Top-down control on major groups of global marine diazotrophs

Dinitrogen(N_(2))fixed by a group of prokaryotes(diazotrophs)is the dominant process adding bioavailable nitrogen into the ocean.Although it has been intensively studied how N_(2) fixation is controlled by resources(bottom-up factors),it is unclear whether the grazing(top-down control)effectively impacts growth and distribution of different diazotroph groups.In this study,we evaluate this question by conducting log-log regression of diazotroph biomass onto corresponding N_(2) fixation rates in the global ocean.The slope of the regression for Trichodesmium is~0.8,indicating that a small portion of the increase in N_(2) fixation does not accumulate as its biomass.This leads to a conclusion that Trichodesmium is under a substantial top-down control,although bottom-up control still dominates.We also analyze the residuals of the regression in the North Atlantic,concluding that free trichomes of Trichodesmium are subject to stronger top-down control than its colonies.The weak correlation between the biomass and N_(2) fixation of unicellular cyanobacterial diazotrophs indicates that the degree of top-down control on this type of diazotrophs varies greatly.The analyses obtain unrealistic results for diatom-diazotroph assemblages due to complicated nitrogen sources of these symbioses.Our study reveals the variability of top-down control among different diazotroph groups across time and space,suggesting its importance in improving our understandings of ecology of diazotrophs and predictions of N_(2) fixation in biogeochemical models.Measurements of size-specific N_(2) fixation rates and growth rates of different diazotroph groups can be useful to more reliably analyze the top-down control on these key organisms in the global ocean.

Effect of Diazotrophs and Chemical Fertilizers on Production and Economics of Okra (<i>Abelmoschus esculentus</i>, L.) Cultivars

The effect of diazotrophs and chemical fertilizers on yield attributing characters and economics of okra cultivation was evaluated. Application of highest dose of NPK @100% in combination with vermicompost (5 t ha-1) and biofertilizers with FYM increased the fruit yield of okra (cultivar Mahyco-10) considerably with yield varying between 80.00 q ha-1 to 227.13 q ha-1 and 80.49 q ha-1 to 229.62 q ha-1 during 2010 and 2011 respectively. In okra cv. Utkal Gaurav the fruit yield varied from 47.68 q ha-1 to 129.84 q ha-1 in 2010 and 47.27 q ha-1 to 131.35 q ha-1 in 2011. As regards the net return, highest net profit of Rs 87,630 and Rs. 89,370 ha-1 from the okra was realized over an investment of Rs. 71,360 ha-1 during both the years with a benefit cost ratio of 2.23 and 2.25 when 100% NPK integrated with vermicompost @5t ha-1 and biofertilizers with FYM applied in okra cv.Mahyco-10. The highest net return of Rs. 14,350 in 2010 and Rs. 15,260 in 2011 with an investment of Rs. 63,550 each year was found in cultivar Utkal Gaurav under highest level of nutrient application.

N2 fixation rate and diazotroph community structure in the western tropical North Pacific Ocean

In the present study, we report N2 fixation rate(15N isotope tracer assay) and the diazotroph community structure(using the molecular method) in the western tropical North Pacific Ocean(WTNP)(13°–20°N, 120°–160°E). Our independent evidence on the basis of both in situ N2 fixation activity and diazotroph community structure showed the dominance of unicellular N2 fixation over majority of the WTNP surface waters during the sampling periods.Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria group B-dominated to Trichodesmium spp.-dominated toward the western boundary current(Kuroshio) was also observed in 2013. We hypothesize that nutrient availability may have played a major role in regulating the biogeography of N2 fixation.In surface waters, volumetric N2 fixation rate(calculated by nitrogen) ranged between 0.6 and 2.6 nmol/(L·d) and averaged(1.2±0.5) nmol/(L·d), with <10 μm size fraction contributed predominantly(88%±6%) to the total rate between 135°E and 160°E. Depth-integrated N2 fixation rate over the upper 200 m ranged between 150 μmol/(m^2·d)and 480 μmol/(m^2·d)average(225±105) μmol/(m^2·d). N2 fixation can account for 6.2%±3.7% of the depthintegrated primary production, suggesting that N2 fixation is a significant N source sustaining new and export production in the WTNP. The role of N2 fixation in biogeochemical cycling in this climate change-vulnerable region calls for further investigations.

