Interactions between Rhizoctonia tuliparum and soil microorganisms

Rhizoctonia tuliparum Whetzel & Arthur is a winter active soil-borne pathogen, which causes the grey bulb rot of tulips and Dutch irises. The fungus survives by means of sclerotia. It was found that the sclerotia only germinated in unsterile soil at low temperatures (0–10 °C). Germination in sterile soil occurred over a wider range of temperatures (0–25 °C) but was inhibited at 20 °C after the addition of a suitable bacterial inoculum. An unidentified pyrone antibiotic was leaked by sclerotia which had been air-dried and re-wetted. This could be detected in vitro using an agar medium seeded with a spore suspension of Bacillus subtilis Cohn emend Prazmowski. In soil, however, material leaked by sclerotia stimulated both bacteria and fungi. Undried, air-dried and oven-dried (non antibiotic) sclerotia persisted equally well in soil over 2 yr. This suggests that the resistance to lysis of sclerotial cell walls may be important in the long term survival of this organism.     

Methylated arsenic species in plants originate from soil microorganisms

? Inorganic arsenic (iAs) is a ubiquitous human carcinogen, and rice (Oryza sativa) is the main contributor to iAs in the diet. Methylated pentavalent As species are less toxic and are routinely found in plants; however, it is currently unknown whether plants are able to methylate As. ? Rice, tomato (Solanum lycopersicum) and red clover (Trifolium pratense) were exposed to iAs, monomethylarsonic acid (MMA(V)), or dimethylarsinic acid (DMA(V)), under axenic conditions. Rice seedlings were also grown in two soils under nonsterile flooded conditions, and rice plants exposed to arsenite or DMA(V) were grown to maturity in nonsterile hydroponic culture. Arsenic speciation in samples was determined by HPLC-ICP-MS. ? Methylated arsenicals were not found in the three plant species exposed to iAs under axenic conditions. Axenically grown rice was able to take up MMA(V) or DMA(V), and reduce MMA(V) to MMA(III) but not convert it to DMA(V). Methylated As was detected in the shoots of soil-grown rice, and in rice grain from nonsterile hydroponic culture. GeoChip analysis of microbial genes in a Bangladeshi paddy soil showed the presence of the microbial As methyltransferase gene arsM. ? Our results suggest that plants are unable to methylate iAs, and instead take up methylated As produced by microorganisms.     

Competition between soil bacteria and fusarium

Summary The phenomenon of competition has been characterized in liquid medium and sterile soil systems using a variety of soil bacteria andFusarium oxysporum f.cubense as test organisms. For most of the bacteria, suppression of the fungus was the result of a biologically induced nitrogen deficiency, this effect being reversed by the addition of excess inorganic nitrogen. High populations of competitors were found in two soils of neutral pH, but no isolates competed in the acid San Alejo loam.Agrobacterium radiobacter was able to compete when San Alejo loam was limed to about pH 6.6. Inhibition of the fungus by a number of gram-positive, spore-forming rods could not be accounted for in terms of competition for nutrients or by antibiotic production in artificial media.The competitive ability ofA. radiobacter when tested in twelve Central American soils was found to be related to pH in acid and neutral environments but was correlated with texture, organic-matter content and total nitrogen in soils of intermediate pH. In all soils where inhibition occurred, the competitive effect was overcome by additions of inorganic nitrogen. Excluding the group ofBacillus spp., the competitive ability of soil bacteria was related to the ability to develop in the absence of amino acids and growth factors but could not be correlated with growth rates of the bacteria in soil or liquid medium.It is suggested that competition for nutrients is a significant means of ecological control among members of the soil microflora and, together with competitive interactions for space and oxygen, may be the major factors governing the biological control of soil-borne fungi.The investigation was supported in part by a grant from the United Fruit Company. Agronomy Paper No. 471.     

