Low Microbial Biomass Research Articles

A comprehensive study of the impact of polycyclic aromatic hydrocarbons (PAHs) contamination on salt marsh plants Spartina alterniflora: implication for plant-microbe interactions in phytoremediation.

These pot experiments aimed to investigate the effects of polycyclic aromatic hydrocarbons (PAHs) on plant uptake, rhizophere, endophytic bacteria, and phytoremediation potentials of contaminated sediments. Salt marsh plant Spartina alterniflora was selected and cultivated in phenanthrene (PHE)- and pyrene (PYR)-contaminated sediments (for 70 days). The results indicated that the amount of PHE removed from the sediments ranged from 13 to 36 %, while PYR ranged from 11 to 30 %. In rhizophere sediment, dehydrogenase activities were significantly (P < 0.05) enhanced by higher concentration of PHE treatments, while polyphenol oxidase activities were prohibited more than 10 % in non-rhizophere sediment. Compared with the control, PHE treatments had also significantly (P < 0.05) lower total microbial biomass; especially for gram-negative bacteria, this decrease was more than 24 %. However, the PYR treatments had little effect on the dehydrogenase, polyphenol oxidase, and total phospholipid fatty acid analysis (PLFA) biomass. The greatest abundance of PAH-ring hydroxylating dioxygenases isolated from gram-negative bacteria (PAH-RHDα-GN) of rhizoplane and endophyte in roots were found at high concentration of PHE treatments and increased by more than 100- and 3-fold, respectively. These results suggested that PAH pollution would result in the comprehensive effect on S. alterniflora, whose endophytic bacteria might play important roles in the phytoremediation potential of PAH-contaminated sediments.

Arsenic-Rich Acid Mine Water with Extreme Arsenic Concentration: Mineralogy, Geochemistry, Microbiology, and Environmental Implications

Extremely arsenic-rich acid mine waters have developed by weathering of native arsenic in a sulfide-poor environment on the 10th level of the Svornost mine in Jáchymov (Czech Republic). Arsenic rapidly oxidizes to arsenolite (As2O3), and there are droplets of liquid on the arsenolite crust with high As concentration (80,000-130,000 mg·L(-1)), pH close to 0, and density of 1.65 g·cm(-1). According to the X-ray absorption spectroscopy on the frozen droplets, most of the arsenic is As(III) and iron is fully oxidized to Fe(III). The EXAFS spectra on the As K edge can be interpreted in terms of arsenic polymerization in the aqueous solution. The secondary mineral that precipitates in the droplets is kaatialaite [Fe(3+)(H2AsO4)3·5H2O]. Other unusual minerals associated with the arsenic lens are běhounekite [U(4+)(SO4)2·4H2O], štěpite [U(4+)(AsO3OH)2·4H2O], vysokýite [U(4+)[AsO2(OH)2]4·4H2O], and an unnamed phase (H3O)(+)2(UO2)2(AsO4)2·nH2O. The extremely low cell densities and low microbial biomass have led to insufficient amounts of DNA for downstream polymerase chain reaction amplification and clone library construction. We were able to isolate microorganisms on oligotrophic media with pH ∼ 1.5 supplemented with up to 30 mM As(III). These microorganisms were adapted to highly oligotrophic conditions which disabled long-term culturing under laboratory conditions. The extreme conditions make this environment unfavorable for intensive microbial colonization, but our first results show that certain microorganisms can adapt even to these harsh conditions.

Reagent and laboratory contamination can critically impact sequence-based microbiome analyses.

BackgroundThe study of microbial communities has been revolutionised in recent years by the widespread adoption of culture independent analytical techniques such as 16S rRNA gene sequencing and metagenomics. One potential confounder of these sequence-based approaches is the presence of contamination in DNA extraction kits and other laboratory reagents.ResultsIn this study we demonstrate that contaminating DNA is ubiquitous in commonly used DNA extraction kits and other laboratory reagents, varies greatly in composition between different kits and kit batches, and that this contamination critically impacts results obtained from samples containing a low microbial biomass. Contamination impacts both PCR-based 16S rRNA gene surveys and shotgun metagenomics. We provide an extensive list of potential contaminating genera, and guidelines on how to mitigate the effects of contamination.ConclusionsThese results suggest that caution should be advised when applying sequence-based techniques to the study of microbiota present in low biomass environments. Concurrent sequencing of negative control samples is strongly advised.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-014-0087-z) contains supplementary material, which is available to authorized users.

