Gregory J. Thoma

Influence of organic and inorganic soil amendments on plant growth in crude oil-contaminated soil

Abstract Phytoremediation can be a viable alternative to traditional, more costly remediation techniques. Three greenhouse studies were conducted to evaluate plant growth with different soil amendments in crude oil‐contaminated soil. Growth of alfalfa (Medicago sativa L., cultivar: Riley), bermudagrass (Cynodon dactylon L., cultivar: Common), crabgrass (Digitaria sanguinalis cultivar: Large), fescue (Lolium arundinaceum Schreb., cultivar: Kentucky 31), and ryegrass (Lolium multiflorum Lam., cultivar: Marshall) was determined in crude oil‐contaminated soil amended with either inorganic fertilizer, hardwood sawdust, papermill sludge, broiler litter or unamended (control). In the first study, the addition of broiler litter reduced seed germination for ryegrass, fescue, and alfalfa. In the second study, bermudagrass grown in broiler litteramended soil produced the most shoot biomass, bermudagrass produced the most root biomass, and crabgrass and bermudagrass produced the most root length. In the third study, soil amended with broiler litter resulted in the greatest reduction in gravimetric total petroleum hydrocarbon (TPH) levels across the six plant treatments following the 14‐wk study. Ryegrass produced more root biomass than any other species when grown in inorganic fertilizer‐ or hardwood sawdust + inorganic fertilizer‐amended soil. The studies demonstrated that soil amendments and plant species selection were important considerations for phytoremediation of crude oil‐contaminated soil.

Tracking the fate and recycling of 13C-labeled glucose in soil

A short-term incubation of soil amended with 13C-glucose was conducted to determine the extent of labeled C recycling that might occur within the microbial community. Changes in the production and isotopic composition of CO2 and biomass suggest that two phases of microbial activity occurred after the glucose addition. The initial phase due directly to the metabolism of the added glucose was characterized by an increase in biomass and a high growth efficiency. A second phase appeared to be driven by less available substrates (e.g., cell wall structures, soil organic matter) and characterized by insignificant changes in biomass but significant generation of CO2 suggestive of low growth efficiency. Glucose-C supported 12 to 73% of the CO2-C evolved and 17 to 21% of biomass-C, suggesting glucose was the principle energy rather than a C source during the 15- to 48-hour phase of the incubation. Variation in &dgr;13C composition of individual phospholipid fatty acids (PLFA) during the incubation indicated that different components of the microbial community played different roles in the cycling of the added glucose. The most enriched &dgr;13C values were initially those PLFA associated with Gram-positive bacteria, suggesting they were responsible for much of the initial incorporation. By contrast, at the end of the 48-hour incubation, 4 of 24 PLFA biomarkers were not labeled with 13C. Actinomycetes, however, probably played a larger role in the use of recycled glucose-derived C, as suggested by the enrichment in 13C of 10-methyl 18:0 PLFA after the exhaustion of glucose. Results from this study show that the element of time needs to be considered carefully in the interpretation of any stable isotope labeling and biomarker study.

Selecting Plants and Nitrogen Rates to Vegetate Crude-Oil–Contaminated Soil

Phytoremediation can be effective for remediating contaminated soils in situ and generally requires the addition of nitrogen (N) to increase plant growth. Our research objectives were to evaluate seedling emergence and survival of plant species and to determine the effects of N additions on plant growth in crude-oil–contaminated soil. From a preliminary survival study, three warm-season grasses—pearlmillet (Pennisetum glaucum [L.] R. Br.), sudangrass (Sorghum sudanense [Piper] Stapf [Piper]), and browntop millet (Brachiaria ramosa L.)—and one warm-season legume—jointvetch (Aeschynomene americana L.)—were chosen to determine the influence of the N application rate on plant growth in soil contaminated with weathered crude oil. Nitrogen was added based on total petroleum hydrocarbon-C:added N ratios (TPH-C:TN) ranging from 44:1 to 11:1. Plant species were grown for 7 wk. Root and shoot biomass were determined and root length and surface area were analyzed. Pearlmillet and sudangrass had higher shoot and root biomass when grown at a TPH-C:TN (inorganic) ratio of 11:1 and pearlmillet had higher root length and surface area when grown at 11:1 compared with the other species. By selecting appropriate plant species and determining optimum N application rates, increased plant root growth and an extended rhizosphere influence should lead to enhanced phytoremediation of crude-oil–contaminated soil.

Petroleum-Degrading Microbial Numbers in Rhizosphere and Non-Rhizosphere Crude Oil-Contaminated Soil

Phytoremediation can be a cost-effective and environmentally acceptable method to clean up crude oil-contaminated soils in situ. Our research objective was to determine the effects of nitrogen (N) additions and plant growth on the number of total hydrocarbon (TH)-, alkane-, and polycyclic aromatic hydrocarbon (PAH)-degrading microorganisms in weathered crude oil-contaminated soil. A warm-season grass, sudangrass (Sorghum sudanense (Piper) Stapf), was grown for 7 wk in soil with a total petroleum hydrocarbon (TPH) level of 16.6 g TPH/kg soil. Nitrogen was added based upon TPH-C:added total N (TPH-C:TN) ratios ranging from 44:1 to 11:1. Unvegetated and unamended controls were also evaluated. The TH-, alkane-, and PAH-degrading microbial numbers per gram of dry soil were enumerated from rhizosphere and non-rhizosphere soil for vegetated pots and non-rhizosphere soil populations were enumerated from non-vegetated pots. Total petroleum-degrading microbial numbers were also calculated for each pot. The TH-, alkane-, and PAH-degrading microbial numbers per gram of dry soil in the sudangrass rhizosphere were 3.4, 2.6, and 4.8 times larger, respectively, than those in non-rhizosphere soil across all N rates. The presence of sudangrass resulted in significantly more TH-degrading microorganisms per pot when grown in soil with a TPH-C:TN ratio of 11:1 as compared to the control. Increased plant root growth in a crude oil-contaminated soil and a concomitant increase in petroleum-degrading microbial numbers in the rhizosphere have the potential to enhance phytoremediation.