Cyanuric acid--a s-triazine derivative as a nitrogen source for some soil microorganisms

Cyanuric acid was not toxic for soil microorganisms examined and was even observed to stimulate the growth of Azotobacter in chernozem. Some isolated fungi were capable of cleaving the ring of cyanuric acid. With the use of 15N-labeled cyanuric acid it was found that the nitrogen taken from this compound by Aspergillus minutus and Pseudogymnoascus sp. was incorporated into their proteins. About 70-90% of 15N derived from cyanuric acid was detected in the biomass of the examined fungi. The ability of soil microorganisms to cleave the triazine ring is of importance in the detoxication of soils treated with triazine herbicides.     

Evolution of parasitic symbioses between plants and filamentous microorganisms

Innate defense in wild plant populations is an invaluable resource for understanding how sustainable disease control can be achieved in crops through research that is rooted in molecular and evolutionary biology. Much progress has been made from molecular research into pathogen detection and defense induction. Bacterial pathology of the wild species Arabidopsis thaliana is at the forefront in revealing parallels with animal innate immunity against infectious diseases. In plants, unlike in animals, however, expansion in biodiversity has been mirrored by tremendous diversification in filamentous parasites. The fungal and oomycete pathology of Arabidopsis is exposing opportunities to investigate the molecular bases of compatibility, plant-driven speciation of parasites, and molecular epidemiology. Such research might reveal evidence that an arms race did occur in the evolution of plant-parasite symbioses.     

Role of soil microorganisms in improving P nutrition of plants

Phosphorus (P) is one of the major plant growth-limiting nutrients although it is abundant in soils in both inorganic and organic forms. Phosphate solubilizing micro-organisms (PSMs) are ubiquitous in soils and could play an important role in supplying P to plants in a more environmentally friendly and sustainable manner. Although solubilization of P compounds by microbes is very common under laboratory conditions, results in the field have been highly variable. This variability has hampered the large-scale use of PSMs in agriculture. Many reasons have been suggested for this variability, but none of them have been extensively investigated. In spite of the importance of PSMs in agriculture, the detailed biochemical and molecular mechanisms of P solubilization are not known. Recent work in our laboratory has shown that the conditions employed to isolate PSMs do not reflect soil conditions and that PSMs capable of effectively releasing P from soil are not so highly abundant as was suggested in earlier studies. These studies have also indicated that the mineral phosphate solubilizing (mps) ability of microbes could be linked to specific genes, and that these genes are present even in non P solubilizing bacteria. Understanding the genetic basis of P solubilization could help in transforming more rhizosphere-competent bacteria into PSMs. Further research should also focus on the microbial solubilization of iron (Fe) and aluminum (Al) phosphates, as well as mobilization of the organic phosphate reserves present in the soils.     

Selecting plants and nitrogen rates to vegetate crude-oil-contaminated soil

Phytoremediation can be effective for remediating contaminated soils in situ and generally requires the addition of nitrogen (N) to increase plant growth. Our research objectives were to evaluate seedling emergence and survival of plant species and to determine the effects of N additions on plant growth in crude-oil-contaminated soil. From a preliminary survival study, three warm-season grasses--pearlmillet (Pennisetum glaucum [L.] R. Br.), sudangrass (Sorghum sudanense [Piper] Stapf [Piper]), and browntop millet (Brachiaria ramosa L.)--and one warm-season legume--jointvetch (Aeschynomene americana L.)--were chosen to determine the influence of the N application rate on plant growth in soil contaminated with weathered crude oil. Nitrogen was added based on total petroleum hydrocarbon-C:added N ratios (TPH-C:TN) ranging from 44:1 to 11:1. Plant species were grown for 7 wk. Root and shoot biomass were determined and root length and surface area were analyzed. Pearlmillet and sudangrass had higher shoot and root biomass when grown at a TPH-C:TN (inorganic) ratio of 11:1 and pearlmillet had higher root length and surface area when grown at 11:1 compared with the other species. By selecting appropriate plant species and determining optimum N application rates, increased plant root growth and an extended rhizosphere influence should lead to enhanced phytoremediation of crude-oil-contaminated soil.     