Linking Soil Biotic and Abiotic Factors to Apple Replant Disease: a Greenhouse Approach

AbstractApple replant disease (ARD) is a frequently occurring plant disease, which causes retarded growth and mortality of young apple trees in replanted orchards. The aetiology is not well understood, but soil‐borne micro‐organisms are often discussed as primary causal agents of the replant problem. A greenhouse study was conducted in Laimburg, Italy, with orchard soils from the region, with the aim of obtaining information about the influence of soil biotic and abiotic factors on the aetiology of the disease. Apple rootstocks (M9) were planted into soils cultivated with apple trees that were either fumigated with chloropicrin or not fumigated, as well as mixtures of fumigated and non‐fumigated soils. In addition, uncultivated soils (from the inter‐row, from a fallow plot and from a meadow) were taken as controls. Various parameters were measured after 62 days in a controlled pot assay. Soils fumigated with chloropicrin resulted in higher apple shoot growth and lower microbial biomass carbon than non‐fumigated soils. Uncultivated soils had generally the highest microbial biomass carbon and the highest ergosterol contents. No considerable differences between basal respiration, ergosterol content, pH, electrical conductivity, and most nutrient and metal contents were observed between fumigated and non‐fumigated soils. Denaturing gradient gel electrophoresis gels of DNA extracted from the soils revealed differences in the fungal, bacterial and actinobacterial communities of the different soils, indicating significant shifts in microbial community composition after chloropicrin treatment. This study indicates biotic factors in soil to be a causal agent of apple replant disease.

Long term effects of nutrient management regimes on abundance of bacterial genes and soil biochemical processes for fertility sustainability in a semi-arid tropical Alfisol

Semi-arid tropical soils have low soil organic carbon and microbial biomass carbon with limited available macro- and micronutrients for crop plants. These soils may have future yield sustainability problems due to intensive cropping, erosion and limited adoption of nutrient management strategies. Therefore monitoring the abundance, community shift and physiological status of the soil microbiome in response to agricultural practices is vital to understand soil fertility sustainability. We measured the long term effects of organic and inorganic nutrient management practices on bacterial communities and soil biochemical processes. Samples were from a long term nutrient experiment field trial on an Alfisol established in 1909. The abundance of bacterial 16S rRNA, nifH and amoA genes was measured in soil samples for two consecutive years, 2009 and 2010 using quantitative real-time polymerase chain reaction (qPCR). The 16S rRNA and nifH genes were abundant in organically managed soil while the inorganic nutrient amendments increased the abundance of amoA gene. Respiration studies indicated that organic management encouraged higher substrate induced respiration rates compared to inorganically fertilized and control soils. However, the metabolic quotient of the soil remained unaffected by long term nutrient management regimes. The inorganic fertilizer amended soil showed higher nitrification potential compared to organically managed and control soils. Multivariate regression analysis revealed that soil organic carbon, microbial biomass carbon and substrate induced respiration rate significantly drive the abundance of 16S rRNA and nifH gene; while for amoA, the drivers were available nitrogen, substrate induced respiration rate and nitrification potential. Our results show that long term organic and inorganic nutrient managements had strong influences on the abundance of key bacterial genes and related biochemical processes in a semi-arid tropical Alfisol. This study further underlines the importance of fertility management options for improving or sustaining the biological properties of tropical soils.

Adaptation of microbial communities to multiple stressors associated with litter decomposition of Pterocarya stenoptera

To understand the further impacts of multiple stressors in freshwater, we investigated the effects of heavy metal (HM, Cu and Zn) and nutrient enrichments (nitrogen and phosphorus, NP) on microbial decomposition of Pterocarya stenoptera litter and the associated extracellular enzyme activities and microbial biomass with microcosms. Results showed that the decomposition rates were slower in the polluted stream waters than those in the unpolluted ones, which corresponded to lower microbial biomass and integrated enzyme activities of cellulose and β-glucosidase. The decomposition rates were accelerated at low HM level, which was associated with the stimulated enzyme activities of hydrolytic enzymes or was stimulated by both NP levels in polluted stream waters. In particular, the hydrolase enzyme activities of microbial communities in polluted stream waters were stimulated by low HM level, suggesting that low HM level-stimulated litter decomposition may be due to the increased enzymatic activities. When microbial communities were exposed to HM and NP simultaneously, the inhibitory effect (in unpolluted stream waters) or the stimulated effect (in polluted stream waters) of low HM concentration was enhanced and attenuated, respectively, which suggests that the NP antagonistic effect against HM toxicity on litter decomposition may contribute to the litter-associated extracellular enzyme activities. These results suggest that the co-occurrence of HM and NP may have antagonistic effects on stream ecosystem functioning.

