the below- and aboveground stems was detected by a fast sequential atomic absorption spectrometry. Na⁺ accumulated in belowground stems, but it was translocated into the aboveground stems to a lesser extent. At the end of the growing season, TWC increased in belowground stems and decreased in aboveground stems. Also anthocyanin concentrations decreased in aboveground stems. The viability of the aboveground stems decreased to 30% along with the Na⁺ accumulation. Despite the strong viability response in aboveground stems, TWC was constant in the middle of the growing season and decreased relatively little at the end of the growing season. It is thus proposed that direct effect of Na⁺ ions, rather than Na⁺-induced water stress, is the primary reason behind the strong viability response and a severe salt stress in bilberry.     

Antibodies to pig kidney (Na++K+)-ATPase inhibit the Na+ pump in human red cells provided they have access to the inner surface of the cell membrane

Antisera from rabbits that had been immunized with a highly active membrane preparation of (Na⁺ + K⁺)-ATPase from the outer medulla of pig kidney strongly inhibited (Na⁺ + K⁺)-ATPase activity in various tissues. When the antiserum was incorporated into released human red cell ghosts, the ouabain-sensitive efflux of Na⁺ into both 15 mM K⁺ and K⁺-free high Na⁺ media was completely abolished. This effect was not observed when non-immune serum was used, or when the immune serum was allowed access only to the outer surface of the red cell membranes.     

An assessment of the relative effects of adverse physical and chemical properties of sodic soil on the growth and yield of wheat (Triticum aestivum L.)

Two wheat varieties were grown in artificially created sodic soils in pots at a range of sodicity levels (exchangeable sodium percentage (ESP) 15–52), with and without an anionic polyacrylamide soil conditioner (PAM) to stabilise soil aggregates. Increasing sodicity decreased the % water stable aggregates (% WSA) in soil and survival, grain and straw yield of wheat. Plants grown at high sodicity also had higher Na⁺, lower K⁺ and Ca²⁺ concentrations and lower K⁺/Na⁺ ratio in flag leaf sap than plants grown in control (non-sodic) soil. Sodicity had no effect on the concentrations of Cu²⁺, Fe²⁺, Mn²⁺ and Zn²⁺ in grains and straw, but total uptake of these micronutrients was deceased due to lower dry weight of these tissues per plant. At all sodicity levels treatment of sodic soil with PAM increased the % WSA to values greater than in the non-sodic control soil, and slightly lowered ESP. Over the range ESP 15–44 the effects of PAM on wheat grain yield increased as sodicity increased, so that at ESP 44 grain yield in the treatment with PAM was only 25% lower than in the non-sodic control. However at ESP 52 the effects of PAM were smaller, and grain yield was 86% lower than in the control. At this sodicity level the decreases in grain yield due to sodicity and the increases in reponse to treatment of sodic soil with PAM were similar in the two varieties tested. At high sodicity levels (ESP 44 and 52) treatment of sodic soil with PAM decreased the concentration of Na⁺ and increased K⁺ and K⁺/Na⁺ ratio in flag leaf sap. However, at the highest sodicity level (ESP 52), flag leaf Na⁺ concentration remained above the level (100 mol m^-3) at which it has been found to be toxic. Concentrations of Cu²⁺, Fe²⁺, Mn²⁺ and Zn²⁺ in grain and straw were unaffected by PAM. These results suggest that at ESP up to 40–50 adverse physical characteristics are the major cause of low wheat yield in sodic soils, either due to their direct effects in decreasing growth, or their indirect effects in increasing uptake of Na⁺ and decreasing uptake of K⁺. Above ESP 50, roots are less able to exclude Na⁺, even in the presence of improved soil physical conditions, so that at these sodicity levels, both adverse physical and adverse chemical properties contribute to the decreased yield. This revised version was published online in June 2006 with corrections to the Cover Date.     

蚯蚓对废纸屑再利用及养分贫瘠土壤综合质量的影响

办公废纸屑作为常见有机废弃物,由于体积小且转化为再生纸成本高,因而再利用很难。但其含有大量有机碳(特别是纤维素)可能有助于退化土壤修复。蚯蚓对土壤有机质分解和其他土壤功能有重要影响,办公废纸屑和蚯蚓共同作用如何影响养分贫瘠土壤质量至今未知。研究以赤子爱胜蚓为接种蚯蚓,将办公废纸屑添加到养分贫瘠土壤中,分别设置纯土壤培养为对照组(S)、单独添加废纸屑(SP)、单独接种赤子爱胜蚓(SE)和添加废纸屑并接种赤子爱胜蚓的处理(SPE),比较培养90 d后各处理理化指标(pH、有机碳、全氮、全磷、全钾、碱解氮、速效磷、速效钾、交换性阳离子钾、钠、钙、镁等)、微生物磷脂脂肪酸(PLFAs)总含量和微生物结构的差异,在此基础上综合评价土壤质量,阐明废纸屑和赤子爱胜蚓在养分贫瘠土壤改良修复中的作用。结果显示：SPE处理较SP处理显著提高废纸屑的分解率89.48%。与对照相比,SP处理能够显著提高土壤pH值2.94个单位,SPE处理能够使其维持在中性水平;前者显著提高土壤有机碳(SOC)125.76%,交换性钠钙镁(Na_Ex、Ca_Ex、Mg_Ex     

Ecophysiological responses of the euhalophyte Suaeda salsa to the interactive effects of salinity and nitrate availability

Effects of salinity and nitrate nitrogen (NO₃⁻-N) on ion accumulation and chlorophyll fluorescence were monitored for two populations of Suaeda salsa grown from seeds in a greenhouse experiment. One population inhabits the intertidal zone and the other occurs on inland saline soils. Ion contents in soils and in leaves of the two populations were also investigated in field. In the greenhouse, seedlings were exposed to a NaCl concentration of 0.6 and 35.1 ppt, with 0.1 or 5 mM NO₃⁻-N treatments for 20 days. The contents of Na⁺ and Cl⁻ were higher, but NO₃⁻ was lower in soils of the intertidal zone than at the inland site. In the field, ion concentrations and the estimated contribution of these ions to osmotic potential in leaves showed no difference between the two populations, except that the estimated contribution of Na⁺ to osmotic potential in leaves of the intertidal population was lower than that in the inland population. In the greenhouse, in contrast, the concentration of Cl⁻ was lower, but NO₃⁻ concentration and the estimated contribution of NO₃⁻ to osmotic potential were higher, in the leaves of plants from the intertidal zone. Salinity had no effect on the maximal efficiency of PSII photochemistry (Fv/Fm) and the actual PSII efficiency (ΦPSII). The results indicated that S. salsa from the intertidal zone was better able to regulate Cl⁻ to a lower level, and accumulate NO₃⁻ even with low soil NO₃⁻ concentrations. Tolerance of the PSII machinery to high salinity stress may be an important characteristic for the studied species supporting growth in highly saline environments.