Bacterial diversity in the bulk soil and rhizosphere fractions of Lolium perenne and Trifolium repens as revealed by PCR restriction analysis of 16S rDNA

The rhizosphere of Trifolium repens and Lolium perenne was divided into three fractions: the bulk soil, the soil adhering to the roots and the washed roots (rhizoplane and endorhizosphere). After isolation and purification of DNA from these fractions, 16S rDNA was amplified by PCR and cloned to obtain a collection of 16S rRNA genes representative of the bacterial communities of these three fractions. The genes were then characterized by PCR restriction analysis. Each different profile was used to define an operational taxonomic unit (OTU). The numbers of OTUs and the numbers of clones among these OTUs allowed to calculate a diversity index. The number of OTUs decreased as root proximity increased and a few OTUs became dominant, resulting in a lower diversity index. In the root fraction of T. repens, the restriction profile of the dominant OTU matched the theoretical profile of the 16S rRNA gene of Rhizobium leguminosarum. This study showed that plant roots create a selective environment for microbial populations.     

Effect of four acid soils on root growth of white clover seedlings using a soil-on-agar procedure

White clover (Trifolium repens L.) is widely distributed in the Appalachian region, except on highly acid soils. We used a procedure where a thin layer of soil is placed on top of solidified water agar to characterize effects of acid soil on seedling root growth. Our objectives were to evaluate the soil-on-agar technique by using four soils (non-limed and limed) with diverse chemical characteristics and to relate root emergence to the chemical properties of the soils. We used three white clover cultivars, Grasslands Huia, Grasslands Tahora and Sacramento. Daily counts of root emergence from soil into agar were made for 12 d. Liming hastened white clover root emergence in three of the four soils. Days to 40% emergence were closely related (P < 0.01) to soil pH and to species of soil solution Al that are associated with Al toxicity in dicotyledonous plants. The r2 values for the regression of days to 40% root emergence on were 0.95, 0.96, 0.94 and 0.96, respectively. Apparently, the primary factor responsible for delayed root emergence in the soil-on-agar procedure was Al toxicity. Because of the close relationship between root emergence and activity of toxic species of soil solution Al, we propose that the soil-on-agar technique should be useful for characterizing the response of many small-seeded species to Al.     

The response of soil microorganisms and roots to elevated CO2 and temperature in a terrestrial model ecosystem

We investigate the response of soil microorganisms to atmospheric CO2 and temperature change within model terrestrial ecosystems in the Ecotron. The model communities consisted of four plant species (Cardamine hirsuta, Poa annua, Senecio vulgaris, Spergula arvensis), four herbivorous insect species (two aphids, a leaf-miner, and a whitefly) and their parasitoids, snails, earthworms, woodlice, soil-dwelling Collembola (springtails), nematodes and soil microorganisms (bacteria, fungi, mycorrhizae and Protista). In two successive experiments, the effects of elevated temperature (ambient plus 2 °C) at both ambient and elevated CO2 conditions (ambient plus 200 ppm) were investigated. A 40:60 sand:Surrey loam mixture with relatively low nutrient levels was used. Each experiment ran for 9 months and soil microbial biomass (Cmic and Nmic), soil microbial community (fungal and bacterial phospholipid fatty acids), basal respiration, and enzymes involved in the carbon cycling (xylanase, trehalase) were measured at depths of 0–2, 0–10 and 10–20 cm. In addition, root biomass and tissue C:N ratio were determined to provide information on the amount and quality of substrates for microbial growth.Elevated temperature under both ambient and elevated CO2 did not show consistent treatment effects. Elevation of air temperature at ambient CO2 induced an increase in Cmic of the 0–10 cm layer, while at elevated CO2 total phospholipid fatty acids (PLFA) increased after the third generation. The metabolic quotient qCO2 decreased at elevated temperature in the ambient CO2 run. Xylanase and trehalase showed no changes in both runs. Root biomass and C:N ratio were not influenced by elevated temperature in ambient CO2. In elevated CO2, however, elevated temperature reduced root biomass in the 0–10 cm and 30–40 cm layers and increased N content of roots in the deeper layers. The different response of root biomass and C:N ratio to elevated temperature may be caused by differences in the dynamics of root decomposition and/or in allocation patterns to coarse or fine roots (i.e. storage vs. resource capture functions). Overall, our data suggests that in soils of low nutrient availability, the effects of climate change on the soil microbial community and processes are likely to be minimal and largely unpredicatable.     

