Average Well Color Development Research Articles

Bacterial soil communities affected by water-repellency

Soil water-repellency is a common phenomenon which occurs throughout the world. It prevents water from infiltrating into the soil and influences soil properties. The effects of water stress induced by water-repellency were investigated with respect to composition and culturability of bacterial communities in soils from an urban park in the centre of Berlin. The bacterial populations of wettable soil areas were compared to populations of repellent soil areas. The genetic diversity was analysed by denaturing gradient gel electrophoresis (DGGE) and the physiological diversity was investigated using plate counts and BIOLOG substrate utilization analysis. The application of cultivation and cultivation independent methods did not allow distinction between the composition of bacterial communities of soil areas with different wettability. Water-repellency did not affect BIOLOG and DGGE fingerprints as investigated by digital image analysis. Similarity analysis of DGGE profiles of the soil community from wettable and repellent zones revealed three distinct clusters of about 74%, 80% and 81% similarity, respectively. BIOLOG fingerprint patterns were similar as determined by average well colour development (AWCD). However, cluster analysis of BIOLOG data generated a dendrogram revealing three distinct clusters showing no separation related to soil wettability. Determination of total cell counts did not result in differences between both zones. Colony forming units (CFUs) showed significantly higher counts in wettable zones than in water-repellent zones. Sequencing of 38 excised bands from different DGGE profiles from wettable and repellent soil samples indicated the presence of typical soil bacteria: the bands were attributed to clones of the classes of Cyanobacteria, Actinobacteria, Chloroflexi and the phylum Firmicutes.

Degradation of phthalate esters (PAEs) in soil and the effects of PAEs on soil microcosm activity

AbstractBACKGROUND: Phthalate esters (PAEs), a class of refractory and toxic organic compounds, are becoming one of the most widespread contaminants in the environment. Degradation of PAEs in soil has been investigated, but limited to one or a few kinds of PAEs. Microorganisms could be regarded as a sensitive bio‐indicator for soil contaminants. Therefore, four commonly used PAEs were chosen to investigate their degradation patterns and potential impacts on soil microbial activity with a series of bioassays.RESULT: PAEs in sterile soils changed slightly, while degradation of PAEs in non‐sterile soil followed a single first‐order kinetic. Higher concentrations of PAEs led to lower β‐glucosidase activity and higher protease activity, with smooth changes of phosphatase and urease activities. PAEs decreased average well color development (AWCD), while Shannon index (H) showed a tendency to increase after a decrease. Carbon utilization profile was affected significantly by PAEs, especially at 10 mg kg−1 soil.CONCLUSION: Degradation of PAEs was driven mainly by microbial mediated processes. PAEs affected carbon, nitrogen and phosphorus cycles variously, and had temporal effects on metabolic diversity, owing to the adaptation of microbes. Carbon substrates utilization changed from easily degradable sugars and carboxylic acids to recalcitrant compounds during the simulation. Copyright © 2010 Society of Chemical Industry

Effect of bioaugmentation by Paracoccus sp. strain HPD-2 on the soil microbial community and removal of polycyclic aromatic hydrocarbons from an aged contaminated soil

A microcosm study was conducted to test the bioremediation potential of Paracoccus sp. strain HPD-2 on an aged PAH-contaminated soil. Bioaugmented microcosms showed a 23.2% decrease in soil total PAH concentrations after 28 days, with a decline in average concentration from 9942 to 7638 μg kg −1 dry soil. The percentage degradation of 3-, 4- and 5(+6)-ring PAHs was 35.1%, 20.7% and 24.3%, respectively. Higher counts of culturable PAH-degrading bacteria, microbial biomass and enzyme activities were observed in bioaugmented soil. The bioaugmented microcosms showed significant increases ( p < 0.05) in the average well-color development (AWCD) obtained by the BIOLOG ecoplate assay and Shannon–Weaver index ( H) compared to the controls. Principal component analysis of BIOLOG data clearly differentiated between the bioaugmented and control microcosms, implying that bioaugmentation restored the microbiological functioning of the PAH-contaminated soil. The results suggest that bioaugmentation by Paracoccus sp. strain HPD-2 may be a promising bioremediation strategy for aged PAH-contaminated soils.