Microbial Community Structures of Novel Icelandic Hot Spring Systems Revealed by PhyloChip G3 Analysis

Microbial community profiles of recently formed hot spring systems ranging in temperatures from 57°C to 100°C and pH values from 2 to 4 in Hveragerði (Iceland) were analyzed with PhyloChip G3 technology. In total, 1173 bacterial operational taxonomic units (OTUs) spanning 576 subfamilies and 38 archaeal OTUs covering 32 subfamilies were observed. As expected, the hyperthermophilic (∼100°C) spring system exhibited both low microbial biomass and diversity when compared to thermophilic (∼ 60°C) springs. Ordination analysis revealed distinct bacterial and archaeal diversity in geographically distinct hot springs. Slight variations in temperature (from 57°C to 64°C) within the interconnected pools led to a marked fluctuation in microbial abundance and diversity. Correlation and PERMANOVA tests provided evidence that temperature was the key environmental factor responsible for microbial community dynamics, while pH, H2S, and SO2 influenced the abundance of specific microbial groups. When archaeal community composition was analyzed, the majority of detected OTUs correlated negatively with temperature, and few correlated positively with pH.

Changes in Soil Microbial Biomass and Residual Indices as Ecological Indicators of Land Use Change in Temperate Permanent Grassland

The relationship between microbial biomass, residues and their contribution to microbial turnover is important to understand ecosystem C storage. The effects of permanent grassland (100 % ryegrass--PG), conversion to modified grassland (mixture of grass and clover--MG) or maize monoculture (MM) on the dynamics of soil organic C (SOC), microbial biomass, fungal ergosterol and microbial residues (bacterial muramic acid and fungal glucosamine) were investigated. Cattle slurry was applied to quantify the effects of fertilisation on microbial residues and functional diversity of microbial community across land use types. Slurry application significantly increased the stocks of microbial biomass C and S and especially led to a shift in microbial residues towards bacterial tissue. The MM treatment decreased the stocks of SOC, microbial biomass C, N and S and microbial residues compared with the PG and MG treatments at 0-40 cm depth. The MM treatment led to a greater accumulation of saprotrophic fungi, as indicated by the higher ergosterol-to-microbial biomass C ratio and lower microbial biomass C/S ratio compared with the grassland treatments. The absence of a white clover population in the PG treatment caused a greater accumulation of fungal residues (presumably arbuscular mycorrhizal fungi (AMF), which do not contain ergosterol but glucosamine), as indicated by the significantly higher fungal C-to-bacterial C ratio and lower ergosterol-to-microbial biomass C ratio compared with the MG treatment. In addition to these microbial biomass and residual indices, the community level physiological profiles (CLPP) demonstrated distinct differences between the PG and MG treatments, suggesting the potential of these measurements to act as an integrative indicator of soil functioning.

Soil aggregation in a semiarid soil amended with composted and non-composted sewage sludge—A field experiment

The addition of exogenous organic matter derived from organic wastes has been widely used to restore semiarid, degraded soils, favoring the development of plant cover. After organic amendment, soil particle aggregation can increase due to the increase of organic matter, plant roots and microbially derived compounds. However, the effect of organic amendment addition over time on aggregate formation is not clear. The aim of this work was to study soil particle aggregation of a semiarid, degraded soil 5years after the addition of a sole dose (120Mgha−1) of non-composted (SS) and composted sewage sludge (cSS). For this purpose, soils were separated into four aggregate-size fractions (8000–2000, 2000–250, 250–53 and <53μm) by wet sieving. Macroaggregates were further separated into coarse particulate organic matter (>250μm), microaggregrates within macroaggregates (250–53μm) and silt and clay (<53μm). In addition, phospholipid fatty acids (PLFAs) of the treated soils were measured. After 5years, the amended soils had a higher plant cover and greater amounts of large macroaggregates and microaggregates within macroaggregates than the control soil. Soil organic carbon (OC) was preferentially protected in the microaggregates within macroaggregates of the SS-treated soils, which tended to have higher fungal biomass than the rest of treatments. In contrast, the cSS-treated soils tended to have a lower microbial biomass than the SS-treated soils and showed the greatest relative enrichment of OC in the coarse particulate organic matter occluded in the macroaggregates. These results indicate that, depending on the pretreatment of the sewage sludge, the protection of OC over time occurs in different soil aggregate-size fractions, promoting a higher aggregate-associated OC in the composted amendments as coarse particulate organic matter.