Competition between siratro and kleingrass for15N labelled mineralized nitrogen

Summary Isotope dilution provides a method for measuring plant competition for mineral N and transfer of biologically fixed N from a legume to a grass. A plant growth medium was enriched with¹⁵N, and used to grow Siratro (Macropitilium atropurpureum D.C. Urb.) and Kleingrass 75 (Panicum coloratum L.) in 20 liter pots for 98 days in a glasshouse. The plants were grown in pure stand and in mixtures. When grown in 50∶50 mixture the grass obtained 59% of the labelled N and the legume obtained 41%. The grass produced nearly as much root mass as the legume even though biomass of the shoots were less than half that of the legume. Reducing the proportion of either plant species in the mixture reduced the proportion of the mineralized N absorbed by that species. The shoots of the grass were significantly more enriched (1.166 atom%¹⁵N excess) than the roots (1.036). The grass received 12% of its N as biologically fixed N from the legume.     

Light affects competition for inorganic and organic nitrogen between maize and rhizosphere microorganisms

Effects of light on the short term competition for organic and inorganic nitrogen between maize and rhizosphere microorganisms were investigated using a mixture of amino acid, ammonium and nitrate under controlled conditions. The amount and forms of N added in the three treatments was identical, but only one of the three N forms was labeled with ¹⁵N. Glycine was additionally labeled with ¹⁴C to prove its uptake by maize and incorporation into microbial biomass in an intact form. Maize out-competed microorganisms for during the whole experiment under low and high light intensity. Microbial uptake of ¹⁵N and ¹⁴C was not directly influenced by the light intensity, but was indirectly related to the impact the light intensity had on the plant. More was recovered in microbial biomass than in plants in the initial 4 h under the two light intensities, although more ¹⁵N-glycine was incorporated into microbial biomass than in plants in the initial 4 h under low light intensity. Light had a significant effect on uptake by maize, but no significant effects on the uptake of or ¹⁵N-glycine. High light intensity significantly increased plant uptake of and glycine ¹⁴C. Based on ¹⁴C to ¹⁵N recovery ratios of plants, intact glycine contributed at least 13% to glycine-derived nitrogen 4 h after tracer additions, but it contributed only 0.5% to total nitrogen uptake. These findings suggest that light intensity alters the competitive relationship between maize roots and rhizosphere microorganisms and that C4 cereals such as maize are able to access small amounts of intact glycine. We conclude that roots were stronger competitor than microorganisms for inorganic N, but microorganisms out competed plants during a short period for organic N, which was mineralized into inorganic N within a few hours of application to the soil and was thereafter available for root uptake.     

Effect of the fungicides tridemorph and vinclozolin on soil microorganisms and nitrogen metabolism

The effects of tridemorph and vinclozolin were studied on different types of microorganisms, urea hydrolysis and nitrification in soil and in culture. The fungicides adversely affected the population of bacteria, fungi and actinomycetes as a function of time of incubation. Urea hydrolysis both in culture and soil were also inhibited by the fungicides, and tridemorph was more detrimental. In soil, 45μg/g of tridemorph inhibited 50 % of ammonification of urea, ID₅₀ for nitrite production was 750μg/g. In urea-hydrolyzing cultures, 80, 75 and 77 mg/L of tridemorph were the ID₅₀ values for urea hydrolysis byMicrococcus sp.,Proteus sp. andP. vulgaris respectively.     

The below-ground perspective of forest plants: soil provides mainly organic nitrogen for plants and mycorrhizal fungi

? Nitrogen (N) availability has a major impact on a wide range of biogeochemical processes in terrestrial ecosystems. Changes in N availability modify the capacity of plants to sequester carbon (C), but despite the crucial importance for our understanding of terrestrial ecosystems, the relative contribution of different N forms to plant N nutrition in the field is not known. Until now, reliably assessing the highly dynamic pool of plant-available N in soil microsites was virtually impossible, because of the lack of adequate sampling techniques. ? For the first time we have applied a novel microdialysis technique for disturbance-free monitoring of diffusive fluxes of inorganic and organic N in 15 contrasting boreal forest soils in situ. ? We found that amino acids accounted for 80% of the soil N supply, while ammonium and nitrate contributed only 10% each. In contrast to common soil extractions, microdialysis revealed that the majority of amino acids are available for plant and mycorrhizal uptake. ? Our results suggest that the N supply of boreal forest soils is dominated by organic N as a major component of plant-available N and thus as a regulator of growth and C sequestration.     