Response of rice to inoculation with genetic engineered strains of associative diazotrophs

Enhancement of nitrogen fixation in the rhizo-sphere of cereals has attracted a wide interestin biological and agricultural research,insteadof chemicals,for supplying higher plants withcombined nitrogen.Bacteria in association withrice plant.s were sensitive to the surroundingfactors in the soil,such as NH~+ or O,whichrepressed associative nitrogen fixation between

硝化抑制剂双氰胺对类珠藻(固氮蓝藻)生长和生理生化特征的影响

为了阐明硝化抑制剂双氰胺(DCD)对稻田固氮蓝藻生长和生理生化特征的影响,为二者在稻田的合理使用提供理论依据,本文研究了DCD胁迫下稻田固氮蓝藻——类珠藻(Aliinostoc sp.YYLX235)的生长、光合作用、固氮酶活性、抗氧化酶活性和氧化损伤特征。结果表明,100~300 mg·L^(-1)DCD对类珠藻的生长具有明显的抑制作用。当DCD浓度为50 mg·L^(-1)时,类珠藻的细胞活力和固氮活性均显著降低。类珠藻光合系统对DCD胁迫表现出应激效应,具体表现为光合色素含量增加、光合系统Ⅱ电子传递效率提高。类珠藻细胞内超氧化物歧化酶(SOD)和过氧化氢酶(CAT)活性与培养体系DCD浓度呈正相关;当DCD浓度为200~300 mg·L^(-1)时,类珠藻细胞内氧化-抗氧化系统失去平衡,藻细胞受到氧化性损伤。DCD对类珠藻的固氮活性具有极强的抑制效应。本研究结果显示,硝化抑制剂DCD对农田固氮蓝藻的生长、细胞活力和固氮活力均具有较强的负面影响,在生产过程中固氮蓝藻菌肥需要与DCD错峰使用。

Community assemblage of free-living diazotrophs along the elevational gradient of Mount Gongga

Mountain systems are unique for studying the responses of species distribution and diversity to environmental changes along elevational gradients.It is well known that free-living diazotrophic microorganisms are important to nitrogen cycling in mountain systems.However,the elevational patterns of free-living diazotrophs and the underlying ecological processes in controlling their turnover along broader gradients are less well documented.Here,we investigated the pattern of diazotrophic diversity along the elevational gradient(1800 m-4100 m)in Mount Gongga of China.The results showed that the α-diversity of diazotrophs did not change with the elevation from 1800 m to 2800 m,but decreased at elevations above 3000 m.Such diversity pattern was driven mainly by soil total carbon,nitrogen,and plant richness.Various diazotrophic taxa showed differential abundance-elevation relationships.Ecological processes determining diazotrophic community assemblage shift along the elevations.Deterministic processes were relatively stronger at both low and high elevations,whereas stochastic processes were stronger at the middle elevation.This study also suggested a strong relationship among aboveground plants and diazotrophs,highlighting their potential interactions,even for free-living diazotrophs.

Soil total organic carbon/total nitrogen ratio as a key driver deterministically shapes diazotrophic community assemblages during the succession of biological soil crusts

The diazotrophic community in biological soil crusts(biocrusts)is the key supplier of nitrogen in dryland.To date,there is still limited information on how biocrust development influences the succession of diazotrophic community,and what are the most important factors mediating diazotrophic communities during biocrust succession.Using the high throughput nifH amplicon sequencing,the diazotrophs in soils at different developmental stages of biocrust were comparatively studied.The results evidenced the decreases of TOC/TN ratio and pH value with biocrust development.Nostoc and Scytonema were the most dominant diazotrophic genera at all biocrust stages,while Azospirillum and Bradyrhizobium were abundant only in bare soil.Diazotrophic co-occurrence networks tended to be less complex and less connected with biocrust succession.The soil TOC/TN ratio was the most dominant factor mediating diazotrophic diversity,community composition and assembly processes,while diazotrophic-diversity and NO3–-N/NH4+-N ratio were positively correlated with the nitrogenase activity during biocrust succession.This study provided novel understandings of nitrogen fixation and succession patterns of diazotrophic community,by showing the effects of biocrust succession on diazotrophic diversity,community composition,community assembly and co-occurrence networks,and recognizing TOC/TN ratio as the most dominant factor mediating diazotrophs during biocrust succession.