Interactions between elevated CO2 concentration, nitrogen and water: effects on growth and water use of six perennial plant species

Two experiments are described in which plants of six species were grown for one full season in greenhouse compartments with 350 or 560 μ mol mol^–1 CO₂. In the first experiment two levels of nitrogen supply were applied to study the interaction between CO₂ and nitrogen. In the second experiment two levels of water supply were added to the experimental set-up to investigate the three-way interaction between CO₂, nitrogen and water. Biomass and biomass distribution were determined at harvests, while water use and soil moisture were monitored throughout the experiments. In both experiments a positive effect of CO₂ on growth was found at high nitrogen concentrations but not at low nitrogen concentrations. However, plants used much less water in the presence of low nitrogen concentrations. Drought stress increased the relative effect of elevated CO₂ on growth. Available soil moisture was used more slowly at high CO₂ during drought or at high nitrogen concentrations, while at low nitrogen concentrations decreased water use resulted in an increase in soil moisture. The response to the treatments was similar in all the species used. Although potentially faster growing species appeared to respond better to high CO₂ when supplied with a high level of nitrogen, inherently slow-growing species were more successful at low nitrogen concentrations.     

Availability of phosphorus and sulfur of insecticide origin by fungi

Thirteen fungal species isolated from soil treated with pesticides were tested for their ability to mineralize and degrade three organophosphate insecticides currently used in Egypt (Cyolan®, Malathion® and Dursban®) in liquid media free from phosporus (P) and sulfur (S). All fungal species grew successfully on the culture media treated with the three used doses of insecticides (10, 50 and 100 ppm active ingredient) but the growth rate varied with the species, the insecticide and the doses. At 10 ppm level, insecticide degradation expressed in term of organic P mineralization (calculated as % of applied P) was the highest with all fungi tested. Organic P mineralization from pesticides was decreased by increasing the dose used to 50 and 100 ppm. The highest amount of P mineralized was observed with Cyolan® followed by Malathion® whilst P mineralization from Dursban® proceeded very slowly. Aspergillus terreus showed the greatest potential to mineralize organic P followed by A. tamarii, A. niger, Trichoderma harzianum and Penicillium brevicompactum whilst the remaining fungi only moderately mineralized the organic P component of the insecticides tested. Organic sulfur mineralization by the used fungal species paralleled, to some extent, organic P mineralization. The extracellular protein content of culture filtrates in the presence of various doses of insecticides was also decreased by increasing insecticide concentrations. The extracellular protein was significantly correlated with P and S mineralization (r = 0.89** and 0.64**, respectively) whilst correlation with cell dry mass was not significant (r = 0.03 and 0.003) suggesting a direct relationship between pesticide degradation and microbial protein production. The addition of P or S to the growth media enhanced extracellular protein excretion, and increased organic P and S mineralization by the most potent species tested (A. niger, A. tamarii, A. terreus and T. harzianum). This increment was significant in most cases, especially at the higher application rates. The relationship between extracellular protein excretion and organic P and S mineralization from insecticides was highly significant with the addition of inorganic phosphorus (r = 0.96** and 0.83**, respectively) or sulfur (r = 0.85** and 0.89**, respectively) to the growth media.     