Competition between grassland plants of different initial sizes

Summary Four species of grassland plant, Plantago lanceolata, Holcus lanatus, Lolium perenne and Rumex acetosa, were grown as monocultures and mixtures in pots of nutrient poor soil in a glasshouse for 8 months. There were four plants per pot and these were arranged in two competition modes: either root and shoot interactions were permitted, or only roots allowed to interact by using above-ground partitions. Time of introduction of seedlings was varied to give a range of plant size ratios at the start of the experiment. The factorial design catered for all combinations of species, competition modes and planting times, replicated in four blocks. The shoots were clipped at a fixed height at each of five harvests. Rumex grew badly and was mostly omitted from analysis of the data.By (i) following the change in the relationship of clip dry weights against planting time with successive harvests, (ii) plotting the change in the logarithm of the ratio of cumulative clip dry weights with time and (iii) the use of de Wit logarithmic ratio plots it was demonstrated that each monoculture and mixture combination's ratios of plant weights converged towards stable equilibrium values. Three hypotheses are put forward to explain why in monocultures a smaller plant was at a competitive advantage relative to a larger neighbour and was not suppressed in its growth by the latter. In mixtures this plant size effect was superimposed to different extents on the relative aggressiveness of the species considered. It was concluded that in a nutrient poor soil, when competition for light was low, root interactions can promote the co-existence of neighbouring plant species.     

Competition between microorganisms for a single limiting resource with cell quota structure and spatial variation

Microbial populations compete for nutrient resources, and the simplest mathematical models of competition neglect differences in the nutrient content of individuals. The simplest models also assume a spatially uniform habitat. Here both of these assumptions are relaxed. Nutrient content of individuals is assumed proportional to cell size, which varies for populations that reproduce by division, and the habitat is taken to be an unstirred chemostat where organisms and nutrients move by simple diffusion. In a spatially uniform habitat, the size-structured model predicts competitive exclusion, such that only the species with lowest break-even concentration persists. In the unstirred chemostat, coexistence of two competitors is possible, if one has a lower break-even concentration and the other can grow more rapidly. In all habitats, the calculation of competitive outcomes depends on a principal eigenvalue that summarizes relationships among cell growth, cell division, and cell size.     

三叶鬼针草与不同本地植物竞争对土壤 微生物和土壤养分的影响

入侵植物三叶鬼针草(Bidens pilosa)对我国农牧业生产造成了重大的损失。本文主要研究三叶鬼针草入侵与不同本地植物竞争对土壤微生物群落结构和土壤养分的影响。利用磷脂脂肪酸方法(phospholipid fatty acids, PLFAs)测定土壤微生物群落组成, 同时测定土壤养分和酶活性, 并利用Canoco4.5软件分析了土壤微生物、土壤养分和土壤酶活性的相关性。结果表明: (1)三叶鬼针草对革兰氏阳性菌、革兰氏阴性菌、丛枝菌根真菌等土壤微生物具有较强的聚集能力, 且其根际土壤聚集的微生物类群与本地植物种类密切相关。(2)三叶鬼针草入侵显著增加了入侵地土壤的有机碳含量, 降低了铵态氮的含量; 土壤中的速效钾、速效磷和硝态氮的含量则与本地植物种类密切相关。(3)相关性分析表明, 16:00和16:1 ω5c对铵态氮的含量影响较大, 而三叶鬼针草入侵地16:00和16:1 ω5c的含量显著高于裸土对照, 进而推测这一状况导致了铵态氮含量的降低。(4) 15:1 anteiso A和18:1 ω5c与速效钾的含量呈显著正相关, 而其含量在狗尾草(Setaria viridis)中显著高于其他处理, 三叶鬼针草与狗尾草混种处理中土壤中速效钾的含量高于其他处理。以上结果说明, 三叶鬼针草通过改变土壤微生物群落结构影响了土壤酶活性和土壤养分, 且这种改变与入侵地本地植物种类有关。     

Transformation of soil and fertilizer nitrogen in paddy soil and their availability to rice plants