Índice de calidad del suelo en áreas cultivadas con banano en Panamá.

The objective of this research was to obtain anindex that could serve as a useful tool for environmentalassessment and managing agricultural banana soils inPanama. The work was performed in five independent farmsfrom Alanje District and six properties belonging to theCooperative COOSEMUPAR in Baru (2005-2006). The soilsbelong to the orders Inceptisols (Andic Dystrudepts, Udichaplustepts and Dystric haplustepts) and Entisols (AndicUdifluvents). For the description of the indicators, four pitswere opened for each area of high and low productivity inthe band of fertilization of recently flowered banana plants.Biometric data were recorded of twenty banana plantsnear harvest. In addition, each site was measured for basicinfiltration, total weight of roots and sampled to determinethe physico-chemical and biological properties of the soil.In all farms high levels of Ca, Mg and K were detected,low concentrations of organic matter associated with lowbiological activity and low microbial biomass. In most farmsinfiltration was slow and degraded structures were present inthe lower horizons. The Minimum Data Set (MDS) that bestpredicted areas of high and low productivity was composedof the following indicators: percentage of sand, pH, Ca, K,organic matter, microbial respiration, mineralization rate andtotal weight of roots.

Effect of wildfires on soil respiration in three typical Mediterranean forest ecosystems in Madrid, Spain

Mediterranean forests are vulnerable to numerous threats including wildfires due to a combination of climatic factors and increased urbanization. In addition, increased temperatures and summer drought lead to increased risk of forest fires as a result of climate change. This may have important consequences for C dynamics and balance in these ecosystems. Soil respiration was measured over 2 successive years in Holm oak (Quercus ilex subsp. ballota; Qi); Pyrenean Oak (Quercus pyrenaica Willd; Qp); and Scots pine (Pinus sylvestris L.; Ps) forest stands located in the area surrounding Madrid (Spain), to assess the long term effects of wildfires on C efflux from the soil, soil properties, and the role of soil temperature and soil moisture in the variation of soil respiration. Soil respiration, soil temperature, soil moisture, fine root mass, microbial biomass, biological and chemical soil parameters were compared between non burned (NB) and burned sites (B). The annual C losses through soil respiration from NB sites in Qi, Qp and Ps were 790, 1010, 1380 gCm−2 yr−1, respectively, with the B sites emitting 43 %, 22 % and 11 % less in Qi, Qp and Ps respectively. Soil microclimate changed with higher soil temperature and lower soil moisture in B sites after fire. Exchangeable cations and the pH also decreased. The total SOC stocks were not significantly altered, but 6–8 years after wildfires, there was still measurably lower fine root and microbial biomass, while SOC quality changed, indicated by lower the C/N ratio and the labile carbon and a relative increase in refractory SOC forms, which resulted in lower Q10 values. We found long term effects of wildfires on the physical, chemical and biological soil characteristics, which in turn affected soil respiration. The response of soil respiration to temperature was controlled by moisture and changed with ecosystem type, season, and between B and NB sites. Lower post-burn Q10 integrated the loss of roots and microbial biomass, change in SOC quality and a decrease in soil moisture.