Diversity of Nitrogenase Systems in Diazotrophs

Nitrogenase is a metalloprotein complex that catalyses the reaction of biological nitrogen fixation. At least three genetically distinct nitrogenase systems have been confirmed in diazotrophs, namely Nil, Vnf, and Anf, in which the active-site central metals are Mo, V, and Fe, respectively. The present review summarizes progress on the genetic, structural, and functional investigations into the three nitrogenases and discusses the possibility of the existence of other novel nitrogenases.

Biochar-induced changes in the soil diazotroph community abundance and structure in a peanut field trial

Biological nitrogen fixation(BNF)can help replenish available nitrogen(N)in cropland and reduce the use of chemical N fertilizers,with diazotrophs playing an important role.However,the response of diazotroph community and BNF activity in biochar amendment soil,especially in the deep soil horizon,are poorly understood.In this study,soil samples were collected from topsoil(0-20 cm)and subsoil(20-40 cm)in the field experiment(established in 2013)comprising treatments with no chemical fertilizer(CK),chemical fertilizer(NPK),biochar(BC),and biochar plus chemical fertilizers(BNPK).Here,we investigated the diazotroph community using real-time PCR and high-throughput sequencing of the nifH gene,and assessed the soil N_(2)fixation rate(R_(Nfix))using acetylene reduction assay(ARA).Results showed that in the topsoil,the treatments with biochar significantly increased nifH gene copies and R_(Nfix),which was consistent with the increased soil organic matter(SOM),total carbon-to-nitrogen ratio(C/N),dissolved organic carbon(DOC)and pH.In the subsoil,applying chemical fertilizers(NPK)strongly decreased R_(Nfix),but had no effect on diazotroph abundance;in contrast,biochar application(BC)had no effect on R_(Nfix),but suppressed the growth of bacteria and diazotrophs while increasing the abundance of Rhizobiales order.Diazotroph and bacterial gene copies were significantly and positively correlated in both top-and sub-soil,and they were mainly influenced by SOM and total nitrogen(TN).In addition,soil nitrate nitrogen(NO_(3)^(−)-N)was the major factor in shaping the vertical stratification of diazotroph community structure.Although nifH gene abundance was significantly cor-related with R_(Nfix)in the topsoil,the structure equation modeling(SEM)showed the highest correlation between diazotroph community structure and R_(Nfix).Hence,we suggested that soil carbon and nitrogen sources were the key factors correlated with changes in the vertical pattern of diazotroph abundance.Biochar induced the dominant diazotroph community succes-sion and increased soil carbon content and pH,which contributed to the BNF activity.Changes in the BNF activity were driven by the variation in diazotroph community structure.

Heterotrophy-coordinated diazotrophy is associated with significant changes of rare taxa in soil microbiome

