Selected forage and soil conservation grasses and legumes were evaluated in the greenhouse for their abilities to stimulate dissipation of PCB, TNT, and pyrene in a soil. The grasses tested were tall fescue (Festuca arundinacea Schreb), reed canarygrass( Phalaris arundinacea L.), switchgrass ( Panicum variegatum L.), and deertongue (Panicum clandestinum L.). The legumes were alfalfa (Medicago sauva L.), crownvetch (Coronilla varia L.), sericea lespedeza (Lespedeza cuneata Dum-Cours.), and flatpea (Wagner pea) ( Lathyrus sylvestris L). The plants were grown in 13.5x15.0 cm pots containing soils that were fortified with 100 mg/kg nominal concentrations of the respective compounds andaged in the laboratory. After six months, 51% or less of an initial 100 mg/kg dose of aroclor 1248 was recovered from soils planted with reed canarygrass, switchgrass, and flatpea. Between 64-70% of the initial dose was recovered from soils planted with tall fescue, deer tongue, and sericea lespedeza, and about 80 % 1646 KUDJO DZANTOR, CHEKOL, AND VOUGH or more was recovered from soils that were planted with alfalfa and crownvetch as well as soil that was left implanted. During the same period, <0.5% of the initial dose of TNT and <3% of pyrene were recovered from soils that were fortified with those compounds, including unplanted controls. Laboratory flask experiments that compared the dissipation of TNT and pyrene in natural soils and soils containing microbial inhibitor suggested that microbial transformation accounted for a major portion of the loss of TNT and pyrene in this soil. A comparison of the dissipation of TNT and pyrene in two different soils reinforces previously well-documented strong role of organic matter in the overall fate of TNT and pyrene in soil.
Phytoremediation has emerged as the method of choice for cleaning up a broad range of environmental contaminants. One process through which plants render some xenobiotic organic contaminants innocuous in soil involves plant-microbe interactions in which root exudates stimulate entire microbial communities, or induce specific enzymes in competent individuals to cause enhanced rhizodegradation. For some contaminants these inherent processes can be slow; however, potentials exist for their improvement through rhizosphere manipulations. Although this requires a greater understanding than currently exists with respect to plant and microbe components and interactions involved in the biodegradation of xenobiotic contaminants, improved understanding is being achieved by advances in biochemical and molecular characterization, and visualization of rhizosphere phenomena. In combination with earlier knowledge of naturally-occurring plant-microbe interactions such as the opine concept, this new knowledge considerably improves the opportunities for manipulating rhizosphere interactions to greatly accelerate rhizodegradation for routine practical implementation in the field.
Climate changes, including chronic changes in precipitation amounts, will influence plant physiology and growth. However, such precipitation effects on switchgrass, a major bioenergy crop, have not been well investigated. We conducted a two-year precipitation simulation experiment using large pots (95 L) in an environmentally controlled greenhouse in Nashville, TN. Five precipitation treatments (ambient precipitation, and -50%, -33%, +33%, and +50% of ambient) were applied in a randomized complete block design with lowland "Alamo" switchgrass plants one year after they were established from tillers. The growing season progression of leaf physiology, tiller number, height, and aboveground biomass were determined each growing season. Precipitation treatments significantly affected leaf physiology, growth, and aboveground biomass. The photosynthetic rates in the wet (+50% and +33%) treatments were significantly enhanced by 15.9% and 8.1%, respectively, than the ambient treatment. Both leaf biomass and plant height were largely increased, resulting in dramatically increases in aboveground biomass by 56.5% and 49.6% in the +50% and +33% treatments, respectively. Compared to the ambient treatment, the drought (-33% and -50%) treatments did not influence leaf physiology, but the -50% treatment significantly reduced leaf biomass by 37.8%, plant height by 16.3%, and aboveground biomass by 38.9%. This study demonstrated that while switchgrass in general is a drought tolerant grass, severe drought significantly reduces Alamo’s growth and biomass, and that high precipitation stimulates its photosynthesis and growth.
Past studies have shown that extremely high concentrations of alachlor in soil can depress microbial biomass and bioactivity; consequently, degradation of alachlor is also inhibited. Studies were undertaken to further characterize the relationship among alachlor concentration in soil, dehydrogenase activity (as an indicator of microbial activity), and degradation rate. Alachlor initially inhibited soil dehydrogenase in soil at concentrations as low as 250 mg/kg with prolonged inhibition through at least 21 d occurring at concentrations 2750 mg/kg. The inhibition of soil dehydrogenase was associated with a prolonged persistence of alachlor beyond the previously reported ranges of half-life observed at normal field rates of application. Amendment of soil with cornmeal caused degradation of 10 to 250 mg/kg alachlor at rates substantially faster than previously reported for laboratory incubations. At alachlor concentrations 2750 mg/kg, dehydrogenase activities in amended soils surpassed levels in corresponding no-pesticide controls after 21 d; coincidentally, >50% of the initially added alachlor had degraded during the same period. These results suggested that stimulation of microbial bioactivity by addition of organic amendments may enhance co-metabolism of high concentrations of pesticides in soil.
Seepage and runoff waters from soils forming in sulfide-bearing dredge materials (SBDM) can have dramatic effects on water quality if they are placed adjacent to open water and do not have adequate containment. Soils forming in SBDM can produce large amounts of acidity upon sulfide oxidation and the oxidation and hydrolysis of released ferrous iron when they are drained or otherwise exposed to air. These soils, under certain environmental conditions, can produce low pH seepage and runoff waters containing large amounts of iron and aluminum, especially after heavy rain that follows a prolonged dry period. During the course of the soil survey update of Somerset County, Maryland (MD), USA, 2 areas of soils forming in SBDM of differing age were identified in close proximity to the sites of recent fish kills on the Pocomoke Sound in Somerset County. Both of these soil areas were initially contained by earthen berms. The dredge materials were deposited directly over the natural tidal marsh soil. Soils forming in SBDM that are approximately 60 years of age were classified as fine-silty, mixed mesic Sulfic Endoaquepts, while the second area of SBDM are 8 years of age and classified as fine-silty, mixed, mesic Typic Sulfaquepts, by Soil Taxonomy. The presence of jarosite was confirmed in both soils by X-ray diffraction, and the presence of ironstone (iron oxyhydroxides) was confirmed in both soils at the effluent discharge points. This is an indication that these soils have undergone intensive acid sulfate weathering (sulfuricization) and that they have released a large quantity of iron to waters leaving the sites. Studies have shown that the 2 mechanisms responsible for acid production from soils forming in SBDM are (i) the oxidation and hydrolysis of mobile ferrous iron; and (ii) the oxidation of the sulfur occurring in the form of pyrite. It is suggested that the resultant low pH, Fe- and Al-enriched water from these soils that entered the Pocomoke Sound may have made fish more susceptible to microbial predation by weakening mucous membranes and/or by promoting the growth of harmful cyannobacteria and flagellates. This paper reports the nature and classification of soils that developed in SBDM at 2 sites of differing age and of the possible environmental impacts of seepage and runoff from these sites entering the Pocomoke Sound.
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