Effects of inoculation with ectomycorrhizal fungi on microbial biomass and bacterial functional diversity in the rhizosphere of Pinus tabulaeformis seedlings

This study investigated the effects of inoculation with three individual ectomycorrhizal (ECM) fungal species on soil microbial biomass carbon and indigenous bacterial community functional diversity in the rhizosphere of Chinese pine ( Pinus tabulaeformis Carr.) seedlings under field experimental conditions. The results showed that ECM fungal inoculation significantly increased the ectomycorrhizal colonization compared with non-inoculated seedlings. ECM fungal inoculations have higher soil microbial biomass carbon than that of control, ranging from 49.6 μg C g −1 dry soil in control to 134.02 μg C g −1 dry soil in treatment inoculated with Boletus luridus Schaeff ex Fr. Multivariate analyses (PCA) of BIOLOG data revealed that the application of ECM fungi significantly influenced bacterial functional diversity in the rhizosphere of P. tabulaeformis seedlings. The highest average well-color development (AWCD) and functional diversity indices were also observed in treatment inoculated with B. luridus. A wider range of sole carbon sources were utilized by the bacterial community in the rhizosphere of inoculated seedlings. The data gathered from this study provides important information for utilization of ECM fungi in forest restoration project in the Northwestern China. The present study will also significantly broaden our understanding of practical importance in the application of ECM fungal inoculum to promote soil microbial community diversity of soil.

Effect of fire and retardant on soil microbial activity and functional diversity in a Mediterranean pasture

The effect of fire and retardants on functional diversity and total activity of soil microorganisms has been studied in a Mediterranean pasture. Soil samples were collected once a month during 1 year after the fire in burnt and un-burnt plots and in one treated with retardant and subsequently burnt. In all cases, samples were taken at two soil depths. Functional diversity was measured by Biolog Eco plates and microbial activity was measured by radioactive labelled thymidine and leucine incorporation technique. In the upper layer, a higher metabolic speed (kinetics of average well colour development (AWCD)) and functional diversity (Shannon's functional diversity) were found irrespective of the treatment (fire or retardant), despite the short distance between them. Results also showed that environmental factors have a greater influence than the treatments (fire and retardant), especially soil moisture. This fact was clearly shown in the seasonal variation of microbial communities throughout the year after the fire, showing a similar successional behaviour irrespective of the treatment. Nevertheless, the techniques used were able to detect changes caused by the fire and, especially, by retardant which is very interesting in public management of fire. Changes caused by the application of retardant to the soils, did not lead to any major adverse effects on the microbial community as compared to the untreated soils. Based on the results, we could hypothesise that bacterial communities studied show a noteworthy grade of fire-accommodation, which can be correlated with the inherent presence of fire in the Mediterranean ecosystems. In particular, we can assume an important resilience and resistance of Mediterranean pastures to fire at the soil bacterial level.

The oxidative degradation of sulfadiazine at the interface of α‐MnO2 and water

AbstractBACKGROUND: Sulfadiazine (SD), a widely used antibiotic chemical, has been detected in contaminated water and soils. Manganese dioxides are common minerals with a high redox potential and can act as active oxidants for SD degradation in wastewater treatment.RESULTS: α‐MnO2 was used to promote SD oxidative degradation. At pH 4.6 and 25 °C, 92.7% of SD (0.02 mmol L−1) was degraded by 1.0 g L−1α‐MnO2. SO42− was detected as the inorganic end product from the mineralization of SD. The ecological toxicity index of average well color development (AWCD) in SD solution was 2.01 after the solution was degraded by α‐MnO2 for 2 h, while the AWCD was 0.48 for the solution without α‐MnO2 treatment. The degradation rate of SD can be improved by increasing the dosages of α‐MnO2 and the reaction temperature, but the rate was limited by increased initial SD concentration and reaction pH.CONCLUSION: SD can be effectively degraded and mineralized with α‐MnO2. These results are helpful for removing antibiotics by manganese dioxides in the environment, and also for exploring new technology for wastewater treatment. Copyright © 2009 Society of Chemical Industry