Soil heterotrophic respiration during decomposition of carbon(C)-rich organic matter plays a vital role in sustaining soil fertility.However,it remains poorly understood whether dinitrogen(N_(2))fixation occurs in support of soil heterotrophic respiration.In this study,^(15)N_(2)-tracing indicated that strong N_(2)fixation occurred during heterotrophic respiration of carbon-rich glucose.Soil organic ^(15)N increased from 0.37 atom%to 2.50 atom%under aerobic conditions and to 4.23 atom%under anaerobic conditions,while the concomitant CO_(2)flux increased by 12.0-fold under aerobic conditions and 5.18-fold under anaerobic conditions.Soil N_(2)fixation was completely absent in soils replete with inorganic N,although soil N bioavailability did not alter soil respiration.High-throughput sequencing of the 16S rRNA gene further indicated that:i)under aerobic conditions,only 15.2%of soil microbiome responded positively to glucose addition,and these responses were significantly associated with soil respiration and N_(2)fixation and ii)under anaerobic conditions,the percentage of responses was even lower at 5.70%.Intriguingly,more than 95%of these responses were originally rare with<0.5%relative abundance in background soils,including typical N_(2)-fixing heterotrophs such as Azotobacter and Clostridium and well-recognized non-N_(2)-fixing heterotrophs such as Sporosarcina,Agromyces,and Sedimentibacter.These results suggest that only a small portion of the soil microbiome could respond quickly to the amendment of readily accessible organic C in a fluvo-aquic soil and highlighted that rare phylotypes might have played more important roles than previously appreciated in catalyzing soil C and nitrogen turnovers.Our study indicates that N_(2)fixation could be closely associated with microbial turnover of soil organic C when available in excess.

Diversity and high nitrogenase activity of endophytic diazotrophs isolated from Oryza rufipogon Griff.

Diversity and nitrogenase activity of endophytic diazotrophs colonized in the wild rice Oryza rufipogon Griff grown in Boluo, Huilai County in Guangdong Province and Lingshui County in Hainan Province were studied. Thirty-seven isolates obtained from Oryza rufipogon were identified as putative endophytic nitrogen-fixing bacteria by ARA (acetylene reduction assay) test and further confirmed by PCR amplification of nifH gene fragments. All obtained strains have ARA activity and the same sized nifH gene fragments. Above the similarity level of 80%, the obtained isolates were assigned as Group Ⅰ to Ⅷ by the clustering of IS-PCR fingerprints. The SDS-PAGE whole-cell protein patterns were similar to those of IS-PCR fingerprints. Components and contents of fatty acid methyl esters (FAMEs) were used to differentiate the representative strains (Ls13, Ls8, BL1, BL12, HL6, Ls4) from Group Ⅰ to Group Ⅵ. The six representative strains showed significant difference in contents and components of cellular fatty acid methyl ester. 16S rDNA sequencing analysis showed that strains of Group Ⅰ to Ⅶ were located in Enterobacteraceae (γ-proteobacteria). Strains of Group Ⅰ and Group II were closely related to Klebsiella sp.; Strain Ls8 of Group Ⅱ was a little far away from the genus of Pantoea (homology level 96% with Pantoea agglomerans), which may represent a new species or genus in Enterobacteraceae; Strains of Groups IV and V belonged to different Enterobacter sp.; Strain Ls4 and Ls 9 representing Group Ⅵ were close to Citrobacter amalonaticus with 98% sequence similarity; Strain Ls15 of Group Ⅶ showed 98% sequence identity with Pantoea sp.; Strains of Group Ⅷ were assigned to the genus Ideonella (β-proteobacteria). Based on the above results, endophytic diazotrophs isolated from O. rufipogon showed great diversity and some diazotrophs showed high nitrogenase activity with 42.52 μmol/mL·h C2H4. Inoculation to rice tests indicated that the isolated endophytic diazotrophs significantly promoted the rice growth.