High concentrations of single-walled carbon nanotubes lower soil enzyme activity and microbial biomass

Nanomaterials such as single-walled carbon nanotubes (SWCNTs) may enter the soil environment with unknown consequences resulting from the development of nanotechnology for a variety of applications. We determined the effects of SWCNTs on soil enzyme activity and microbial biomass through a 3-week incubation of urban soils treated with different concentrations of SWCNTs ranging from 0 to 1000μgg−1 soil. The activities of cellobiohydrolase, β-1,4-glucosidase, β-1,4-xylosidase, β-1,4-N-acetylglucosaminidase, l-leucine aminopeptidase, and acid phosphatase and microbial biomass were measured in soils treated with powder and suspended forms of SWCNTs. SWCNTs of concentrations at 300–1000μgg−1 soil significantly lowered activities of most enzymes and microbial biomass. It is noteworthy that the SWCNTs showed similar effects to that of multi-walled carbon nanotubes (MWCNTs), but at a concentration approximately 5 times lower; we suggest that this is mainly due to the higher surface area of SWCNTs than that of MWCNTs. Indeed, our results show that surface area of CNTs has significant negative relationship with relative enzyme activity and biomass, which suggests that greater microorganism—CNT interactions could increase the negative effect of CNTs on microorganisms. Current work may contribute to the preparation of a regulatory guideline for the release of CNTs to the soil environment.

SOIL MICROBIAL BIOMASS AND NITROGEN MINERALIZATION RATES ALONG AN ALTITUDINAL GRADIENT ON THE COFRE DE PEROTE VOLCANO (MEXICO): THE IMPORTANCE OF LANDSCAPE POSITION AND LAND USE

A study was conducted to examine the responses of microbial activity and nitrogen (N) transformations along an altitudinal gradient. The gradient was divided into three parts. Three areas were sampled: upper part (UP): coniferous forest, corn field, and abandoned corn field; middle part (MP): tropical cloud forest, grassland, and corn field (COL); and lower part (LP): tropical deciduous forest and sugarcane. The results showed that soil microbial biomass carbon (C) and basal respiration were significantly higher in MP and UP than in LP, whereas the microbial quotient (Cmic/Corg) was higher in LP and MP than in UP. The metabolic quotient (qCO2) was similar among gradient parts evaluated. Net N mineralization, ammonification, and nitrification rates were higher in UP than MP and LP. We found that in UP, the forest conversion to cropland resulted in no significant differences in microbial activity and N transformation rates between land uses. In MP, microbial biomass C, ammonification, and net N mineralization rates decreased significantly with conversion to cropland, but Cmic/Corg and nitrification were higher in COL. Basal respiration and qCO2 were significantly lower in COL when compared with other land uses. In LP, lower microbial biomass C, Cmic/Corg, and nitrification rates but higher ammonification and net N mineralization rates were observed in tropical deciduous forest than in sugarcane. No significant differences in basal respiration and qCO2 were found between uses of LP. Clearly, then, soil organic C is not equally accessible to the microbial community along the gradient studied. Copyright © 2012 John Wiley & Sons, Ltd.

Response of microbial activity and biomass to increasing salinity depends on the final salinity, not the original salinity

Salinization is a global land degradation issue which inhibits microbial activity and plant growth. The effect of salinity on microbial activity and biomass has been studied extensively, but little is known about the response of microbes from different soils to increasing salinity although soil salinity may fluctuate in the field, for example, depending on the quality of the irrigation water or seasonally. An incubation experiment with five soils (one non-saline, four saline with electrical conductivity (ECe) ranging from 1 to 50 dS m−1) was conducted in which the EC was increased to 37 ECe levels (from 3 to 119 dS m−1) by adding NaCl. After amendment with 2% (w/w) pea straw to provide a nutrient source, the soils were incubated at optimal water content for 15 days, microbial respiration was measured continuously and chloroform-labile C was determined every three days. Both cumulative respiration and microbial biomass (indicated by chloroform-labile C) were negatively correlated with EC. Irrespective of the original soil EC, cumulative respiration at a given adjusted EC was similar. Thus, microorganisms from previously saline soils were not more tolerant to a given adjusted EC than those in originally non-saline soil. Microbial biomass in all soils increased from day 0 to day 3, then decreased. The relative increase was greater in soils which had a lower microbial biomass on day 0 (which were more saline). Therefore the relative increase in microbial biomass appears to be a function of the biomass on day 0 rather than the EC. Hence, the results suggest that microbes from originally saline soils are not more tolerant to increases in salinity than those from originally non-saline soils. The strong increase in microbial biomass upon pea straw addition suggests that there is a subset of microbes in all soils that can respond to increased substrate availability even in highly saline environments.