Evaluation of the metabolic diversity of microbial communities in four different filter layers of a constructed wetland with vertical flow by Biolog™ analysis

The community-level substrate utilization test based on direct incubation of environmental samples in Biolog EcoPlates™ is a suitable and sensitive tool to characterize microbial communities. The aim of this study was to investigate the influence of plant roots and soil structure on the metabolic diversity of microorganisms in a constructed wetland with vertical flow. Sediment samples were taken from different filter depths representing specific filter layers. The color development representing the substrate utilization was measured with the samples over a period of 10 days. The average well color development (AWCD) for all carbon sources was calculated as an indicator of total activity and in order to compensate the influence of the inoculum's density on the color development in the plates. After transformation by dividing by the AWCD, the optical density data were analysed by principal component analysis (PCA). An analysis of the kinetic profile of the AWCD was carried out to increase the analytical power of the method. The corrected data have been successfully fit to the logistic growth equation. Three kinetic model parameters, the asymptote ( K), the exponential rate of color change ( p) and the time to the midpoint of the exponential portion of the curve ( s), were used for statistical analysis of the physiological profile of the microbial community in the different filter layers of the constructed wetland. We found out that in the upper two horizons, which were rooted most densely, mainly easily degradable materials like specific carbohydrates were utilized, while in the lower layers, where only single roots occur, more biochemically inert compounds, e.g. 2-hydroxy benzoic acid, were utilized. Furthermore it could be shown that microorganisms in the surface layer benefited from the plant litter because they can utilize decay products of these. In the lower filter layers specialists took advantage because they had to cope with the biochemically inert materials and the lower nutrient supply.

Effects of the invader Solidago canadensis on soil properties

A perennial goldenrod weed, Solidago canadensis, is rapidly spreading in China and now poses a serious threat to native ecosystem structure and function. Little is known about the effects of S. canadensis invasion on rhizosphere physico-chemical properties and microbial communities. The objective of this study was to compare the soil physico-chemical properties and microbial communities of invaded (two ecotone sites and one monoculture site) and native plant rhizospheres in field areas of Zhejiang Province, Eastern China. Compared with those in the native site, soil total nitrogen, total phosphorus, NO 3-N, available phosphorus content, and aggregate stability consistently decreased with S. canadensis invasion, while soil organic carbon, NH 4-N content, pH, and bulk density in the invaded sites significantly increased. Soil microbial biomass (expressed by carbon, nitrogen, or phosphorus content), activity (basal respiration and substrate induced respiration), and functional diversity (calculated from the average well color development (AWCD) of 31 carbon sources in a BIOLOG Ecoplate) significantly increased with S. canadensis invasion. Microbial utilization of carbohydrate groups significantly increased in the invaded sites, while the utilization of carboxylic acids and amines/amides groups significantly decreased. Principal components analysis (PCA) of the AWCD data indicated that the heavily invaded site (monoculture) was clearly separated from the native site. Redundancy analysis (RDA) indicated that soil organic carbon, NH 4-N, NO 3-N, and pH significantly impacted the dynamics of microbial parameters across the invaded sites. These results suggested that several soil chemical properties (e.g., organic carbon, NH 4-N, and pH) and microbial parameters (e.g., microbial biomass, basal respiration, substrate induced respiration, and functional diversity) might be used as indicators of S. canadensis invasion density.

