A large-scale mesocosm study was conducted to determine if vegetation with willow trees enhances biodegradation and to evaluate the mechanisms of natural biodegradation of weathered petroleum compounds under field conditions. The mesocosms were designed to model conditions at a former oil field where mid-range petroleum distillates were used as a diluent for pumping crude oil contaminated the soil and groundwater at the site with petroleum compounds. Ten mesocosms were constructed at the field site using un-impacted soil and diluent-impacted groundwater from the site. Five of the mesocosms were planted with Arroyo Willow trees native to the field site and the other five served as controls without trees. Since these willow trees are phreatophytes, their roots are capable of consuming water from the water table. A previous study was conducted using these mesocosms, however the willow trees then were in poor condition.In this study, fertilizer was added to the mesocosms to promote healthy growth of the willows. Fertilizer was added equally to mesocosms with and without willow trees to avoid introducing bias. Groundwater was circulated through the mesocosms for two 109 to 126 days runs, while the total petroleum hydrocarbon (TPH) concentrations of the groundwater were measured periodically. Dissolved oxygen concentrations were also monitored in each of the mesocosms to determine if the willow trees had any impact on oxygen transfer to the subsurface.In the first run without nutrient amendments the trees did not enhance biodegradation.All the mesocosms started with an average TPH concentration of 6.3 mg/L and ended vi with a concentration of 1.0 mg/L. After this first run, nutrient amendments were added to all the mesocosms, resulting in healthy trees with robust growth. With healthy willow trees, the planted mesocosms resulted in a statistically significant increase in long-term biodegradation of dissolved-phase petroleum compounds. The planted mesocosms resulted in 29 percent more degradation. These results agree with prior lab studies using bench-scale microcosms with media from the former oil field which indicated that TPH concentrations after 100 days were lower in containers with willows or lupines compared to controls without plants. Microtox ® toxicity decreased for both planted and control mesocosms, showing no toxic root exudates or by-products.There are several potential mechanisms of the observed phytoremediation. Terminal restriction fragment analyses showed that the planted mesocosms had different microbial communities than the unplanted mesocosms. Thus, a possible mechanism of the phytoremediation is stimulation of a rhizobial microbial community that biodegrades petroleum compounds. The dissolved oxygen (DO) concentrations were actually lower in the planted mesocosms, possibly due to consumption of oxygen during biodegradation of root exudates. The reduced DO concentrations in the planted mesocosm discounts the possibility that the plants stimulated biodegradation by increasing oxygen transfer to the ...