Effects of a triazole fungicide and a pyrethroid insecticide on the decomposition of leaves in the presence or absence of macroinvertebrate shredders

Previously, laboratory experiments have revealed that freely diluted azole fungicides potentiate the direct toxic effect of pyrethroid insecticides on Daphnia magna. More ecologically relevant exposure scenarios where pesticides are adsorbed have not been addressed. In this study we exposed beech leaves (Fagus sylvatica) to the azole fungicide propiconazole (50 or 500μgL−1), the pyrethroid insecticide alpha-cypermethrin (0.1 or 1μgL−1) or any combination of the two for 3h. Exposed leaves were transferred to aquaria with or without an assemblage of macroinvertebrate shredders, and we studied treatment effects on rates of microbial leaf decomposition, microbial biomass (using C:N ratio as a surrogate measure) and macroinvertebrate shredding activity during 26 days post-exposure. Microbial leaf decomposition rates were significantly reduced in the propiconazole treatments, and the reduction in microbial activity was significantly correlated with loss of microbial biomass (increased C:N ratio). Macroinvertebrate shredding activity was significantly reduced in the alpha-cypermethrin treatments. In addition, the macroinvertebrate assemblage responded to the propiconazole treatments by increasing their consumption of leaf litter with lower microbial biomass, probably to compensate for the reduced nutritional quality of this leaf litter. We found no interaction between the two pesticides on macroinvertebrate shredding activity, using Independent Action as a reference model. In terms of microbial leaf decomposition rates, however, alpha-cypermethrin acted as an antagonist on propiconazole. Based on these results we emphasise the importance of considering indirect effects of pesticides in the risk assessment of surface water ecosystems.

Nutrient addition retards decomposition and C immobilization in two wet grasslands

Eutrophication is one of the biggest environmental problems facing wetlands. However, its effect on soil functioning is not yet well understood. We tested the hypothesis that increased nutrient loading into wet grassland ecosystems accelerates soil C and N cycles and decreases microbial immobilization of C and N. Experimental sites were established on two wet grasslands, with either mineral or peaty soils, and fertilized by NPK fertilizer for 3 years. Soils were analyzed for soluble and microbial C and N contents and their transformations, profile of phospholipid fatty acids and number of nirK denitrifiers. Fertilization affected C more than N transformations. Opposite to what was predicted, decomposition was retarded, the soil C cycle was based more on labile C compounds, and the soil was more susceptible to C losses in fertilized versus unfertilized treatments in both soils. Fertilization resulted in lower microbial biomass C and microbial C immobilization and also decreased the activity of lignin-degrading enzymes. Shifts in the composition of the microbial communities led to decreased (1) decomposition of complex organic compounds and (2) immobilization of transformed C. Net nitrification and microbial N immobilization tended to increase in fertilized treatments indicating an acceleration of soil N cycling and losses, but only in the more vulnerable organic soil.

Aging of methabenzthiazuron, imidacloprid, and N,N‐dimethylsulfamide in silty soils and effects on sorption and dissipation

Differences in soil properties can influence the fate of plant protection agents in the environment. The present study aims to investigate the sorption behavior and related aging processes of imidacloprid (IMI; insecticide), methabenzthiazuron (MBT; herbicide), and N,N-dimethylsulfamide (DMSA; degradate of the fungicide tolylfluanid) in six soils of silty texture but otherwise varying properties. The sorption behavior of these ¹⁴C-labeled compounds exhibiting different physicochemical properties was characterized by applying a three-step sequential extraction procedure. After 119 d, MBT revealed strongest sorption (K'(tot) 47.4-200.4 L/kg), followed by IMI (K'(tot) 11.7-30.6 L/kg), and DMSA with K'(tot) close to zero. Aged sorption factors (AFs) were calculated to characterize aging processes over time exhibiting a 2.6-3.5-fold (IMI), a 1.8-4.5-fold (MBT), and no (DMSA) increase of sorbed amounts within 84 d. Sorption and aging varied widely in the group of silty soils, which differed with respect to organic matter content, C/N-ratio, and microbial soil parameters. The time-dependent increase of adsorption of MBT and IMI was more pronounced in those soils that had a lower organic carbon and low microbial biomass content. Concomitantly, MBT and IMI degradation decelerated, presumably because of aged sorption at inner binding sites leading to a lower accessibility. In contrast, in the soils with a higher organic carbon content a strong initial (but later still reversible) sorption of MBT and IMI, occurring presumably at outer surface sites, reduced the extent of time-dependent diffusion toward inner binding sites.