Contribution of micro-organisms to the carbon dynamics in black spruce (Picea mariana) forest soil in Canada

In order to clarify the role of micro-organisms in the carbon cycle of the boreal forest ecosystem, the vertical distribution of soil carbon, soil microbial biomass and respiratory activity was studied in a black spruce forest near Candle Lake in Saskatchewan, Canada. The total amount of carbon contained in moss and soil layers (to the depth of 50cm beneath the mineral soil surface) was 7.2kgm^–2, about 47% of which was in the L and FH horizons of the soil. Soil microbial biomass per dry weight of soil was largest in the L horizon, while the biomass per ground area was largest in the FH horizon. Soil respiration rate, measured using a portable infrared gas analyzer, was highest in the FH horizon, exceeding 50% of the total soil respiration. Low but significant CO₂ emission was detected even in deeper soil horizon (E horizon). We also examined the respiration rate of cut roots and the effect of root excision on respiration. The contribution of root respiration to total soil respiration, calculated from root biomass and respiration rate of cut roots, was about 54%. The amount of carbon evolved through microbial respiration during the snow-free season (June–October) was estimated as 221gCm^–2. Micro-organisms in the L horizon showed high respiratory activity as compared with those in deeper soil horizons.     

The use of forest inventory data for a National Protected Area Strategy in Guyana

Forest inventories are largely neglected in the debate of national parks selection in Guyana (and probably elsewhere). Because taxonomic data are often scant and biased towards are as of high collecting effort, large scale forest inventory data can be a useful tool adding to a knowledge database for forests. In this paper the use of forest inventories to select national parks in Guyana is assessed. With the data of a large scale inventory five forest regions could be distinguished and two were added on the base of existing other information. Forest composition in Guyana is largely determined by geology at a national level and soil type at regional level. Species diversity is higher in the south of Guyana, possibly due to higher disturbance and is also higher on the better soils. It is concluded that a selection of national parks in Guyana should include a sample of all seven regions, including as much soil variation as possible. Because of land use conflicts in central Guyana, this area is in need of quick attention of Guyana's policy makers.     

江淮丘陵地区油-稻轮作条件下土壤硫库变化研究

江淮丘陵地区下蜀系黄土母质发育的水稻土油2稻轮作试验表明, 油菜种植期间, 耕层土壤硫主要来源于耕层以下土层的补给, 其次是大气干湿沉降;该时期耕层土壤硫输出主要是油菜吸收, 其次是淋失.油菜种植期间耕层土壤硫输入量小于输出, 导致耕层土壤硫库下降8.76kg·hm^-2, 22%来自无机硫库的下降.水稻种植期间, 耕层土壤硫输入主要来自灌溉水, 其次是底土层的补给和大气干湿沉降;而硫输出主要是淋失, 其次是水稻吸收.耕层土壤硫输入量大于输出, 导致耕层土壤硫库增加18.69kg·hm^-2, 18%来自无机硫库的增加.全年油2稻轮作期间耕层土壤硫输入量大于输出, 导致耕层土壤硫库增加9.93kg·hm^-2, 13%来自无机硫库的增加.     

土壤大气湿度组合对玉米生长和WUE效应研究

采用人工气候生长箱盆栽试验,模拟土壤和大气湿度不同组合处理,发现土壤干旱和大气干旱显著抑制玉米生长。大气湿度提高,在一定程度能改善作物的水分状况,降低蒸腾,解除土壤干旱危害,产生生长和生理补偿效应,明显提高水分利用效率。     

红壤丘陵区土地利用方式变更后土壤有机碳动态变化的模拟

采用双组分模型模拟土地利用方式变更后土壤有机碳储量的变化,并用一些调查和监测数据进行了初步验证．此双组分模型将土壤有机碳分为新形成有机碳和原有有机碳两个组分．每个组分有机碳的形成转化用一级动力学方程描述．本文用此模型对亚热带土壤开垦利用为马尾松林地、湿地松林地、柑桔园和牧草地４种方式１０年来土壤有机碳储量的变化过程进行了模拟,初步结果表明,模拟值与实测值拟合较好．可见,此方法适于用来模拟不同土壤类型下土地利用系统变更初期的土壤有机碳储量动态变化过程．