The effects of mineral fertilizer and organic manure on soil microbial community and diversity

The effects of mineral fertilizer (NPK) and organic manure on phospholipid fatty acid profiles and microbial functional diversity were investigated in a long-term (21-year) fertilizer experiment. The experiment included nine treatments: organic manure (OM), organic manure plus fertilizer NPK (OM + NPK), fertilizer NPK (NPK), fertilizer NP (NP), fertilizer NK (NK), fertilizer N (N), fertilizer P (P), fertilizer K (K), and the control (CK, without fertilization). The original soil was extremely eroded, characterized by low pH and deficiencies of nutrients, particularly N and P. The application of OM and OM + NPK greatly increased crop yields, soil pH, organic C, total N, P and K, available N, P and K content. Crop yields, soil pH, organic C, total N and available N were also clearly increased by the application of mineral NPK fertilizer. The amounts of total PLFAs, bacterial, Gram-negative and actinobacterial PLFAs were highest in the OM + NPK treatment, followed by the OM treatment, whilst least in the N treatment. The amounts of Gram-positive and anaerobic PLFAs were highest in the OM treatment whilst least in the P treatment and the control, respectively. The amounts of aerobic and fungal PLFAs were highest in the NPK treatment whilst least in the N and P treatment, respectively. The average well color development (AWCD) was significantly increased by the application of OM and OM + NPK, and the functional diversity indices including Shannon index (H ′ ), Simpson index (D) and McIntosh index (U) were also significantly increased by the application of OM and OM + NPK. Principal component analysis (PCA) of PLFA profiles and C source utilization patterns were used to describe changes in microbial biomass and metabolic fingerprints from nine fertilizer treatments. The PLFA profiles from OM, OM + NPK, NP and NPK were significantly different from that of CK, N, P, K and NK, and C source utilization patterns from OM and OM + NPK were clearly different from organic manure deficient treatments (CK, N, P, K, NP, NK 6 and NPK). Stepwise multiple regression analysis showed that total N, available P and soil pH significantly affected PLFA profiles and microbial functional diversity. Our results could provide a better understanding of the importance of organic manure plus balanced fertilization with N, P and K in promoting the soil microbial biomass, activity and diversity and thus enhancing crop growth and production.

Evaluation of in situ functional activity of casing soils during growth cycle of mushroom (Agaricus bisporus (Lange) Imbach) employing community level physiological profiles (CLPPs)

Community level physiological profiles (CLPPs) have been rarely applied to mushroom compost ecosystem, probably for the lack of standardized methodology. Recently, however CLPPs have been employed as a tool to investigate the degree of maturity of compost (Mondini and Insam, 2005, Compost Science and Utilization, 13(1): 27-33). The potential of CLPPs to detect compost maturity test is considerably significant in that it provides sensitivity and the simplicity of the assay. The aim of this work was to investigate the maturity of casing that comprised of farm yard manure and spent compost and influence of casing type on the behaviour of bacterial community during the growth cycle of mushroom Agaricus bisporus (Lange) Imbach employing standardized inoculum density and effects of different data interpretation based on the kinetics of colour formation. Casing samples of different age were extracted at a particular dilution and then inoculated in 96 well microtitre plates. Optical density (OD) in well was measured at 590 nm every 24 hours for 5 days. Principal component analysis (PCA) was performed by employing OD values at fixed average well colour development (AWCD). PCA of fresh samples showed that classification and ordination of samples according to their age were significant with fixed AWCD.

Degradation of chlorpyrifos in laboratory soil and its impact on soil microbial functional diversity

Degradation of chlorpyrifos at different concentrations in soil and its impact on soil microbial functional diversity were investigated under laboratory condition. The degradation half-live of chlorpyrifos at levels of 4, 8, and 12 mg/kg in soil were calculated to be 14.3, 16.7, and 18.0 d, respectively. The Biolog study showed that the average well color development (AWCD) in soils was signifficantly ( P < 0.05) inhibited by chlorpyrifos within the ffirst two weeks and thereafter recovered to a similar level as the control. A similar variation in the diversity indices (Simpson index 1/ D and McIntosh index U) was observed, but no signifficant difference among the values of the Shannon-Wiener index H′ was found in chlorpyrifos-treated soils. With an increasing chlorpyrifos concentration, the half-life of chlorpyrifos was signifficantly ( P ≤ 0.05) extended and its inhibitory effect on soil microorganisms was aggravated. It is concluded that chlorpyrifos residues in soil had a temporary or short-term inhibitory effect on soil microbial functional diversity.