黄土高原水蚀风蚀交错区固氮微生物群落多样性在生物结皮中的演变规律

生物土壤结皮在黄土高原的水土保持、养分补给等方面发挥着重要生态功能。固氮微生物是固氮过程的关键功能群,影响着土壤氮素平衡,然而黄土高原水蚀风蚀交错区严重的水土流失导致土壤养分大量损失,尤其是氮素。该区域生物结皮广泛发育,它们在干旱半干旱区土壤氮素积累过程中起着重要作用。但是该区域生物结皮不同发育阶段中的固氮潜力、固氮微生物群落多样性及其关键环境控制因子尚不清楚。以黄土高原水蚀风蚀交错区为研究区,三种生物结皮类型和裸地对照为研究对象,采用Illumina HiSeq平台对nifH基因扩增产物进行生物信息学分析,揭示了固氮潜力及微生物群落多样性的分布规律。结果表明,生物结皮演替显著影响固氮酶活性、nifH基因丰度和固氮微生物α-多样性,各值均在苔藓结皮最高,显著高于其它演替阶段,且生物结皮层高于结皮下层土壤。在固氮微生物群落组成上,裸地以变形菌门的红螺菌目、酸硫杆菌目和根瘤菌目为主;而在藻结皮、地衣结皮和藓结皮阶段,均表现为蓝藻门的念珠藻目占绝对优势,是固氮微生物的关键类群,其中Trichormus、念珠藻属是优势固氮物种;与藻结皮、地衣结皮相比,藓结皮中念珠藻目的相对丰度显著降低,而变形菌门的红螺菌目、酸硫杆菌目和根瘤菌目的丰度明显上升。除此之外,蓝杆藻属、鱼腥藻属、斯克尔曼氏菌属、瘤胃梭菌属在藻结皮、地衣结皮、藓结皮也占明显优势。念珠藻目以及Trichormus的丰度与固氮酶活性、nifH基因丰度呈显著正相关关系,而与红螺菌目、酸硫杆菌目、梭菌目、根瘤菌目呈显著负相关;Trichormus相关的物种在黄土高原生物结皮氮输入过程中可能发挥着关键作用。生物结皮演替通过改变土壤pH值、有机质、全氮、含水量等理化特征而影响了微生境,通过环境筛选和物种重排作用对固氮微生物群落进行综合调控,其中pH与土壤全氮(TN)发挥着关键作用。

氮肥对大豆根际土壤微生物群落和代谢的影响

探究不同施氮水平下大豆根际固氮微生物群落和代谢差异,对于合理有效的调控根际生态系统、提高土壤养分利用效率等具有重要意义。采用土壤代谢组学和高通量测序技术,研究了不同施氮处理对大豆根际土壤代谢产物与固氮微生物群落结构的影响。结果表明,无氮和施氮处理下大豆根际土壤代谢产物以及土壤固氮菌群落结构差异显著;共检测到29种差异代谢物,包括有机酸、聚酮类化合物、脂肪酰基类化合物、有机含氮化合物等物质;施氮促进大多数代谢物含量上调,且以有机酸及其衍生物含量最高,而下调代谢物仅1种为脂肪酰基类化合物。差异代谢物通路分析表明,差异代谢产物主要富集在氨基酸合成与碳水化合物代谢等途径;此外,施氮处理显著改变固氮微生物群落结构组成,与无氮处理相比大豆根际土壤中慢生根瘤菌属(Bradyrhizobium)、固氮氢自养单胞菌属(Azohydromonas),弗兰克氏菌属(Frankia)以及中华根瘤菌属(Sinorhizobium)等相对丰度显著下降。相关性分析表明根际优势固氮菌慢生根瘤菌属、固氮氢自养单胞菌属以及弗兰克氏菌属同大多数上调代谢物呈显著负相关关系,同下调差异代谢物呈显著正相关关系;中华根瘤菌属同大多数上调代谢物呈显著负相关关系。施氮可能通过影响根系分泌物,从而直接或间接对根际土壤固氮微生物群落结构产生影响。研究结果为不同氮肥处理对大豆根际土壤微生物群落及代谢产物的影响提供了理论基础。

对苯二酚对类珠藻的生物毒性

以从酸化稻田中分离的固氮蓝藻类珠藻(Aliinostoc sp.YYLX235)为研究对象,将类珠藻于脲酶抑制剂对苯二酚最终质量浓度分别为0.0、2.5、5.0、10.0 mg/L的培养液中培养,培养2、6 d时分别对该藻株的生长、光合作用、固氮酶活性、抗氧化酶活性和氧化损伤特征等进行分析,探究对苯二酚对农田类珠藻的生物毒性。结果表明:2.5 mg/L对苯二酚处理,短时间(2 d)内对类珠藻的生长和藻胆蛋白具有刺激效应,但随着处理时间的延长,刺激效应转变为抑制效应;对苯二酚处理对藻细胞内叶绿素a和类胡萝卜素含量的影响不明显;对苯二酚处理下类珠藻的光合系统Ⅱ电子传递效率表现敏感,造成光合系统Ⅱ反应中心电子供体侧受损,短时间(2 d)内5.0 mg/L对苯二酚处理的光合系统Ⅱ最大光化学效率仅为对照的79.1%;对苯二酚处理条件下类珠藻细胞内的过氧化氢酶(CAT)活性升高,6 d时10.0 mg/L对苯二酚处理的CAT活性是对照的9.96倍,但是藻细胞的脂质氧化程度并未因对苯二酚处理而加剧;对苯二酚对类珠藻固氮酶活性具有显著的抑制效应。

