植物病原真菌对土壤种子库的影响

植物病原真菌对土壤种子库的影响是国际新兴的研究领域之一。本研究回顾与综述了近20年来该领域的研究进展。大多数已报道的研究均采用"种子袋埋藏+杀菌剂处理"的方法。应用杀菌剂处理埋藏种子能提高多种生境下土壤种子库的存活或降低其损失,其效果因生境或植物种而异。小粒种子与老龄种子更易受到病原真菌的威胁,种子内的化学防御物质有助于种子抵御病原真菌的侵袭而延长其寿命。病原真菌对种子库的影响亦与土壤水分、温度及种子埋深等土壤非生物因素及生物因素有关:在湿润条件下病原真菌作用增强,浅土层或高密度的种子库受其影响更甚,食真菌动物与非致病性细菌能够保护种子免于受害。分析了现有研究方法的特点并提出了改进的意见,建议了该领域的研究思路与方法及尚需加强研究的内容。

Interactive effects of soils,local environmental conditions and herbivores on secondary chemicals in tallow tree

Plants produce secondary chemicals that may vary along with latitude due to changing abiotic and biotic stress gradients and local environmental conditions.Teasing apart the individual and combined effects of these different abiotic,such as soil nutrients,and biotic factors,such as soil biota and herbivores,on secondary chemicals is critical for understanding plant responses to changing environments.We conducted an experiment at different latitudes in China,using tallow tree(Triadica sebifera)seedlings sourced from a population at 31°N.These seedlings were cultivated in gardens located at low,middle and high latitudes,with either local soil or soil from the original seed collection site(origin soil).The seedlings were exposed to natural levels of aboveground herbivores or had them excluded.Plant secondary chemicals(both foliar and root),aboveground herbivores and soil characteristics were measured.Results showed that most leaf and root secondary metabolites depended on the interaction of the experimental site and soil type.Leaf and root phenolic and tannin concentrations were higher at the middle latitude site,especially in the origin soil.Root and foliar flavonoid concentrations increased when aboveground herbivores were excluded.Microbial communities depended strongly on soil treatment.The different responses of tannins versus flavonoids suggest that these two chemical classes differ in their responses to the varying abiotic and biotic factors in these sites along latitudes.Taken together,our results emphasize the importance of considering the interactive effects of local environmental conditions,soil properties and herbivory in regulating plant chemical defenses.

Soil CO_2 Flux in Different Types of Forests Under a Subtropical Microclimatic Environment

The flux of carbon dioxide （CO2） from soil surface presents an important component of carbon （C） cycle in terrestrial ecosystems and is controlled by a number of biotic and abiotic factors. In order to better understand characteristics of soil CO2 flux （FCO2） in subtropical forests, soil FCO2 rates were quantified in five adjacent forest types （camphor tree forest, Masson pine forest, mixed camphor tree and Masson pine forest, Chinese sweet gum forest, and slash pine forest） at the Tianjiling National Park in Changsha, Hunan Province, in subtropical China, from January to December 2010. The influences of soil temperature （Tsoil）, volumetric soil water content （0soiI）, soil pH, soil organic carbon （SOC） and soil C/nitrogen （N） ratio on soil FCO2 rates were also investigated. The annual mean soil FCO2 rate varied with the forest types. The soil FCO2 rate was the highest in the camphor tree forest （3.53 ± 0.51 μmol m-2 s-I）, followed by, in order, the mixed, Masson pine, Chinese sweet gum, and slash pine forests （1.53 ± 0.25 μmol m-2 sl）. Soil FCO2 rates from the five forest types followed a similar seasonal pattern with the maximum values occurring in summer （July and August） and the minimum values during winter （December and January）. Soil FCO2 rates were correlated to Tsoil and 0soil, but the relationships were only significant for Tsoil. No correlations were found between soil FCO2 rates and other selected soil properties, such as soil pH, SOC, and C/N ratio, in the examined forest types. Our results indicated that soil FCO2 rates were much higher in the evergreen broadleaved forest than coniferous forest under the same microclimatic environment in the study region.

Soil Microbial Activities in Beech Forests Under Natural Incubation Conditions as Affected by Global Warming

Microbial activity in soil is known to be controlled by various factors. However, the operating mechanisms have not yet been clearly identified, particularly under climate change conditions, although they are crucial for understanding carbon dynamics in terrestrial ecosystems. In this study, a natural incubation experiment was carried out using intact soil cores transferred from high altitude(1 500 m) to low(900 m) altitude to mimic climate change scenarios in a typical cold-temperate mountainous area in Japan. Soil microbial activities, indicated by substrate-induced respiration(SIR) and metabolic quotient(q CO2), together with soil physicalchemical properties(abiotic factors) and soil functional enzyme and microbial properties(biotic factors), were investigated throughout the growing season in 2013. Results of principal component analysis(PCA) indicated that soil microbial biomass carbon(MBC) andβ-glucosidase activity were the most important factors characterizing the responses of soil microbes to global warming. Although there was a statistical difference of 2.82 ℃ between the two altitudes, such variations in soil physical-chemical properties did not show any remarkable effect on soil microbial activities, suggesting that they might indirectly impact carbon dynamics through biotic factors such as soil functional enzymes. It was also found that the biotic factors mainly controlled soil microbial activities at elevated temperature,which might trigger the inner soil dynamics to respond to the changing environment. Future studies should hence take more biotic variables into account for accurately projecting the responses of soil metabolic activities to climate change.

Factors Affecting Phytoextraction： A Review

Hyperaccumulators concentrate trace metals and heavy metals in their shoots when grown in metal-contaminated soils and these trace metal-loaded plants may be removed by harvesting the fields. Studies exploring the beneficial role of these hyperaccumulators to clean up the environment have led to the development of phytoextraction. The success of phytoextraction depends upon the high biomass of plant species and bioavailability of metals for plant uptake. The phytoavailability of metals is influenced by soil- associated factors, such as pH, redox potential, cation exchange capacity, soil type, and soil texture, and by plant-associated factors, such as root exudates and root rhizosphere processes （microorganisms）. Efficiency of phytoextraction can be improved by advanced agronomic practices including soil and crop management by application of genetic engineering to enhance the metal tolerance, shoot translocation, accumulation, and sequestration and by application of chelate treatments to enhance metal bioavailability. Application of microorganisms including bacteria and mycorrhiza may facilitate the phytoextraction application at commercially large scale.

Effects of Inoculum Density on Plant Growth and Hydrocarbon Degradation

The combined use of plants and bacteria is a promising approach for the remediation of soil contaminated with organic pollutants. Different biotic and abiotie factors can affect the survival and activity of the applied bacteria and consequently plant growth and phy- toremediation efficiency. The effect of inoculum density on the abundance and expression of alkune-degrading genes in the rhizosphere of plant vegetated in hydrocarbon-contaminated soil has been rarely observed. In this study, an alkane-degrading bacterium （Pantoea sp. strain BTRH79）, at different inoculum densities （10^5 to 10^8 cells cm^-3 soil）, was inoculated to ryegrass （Lolium perenne） vegetated in diesel-contaminated soil to find the optimum inoculum density needed for its efficient colonization and hydrocarbon degradation activity. Bacterial inoculation improved plant growth and hydrocarbon degradation. Maximum plant growth and hydrocarbon degra- dation were observed with the inoculum having the highest cell density （10^8 cells cm^-3 soil）. Moreover, the inoculum with higher cell density exhibited more abundance and expression of alkane hydroxylase gene, CYP153. This study suggests that the inoculum density is one of the main factors that can affect bacterial colonization and activity during phytoremediation.