Bacterivory by phototrophic picoplankton and nanoplankton in Arctic waters

Mixotrophy, the combination of phototrophy and heterotrophy within the same individual, is widespread in oceanic systems. Yet, neither the presence nor ecological impact of mixotrophs has been identified in an Arctic marine environment. We quantified nano- and picoplankton during early autumn in the Beaufort Sea and Canada Basin and determined relative rates of bacterivory by heterotrophs and mixotrophs. Results confirmed previous reports of low microbial biomass for Arctic communities in autumn. The impact of bacterivory was relatively low, ranging from 0.6 × 10(3) to 42.8 × 10(3) bacteria mL(-1) day(-1) , but it was often dominated by pico- or nanomixotrophs. From 1% to 7% of the photosynthetic picoeukaryotes were bacterivorous, while mixotrophic nanoplankton abundance comprised 1-22% of the heterotrophic and 2-32% of the phototrophic nanoplankton abundance, respectively. The estimated daily grazing impact was usually < 5% of the bacterial standing stock, but impacts as high as 25% occurred. Analysis of denaturing gradient gel electrophoresis (DGGE) band patterns indicated that communities from different depths at the same site were appreciably different and that there was a shift in community diversity at the midpoint of the cruise. Sequence information from DGGE bands reflected microbes related to those from other Arctic studies, particularly from the Beaufort Sea.

Plant and Soil Responses to High and Low Diversity Grassland Restoration Practices

The USDA's Conservation Reserve Program (CRP) has predominantly used only a few species of dominant prairie grasses (CP2 practice) to reduce soil erosion, but recently has offered a higher diversity planting practice (CP25) to increase grassland habitat quality. We quantified plant community composition in CP25 and CP2 plantings restored for 4 or 8years and compared belowground properties and processes among restorations and continuously cultivated soils in southeastern Nebraska, USA. Relative to cultivated soils, restoration increased soil microbial biomass (P=0.033), specifically fungi (P<0.001), and restored soils exhibited higher rates of carbon (C) mineralization (P=0.010). High and low diversity plantings had equally diverse plant communities; however, CP25 plantings had greater frequency of cool-season (C(3)) grasses (P=0.007). Older (8 year) high diversity restorations contained lower microbial biomass (P=0.026), arbuscular mycorrhizal fungi (AMF) biomass (P=0.003), and C mineralization rates (P=0.028) relative to 8year low diversity restorations; older plantings had greater root biomass than 4year plantings in all restorations (P=0.001). Low diversity 8year plantings contained wider root C:N ratios, and higher soil microbial biomass, microbial community richness, AMF biomass, and C mineralization rate relative to 4year restorations (P<0.050). Net N mineralization and nitrification rates were lower in 8year than 4year high diversity plantings (P=0.005). We attributed changes in soil C and N pools and fluxes to increased AMF associated with C(4) grasses in low diversity plantings. Thus, reduced recovery of AMF in high diversity plantings restricted restoration of belowground microbial diversity and microbially-mediated soil processes over time.

Nitrification denitrification enhanced biological phosphorous removal (NDEBPR) occurs in a lab-scale alternating hypoxic/oxic membrane bioreactor

Strict anaerobic or anoxic maintenance of the system and process susceptibility to low organic loading are major concerns in nitrification denitrification enhanced biological phosphorous removal (NDEBPR). The study has initiated NDEBPR in a lab-scale alternating hypoxic/oxic membrane bioreactor by developing an enhanced mixed microbial culture capable of removing 97±2% COD, 99±0.84% NH3-N, 90±3% TN, and 96±1% TP-PO43- with 20-day SRT. The viable cells ranging from 1.6×108 to 2.0×108cells/ml estimated from the total bacterial genomic DNA (6.43–7.83μgDNA/ml) represented only 5% of the MLVSS indicating low microbial biomass concentration. Reducing the organic load from 1250 to 750mgCOD/ml as glucose did not deteriorate the effluent quality (3.77±1.0mgN-TN/l; 0.08±0.24mgNH3-N/l; and 0.32±0.10mgPO43--P/l). These observations are characteristics of activated sludge that harbors denitrifying polyphosphate accumulating organisms (DPAOs). The results showed that NDEBPR can be achieved under alternating hypoxic/oxic conditions.