Effects of repeated applications of fungicide carbendazim on its persistence and microbial community in soil

Carbendazim, a systemic benzimidazole fungicide, is applied repeatedly to control plant diseases including soilborne diseases, over a growing season. Studies were carried out under laboratory conditions to assess the effects of repeated carbendazim applications on its persistence and microbial community in soil. The results indicate that dissipation of carbendazim in soil was accelerated with its application frequency. The degradation rate constant of carbendazim was increased significantly from 0.074 d(-1) to 0.79 d(-1). The corresponding half-life was shorten markedly from 9.3 d to 0.9 d after four repeated applications. No significant inhibitory effect of carbendazim on soil microbial utilization of the carbon sources was observed after first treatment, but a slight increase in average well color development (AWCD) was shown after second, third, and fourth applications. It suggested that soil microorganisms become adapted to carbendazim after repeated application. Simpson and Shannon indexes of soil microbial community from carbendazim treated soil were also similar to those from the control soil, indicating that the richness and dominant character of soil microorganisms remain unchangeable after repeated application. However, after first, second, and third addition of carbendazim, McIntosh indexes on day 21 were significantly higher compared with the control, suggesting that balance of soil microorganisms was altered due to the enrichment of the specific carbendazim-adapting strains in soil.

Soil Microbial Community Structure in Diverse Land Use Systems: A Comparative Study Using Biolog, DGGE, and PLFA Analyses

Biolog, 16S rRNA gene denaturing gradient gel electrophoresis (DGGE), and phospholipid fatty acid (PLFA) analyses were used to assess soil microbial community characteristics in a chronosequence of tea garden systems (8-, 50-, and 90 year-old tea gardens), an adjacent wasteland, and a 90-year-old forest. Biolog analysis showed that the average well color development (AWCD) of all carbon sources and the functional diversity based on the Shannon index decreased ( P < 0.05) in the following order: wasteland > forest > tea garden. For the DGGE analysis, the genetic diversity based on the Shannon index was significantly lower in the tea garden soils than in the wasteland. However, compared to the 90-year-old forest, the tea garden soils showed significantly higher genetic diversity. PLFA analysis showed that the ratio of Gram positive bacteria to Gram negative bacteria was significantly higher in the tea garden soils than in the wasteland, and the highest value was found in the 90-year-old forest. Both the fungal PLFA and the ratio of fungi to bacteria were significantly higher in the three tea garden soils than in the wasteland and forest, indicating that fungal PLFA was significantly affected by land-use change. Based on cluster analysis of the soil microbial community structure, all three analytical methods showed that land-use change had a greater effect on soil microbial community structure than tea garden age.

Leaf litter decomposition in western Iberian forested wetlands: lentic versus lotic response

Leaf litter decomposition in western Iberian forested wetlands: lentic versus lotic response Leaf litter breakdown is frequently used to measure both structural and functional integrity in aquatic ecosystems. Forested wetlands are interface systems that received little attention, especially in the Iberian Peninsula. The present study compares the decomposition of alder and willow in two biotopes (lentic and lotic) located in two different sites (Caxarias and Alpiarca). Litterbags were used to compare decomposition among the different treatments. Throughout decay, dry weight loss, nitrogen and phosphorus content, microbial community physiological profiles, microbial and macro-invertebrate colonization were measured at days 2, 4, 8, 16, 32 and 64. Decomposition rates ranged from intermediate to rapid for alder (0.006 day −1 to 0.0338 day −1 ) and slow to rapid for willow (0.0024 day −1 to 0.0272 day −1 ), depending on site and biotopes type. The com- bined effects biotope × time was significant for heterotrophs (P < 0.05), molds (P < 0.05) and yeasts (P < 0.001). The analysis of average well colour development (AWCD), richness (R) and Shannon-Wiener index (H � ) based on the oxidation of carbon and nitrogen sources by the microbial community, showed differences between the two biotopes, lotic and lentic, and sites. These differences were also shown by the principal component analysis (PCA). Moreover, the macroinvertebrate commu- nities clearly distinguished between lentic and lotic systems. The invertebrates belonging to Plecoptera, Ephemeroptera and Trichoptera were absent from lentic habitats.