库布齐沙漠不同演替阶段土壤结皮固氮菌群特征

运用nifH基因高通量测序及实时荧光定量PCR技术,分析库布齐沙漠演替初期物理结皮、演替中期藻类结皮、演替后期藓类结皮土壤固氮菌群组成、多样性及丰度;通过共现性网络及斯皮尔曼相关性分析关键类群及其对土壤理化因子的响应。结果表明,物理结皮、藻类结皮、藓类结皮土壤的优势固氮菌属皆为斯科曼氏球菌属(32.33%~50.72%);藻类结皮α多样性最低;藻类结皮、藓类结皮均与物理结皮间固氮菌群nifH基因丰度存在显著性差异(P<0.05);物理结皮与藻类结皮中关键固氮类群均为斯科曼氏球菌属,其丰度与土壤全磷含量呈显著负相关(P<0.05),而藓类结皮中关键固氮类群为固氮螺菌属,其丰度与速效氮含量呈显著正相关(P<0.05)。固氮菌群中斯科曼氏球菌属和固氮螺菌属分别是物理藻类结皮和藓类结皮的指示生物。

小麦、水稻、玉米、白菜、芹菜内生固氮菌及其系统发育

【目的】了解小麦、水稻、玉米、白菜、芹菜内生固氮菌的主要类群,确定内生固氮菌的系统发育地位。【方法】样品表面灭菌后采用无氮培养基分离、培养内生固氮菌,乙炔还原法测定菌株固氮酶活性;PCR扩增得到菌株16S rDNA,通过序列测定和相似性分析研究菌株的系统发育。【结果】从大田小麦体内分离到内生固氮菌34株,固氮酶活性在0.30—30.24 nmol C2H4/h.mg蛋白,基于16S rDNA序列最大相似性,这些菌株分属于假单胞菌(Pseudomonas)、根瘤菌(Rhizobium)、芽孢杆菌(Bacillus)、黄杆菌(Flavobacterium)等13属21种,种群分布较为广泛;从大田水稻体内分离到内生固氮菌25株,固氮酶活性在3.12—254.12 nmol C2H4/h.mg蛋白,属于芽孢杆菌、伯克霍尔德氏菌(Burkholderia)、肠杆菌(Enterobacter)、克雷伯氏菌(Klebsiella)等9属16种,伯克霍尔德氏菌、肠杆菌和克雷伯氏菌是水稻内生固氮菌特有种群;从大田玉米体内分离到内生固氮菌9株,固氮酶活性在7.27—59.58 nmol C2H4/h.mg蛋白,属于根瘤菌、鞘氨醇单胞菌(Sphingomonas)等5属6种;从盆栽试验小白菜体内分离到内生固氮菌14株,固氮酶活性在2.33—205.21 nmol C2H4/h.mg蛋白,属于根瘤菌、节杆菌(Arthrobacter)等6属8种;从市售芹菜体内分离到内生固氮菌10株,固氮酶活性在1.23—46.70 nmol C2H4/h.mg蛋白,属于鞘氨醇单胞菌、假单胞菌等5属8种。【结论】在生长期小麦、水稻、玉米和部分蔬菜体内普遍存在内生固氮菌,菌株固氮酶活性差异较大,在0.30—254.12 nmol C2H4/h.mg蛋白,系统发育地位分属于假单胞菌、根瘤菌、芽孢杆菌等25个属的56个种,这些内生固氮菌对于农业生产有巨大潜能。