Summary A pot experiment in the field showed that addition of ammonium sulfate increased the uptake of soil nitrogen. A-value was found to be independent of the rate of nitrogen application. The rice plant took up about 13 percent of the nitrogen in rice straw which was incorporated into the soil when nitrogen fertilizer was not added, and about 15 percent when 50 ppm N was added. Addition of different levels of fertilizer did not affect the release of immobilized fertilizer nitrogen. Recovery of fertilizer by the rice plant was low when nitrogen was added as basal (broadcast). Recovery was improved by incorporating fertilizer nitrogen before transplanting. Recovery of fertilizer nitrogen when topdressed at reproductive stages was much higher than when applied as basal. A fairly large portion of fertilizer nitrogen was immobilized into the soil. Availability of immobilized nitrogen in the soil appeared low. re]19751117     

Competition between native Antarctic vascular plants and invasive <Emphasis Type="Italic">Poa annua</Emphasis> changes with temperature and soil nitrogen availability

Over the last decades human have introduced non-native organisms to Antarctica, including the grass species Poa annua. This non-native grass under constant growth temperatures has been shown negatively affect the growth of the only two native Antarctic vascular plants, Deschampsia antarctica and Colobanthus quitensis, under constant growth temperatures. However, whether there are changes in the interaction between these species under warmer conditions is an important question. In cold ecosystems, soil nutrient status directly affects plant responses to increases in temperature and Antarctic soils are highly variable in nutrient supply. Thus, in this study we experimentally assessed the interaction between the non-native Poa with the two native Antarctic vascular plant species at two different temperatures and levels of nutrient availability. Individual mats of the study species were collected in King George Island, and then transported to Concepcion where we conducted competition experiments. In the first experiment we used soil similar to that of Antarctica and plants in competition were grown at two temperatures: 5°/2° and 11°/5 °C (day/night temperature). In a second experiment plants were grown in these two temperature regimes, but we varied nitrogen (N) availability by irrigating plants with Hoagland solutions that contained 8000 or 300 µM of N. Overall, Poa exerted a competitive effect on Deschampsia but only at the higher temperature and higher N availability. At 5°/11 °C the competitive response of Deschampsia to Poa was of similar magnitude to the competitive effect of P. Deschampsia, and the competitive effect was greater with at low N. The competitive effect of Poa was similar to the competitive response of Colobanthus to Poa at both temperatures and N levels. Thus, at low temperatures and N soil content the native Antarctic species might withstand Poa invasion, but this might change with climate warming.     

Competition for nitrogen between Australian native grasses and the introduced weed Nassella trichotoma

BACKGROUND: and Aims Nassella trichotoma is an unpalatable perennial grass weed that invades disturbed native grasslands in temperate regions of south-eastern Australia. This experiment investigated whether elevated N levels, often associated with disturbance, increases the competitiveness of N. trichotoma relative to C3 and C4 native Australian grasses. METHODS: A pot experiment investigated competitive interactions between four native grasses, two C3 species (Microlaena stipoides and Austrodanthonia racemosa) and two C4 species (Themeda australis and Bothriochloa macra), and N. trichotoma at three different N levels (equivalent to 0, 60 and 120 kg ha-1) and three competing densities (zero, one and eight neighbouring plants), using an additive design. KEY RESULTS: All native grasses were competitive with N. trichotoma at low N levels, but only M. stipoides was competitive at high N. High densities of native grasses (8:1) had a major competitive effect on N. trichotoma at all N levels. The competitive ranking of native grasses, across all N levels, on N. trichotoma was: M. stipoides>A. racemosa>B. macra>T. australis. The C3 species were generally more competitive than the C4 species and C4 grasses were not inherently more productive at low N levels, in contrast to the results of other studies. CONCLUSION: To resist invasion from N. trichotoma, these native grasses need to be maintained at a high density and/or biomass. The results do not support the theory that species such as N. trichotoma, with high tissues density, are always less competitive than those of low tissue density; in this case competitiveness depended on N levels. The ability of N. trichotoma to accumulate biomass at a higher rate than these native grasses, helps to explain why it is a major weed in disturbed Australian native grasslands.