Functional microbial diversity of the railway track bed

Railways constitute relatively unexplored microbial habitats. Little is known about the amounts, activities or distributions of microorganisms and their associated heterotrophic capabilities on railway embankments. The aim of this study was to investigate the microbiology of two Swedish railway tracks in order to fill some of the gaps in the available information. We estimated microbial biomass by means of substrate-induced respiration, microbial activity as basal respiration (BR) and as a kinetically derived parameter ( r) hypothesised to correspond to the active fraction of the microbial biomass. It was confirmed that the microbial biomass and activity were low as compared with agricultural soils and that their distributions were distinctly positively skewed. Spatial Kriging revealed that covariance structures were sustained on a scale smaller than the employed sampling grid (< 1 m). Substrate richness (SR), as measured with Biolog ECO plates, was used as a quantitative measure of functional diversity. SR correlated to microbial activity and SIR, indicating that functional traits were lacking where the microbial biomass was low or less active. The dependence of microbial activities on basic soil characteristics were inferred by separately designed general linear models. Water content was found to be the most important factor moderating basal respiration and functional diversity, whereas the organic matter content was identified as the most important covariate for SIR. Multivariate analysis of the carbon source utilisation patterns of the Biolog plates with equivalent average well-colour development revealed homogenous substrate utilisation among samples. This indicates that the microbial functional potential is randomly distributed in the railway track bed. In combination, our findings imply that the ecosystem functionality of railway embankments may be seriously hampered as compared with agricultural soils. This has consequences for the risk assessment of herbicides applied to railways.

Land use intensification affects soil microbial populations, functional diversity and related suppressiveness of cucumber Fusarium wilt in China’s Yangtze River Delta

The extent of soil microbial diversity in agricultural soils is critical to the maintenance of soil health and quality. The aim of this study was to investigate the influence of land use intensification on soil microbial diversity and thus the level of soil suppressiveness of cucumber Fusarium wilt. We examined three typical microbial populations, Bacillus spp., Pseudomonas spp. and Fuasarium oxysporum, and bacterial functional diversity in soils from three different land use types in China’s Yangtze River Delta, and related those to suppressiveness of cucumber Fusarium wilt. The land use types were a traditional rice wheat (or rape) rotation land, an open field vegetable land, and a polytunnel greenhouse vegetable land that had been transformed from the above two land use types since 1995. Results generated from the field soils showed similar counts for Bacillus spp. (log 5.87–6.01 CFU g−1 dw soil) among the three soils of different land use types, significantly lower counts for Pseudomonas spp. (log 5.44 CFU g−1 dw soil) in the polytunnel greenhouse vegetable land whilst significantly lower counts for Fusarium oxysporum (log 3.21 CFU g−1 dw soil) in the traditional rice wheat (or rape) rotation land. A significant lower dehydrogenase activity (33.56 mg TPF kg−1 dw day−1) was observed in the polytunnel greenhouse vegetable land. Community level physiological profiles (CLPP) of the bacterial communities in soils showed that the average well color development (AWCD) and three functional diversity indices of Shannon index (H′), Simpson index (D) and McIntosh index (U) at 96 h incubation in BIOLOG Eco Micro plates were significantly lower in the polytunnel greenhouse vegetable land than in both the traditional rice wheat (or rape) rotation land and the open field vegetable land. A further greenhouse experiment with the air-dried and sieved soils displayed significantly lower plant growth parameters of 10-old cucumber seedlings as well as significantly lower biomass and total fresh fruit yield at the end of harvesting at day 70 in the polytunnel greenhouse vegetable soil sources. The percentages of Fusarium wilt plant death were greatly increased in the polytunnel greenhouse vegetable plants, irrespective of being inoculated with or without Fusarium oxysporum f. sp. cucumerinum. Our results could provide a better understanding of the effects of land use intensification on soil microbial population and functional diversity as well as the level of soil suppressiveness of cucumber Fusarium wilt.