陵水普通野生稻(Oryza rufipogon)内生菌的固氮及溶磷特性

从陵水普通野生稻中分离了36株内生菌,经乙炔还原法确定22株菌为内生固氮菌,并测定了其溶磷特性.在22株内生固氮菌中,除菌株Ls2不具有溶磷能力外,其余21株菌溶磷能力差异较大,为(0.002±0.01)^(12.7±0.04)μgmL-1.将这些内生固氮菌接种籼型水稻华航1号,除菌株Ls23外,其余菌株均能使接种水稻苗的根、茎长度增加(在α=0.05水平上差异显著).以上研究结果表明,这些菌株在水稻菌肥研制方面具有较大的开发潜力.

糖蜜草(Melinis minutiflora Beauv.)内生固氮菌分离鉴定

糖蜜草(MelinisminutifloraBeauv.)是热带地区的一种优良牧草。采用选择性培养基在厌氧和好氧两种培养条件下,从糖蜜草根、茎中都可分离得到具有较强固氮酶活性的菌株。通过SDS-PAGE全细胞蛋白电泳技术快速聚类分析表明,来源于糖蜜草中的菌株为同一类群。16SrDNA序列分析和总DNA的G+C%含量进一步确定糖蜜草中所分离的菌株属于固氮螺菌属(Azospirillum),与产脂固氮螺菌(Azospirillumlipoferum)亲缘关系较近。BIOLOG板测定结果显示,糖蜜草菌株TMCY243对多种碳源具有很强的适应性,与产脂固氮螺菌(A.lipoferum)的模式菌株DSM1691存在着较大的差异。以上结果表明,糖蜜草内生固氮菌为固氮螺菌属的一个新类群。

小麦-甘薯轮作长期增施有机肥对碱性土壤固氮菌群落结构及多样性的影响

生物固氮为农业生态系统提供天然的氮素来源,探究长期增施有机肥对土壤固氮菌群落的影响,为合理增施有机肥和维持土壤固氮微生物群落多样性提供科学依据。选取小麦-甘薯轮作中连续37a不施肥对照(CK)、单施化肥(NPK)、化肥+有机肥(NPKM)处理的甘薯季碱性土壤样品为研究对象。采用Illumina MiSeq高通量测序技术,研究土壤固氮菌群落的组成、多样性及其与土壤特性的关系。结果表明:与对照和单施化肥相比,长期增施有机肥降低土壤固氮菌群落丰富度和多样性,且丰富度与土壤pH显著正相关,与有机碳、全氮和有效养分(硝态氮、有效磷和速效钾)显著负相关。主坐标分析表明长期施肥显著改变土壤固氮菌群落结构,与对照相比,增施有机肥比单施化肥对固氮菌群落结构的影响更大。冗余分析表明土壤有机碳和速效钾是影响固氮菌群落结构改变最主要的因素。长期增施有机肥显著降低变形菌门、蓝藻菌门、Beta-变形菌和固氮弧菌属的相对丰度,显著增加硝化螺旋菌门、酸杆菌门和硝化螺菌属的相对丰度,这与土壤pH、有机碳和有效养分显著相关。因此,在碱性土壤上长期增施有机肥对固氮菌群落结构的改变更大,对群落多样性的抑制作用更强。

禾本科植物联合固氮研究及其应用现状展望

综述了近年来从禾本科植物体内和根际发现的内生固氮菌和根际固氮菌的种类、特征及对宿主的促生机理 ,以及固氮菌接种剂在农业生产中的应用现状和存在的问题 ,指出影响联合固氮菌接种效果的主要因素有土著微生物的竞争 ;植物基因型差异和环境条件的变化 ,如结合态氮 (氨、亚硝酸盐、硝酸盐等 )对固氮酶的合成阻遏和较高的氧分压对联合固氮菌的固氮效率影响 .提出了发掘和利用禾本科植物的生物固氮潜力的努力方向 :从自然界分离筛选获得广谱高效固氮菌株 ;应用基因工程构建耐铵、泌铵型联合固氮菌 ;诱导禾本科植物形成固氮根瘤 ;充分发挥植物内生固氮菌的优势 .