Impact of allelopathic rice seedlings on rhizospheric microbial populations and their functional diversity

Field performance of rice allelopathic potential is indirectly regulated by the microflora in the rhizosphere. The present study aimed to investigate the dynamics of microbial populations and their functional diversities in the seedling rhizospheres of rice cultivars with varied allelopathic activities by employing agar plate bioassay, fumigation and BIOLOG analysis. Rice cultivars significantly affected the microbial carbon content in their associated rhizospheric soil. The microbial carbon contents were ranked in a decreasing order as Iguape Cateto (441.0 mg·kg –1) > IAC47 (389.7 mg·kg –1) > PI312777 (333.2 mg·kg –1) > Lemont (283.8 mg·kg –1) with the nil-rice control soil of 129.3 mg·kg –1. Similarly, the respiration rate of the soils was 1.404, 1.019, 0.671 and 0.488 μgC·g –1· h –1 for PI312777, Iguape Cateto, IAC47 and Lemont, respectively. The respiration rate was only 0.304 μ gC·g –1·h –1 for the control soil. The microbial flora in the rhizospheric soil of different rice cultivars was dominated by bacteria (58.4%–65.6%), followed by actinomycete (32.2%–39.4%) and fungi (2.2%–2.8%). BIOLOG analysis showed that the value of Average Well Color Development (AWCD) differed significantly among rice cultivars. It was always the highest in the rhizospheric soil of the strongly allelopathic rice cv. PI312777, and the lowest in the rhizospheric soil of the poorly allelopathic rice cv. Lemont. The AWCD value reached the maximum in all the sampled soils after 144 hours of incubation. The AWCD values from the rhizospheric soils of PI312777, IAC47, Iguape Cateto and Lemont were 1.89, 1.79, 1.60 and 1.43 times higher than that of the control soil. Principal Component Analysis (PCA) identified 3 principal component factors (PCF) in relation to carbon sources, accounting for 70.1%, 11.3% and 7.0% of the variation, respectively. 19 categories of carbon sources were significantly positively correlated to the 3 principal components. Phenolic acids, carbohydrates, amino acids and amides were significantly correlated to the principal component 1, phenolic acids, carbohydrates and fatty acids to the principal component 2, and carbohydrates and hydroxylic acids to the principal component 3. Amino acids and amides were the two main carbon sources separating the 3 principal component factors. In addition, the total microbial population in the rhizospheric soil was significantly positively correlated with AWCD, microbial biomass carbon, microbial respiration and Shannon index. There was a significantly positive correlation between the total microbial population and the inhibition rate (IR) on the root length of lettuce owing to the different allelopathic activities of the rice cultivars. These results suggest that changes in microbial population, activity and functional diversity in the rhizospheres are highly cultivar-dependent. These changes might play an important role in governing the rice allelopathic activity in the field.

Influence of tillage, residue management, and crop rotation on soil microbial biomass and catabolic diversity

The densely populated, intensively cropped subtropical highlands of the world have agricultural sustainability problems from soil erosion and fertility decline. In 1991, the International Maize and Wheat Improvement Center (CIMMYT) initiated a long-term field experiment at its semi-arid highland experiment station in Mexico (2240 masl; 19.31°N, 98.50°W; Cumulic Phaeozem) to investigate the long-term effects of tillage/seeding practices, crop rotations, and crop residue management on maize and wheat grown under rainfed conditions. Soil ecology status contributes to agricultural system sustainability, and evaluations were made to determine the effect of different management practices on soil microbial biomass (SMB) (substrate-induced respiration (SIR) and chloroform fumigation incubation (CFI)) and micro-flora physiological and catabolic diversity (BIOLOG™ ecoplate well system). SMB-C (CFI, SIR) was significantly and respectively 1.2 and 1.3 times higher for residue retention (average 387 mg C kg −1 dry soil and 515 mg C kg −1 dry soil , respectively) compared to residue removal. SMB-C (CFI) was significantly higher for wheat (369 mg C kg −1 dry soil ) compared to maize (319 mg C kg −1 dry soil ). SMB-N (CFI) was significantly 1.3 times higher for residue retention (average 28 mg N kg −1 dry soil ) compared to residue removal. The average well color development (AWCD) obtained by the BIOLOG™ ecoplate essay indicated there were large differences in the catabolic capability of soil microbial communities after 15 years of contrasting management practices. While maize and wheat rotation under conventional tillage with residue retention showed a significantly higher overall AWCD value compared to the other treatments, AWCD of maize with zero tillage and residue removal was significantly lower than in the other treatments. AWCD was significantly higher for residue retention compared to residue removal and for wheat as compared to maize. For maize, the management practices were divided into two groups; zero tillage with residue removal was separate from all other treatments. For wheat, conventional tillage was separate from all zero tillage treatments. This study suggests that in the target area, a cropping system that includes zero tillage, crop rotation, and crop residue retention can increase overall biomass and micro-flora activity and diversity compared with common farming practices. In the long term, zero tillage combined with residue retention creates conditions favourable for the development of antagonists and predators, and fosters a new ecological stability. Zero tillage without residue retention is an unsustainable practice that leads to poor soil health in the long run.

Structural and functional diversity of a culturable bacterial community during the early stages of revegetation near a Pb/Zn smelter in Guangdong, PR China

The land near a Pb/Zn smelter in the vicinity of Shaoguan city of Southern China, contaminated with heavy metals by smelting activities, was revegetated using an indigenous plant ( Paulownia fortunei). The structure and function of the soil bacterial community were investigated to examine whether such microbial indexes could be used as indicators of changes in abiotic environment and vegetative parameters. Soil samples were analyzed for bacterial community level physiological profiles (CLPP) and molecular fingerprinting of 16S rRNA genes. The average well color development (AWCD), functional diversity ( H F), and carbon substrate richness ( R S) were high in the two young revegetated sites (revegetated for 2 and 3 years, respectively), and low in the old revegetated site (revegetated for 5 years). The structural diversity ( H S) and ribotype ( R T) showed an identical trend. Changes in the culturable bacterial community structure and function were not correlated with the revegetation age or heavy metal content, but were closely correlated with a number of soil physico-chemical and vegetative parameters. Therefore, the results suggest that the structural and functional parameters of soil culturable bacterial community may be used as sensitive indicators of changes in soil physico-chemical and vegetative parameters during the early stage of revegetation in this area that has been chronically contaminated by heavy metals.

Effects of soil moisture and plant interactions on the soil microbial community structure

A greenhouse pot experiment was conducted to investigate the influence of soil moisture content and plant species on soil microbial community structure using cultivation-independent methods. White clover and ryegrass were grown individually or in a mixture. Plants were subjected to soil moisture content corresponding to 60% field capacity (FC) and 80% FC. Total plant biomass of white clover and ryegrass increased with increasing soil moisture contents. At a given soil moisture content, total biomass of white clover was lower in the ryegrass–clover (RC) mixture compared with those grown individually, while total biomass of ryegrass was higher. Microbial community structure assessed by phospholipid fatty acid analysis (PLFA) was more affected by plant species than soil moisture. Community level physiological profiles (CLPP), in terms of diversity of substrate utilization and average well colour development (AWCD) were affected by plant species and soil moisture. Soil moisture effects were more pronounced in clover than in ryegrass. AWCD and diversity of substrate utilization in the ryegrass–clover mix were similar to those of sole clover while they differed from that of ryegrass suggesting a dominant effect of clover in the mix.