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Under greenhouse conditions, we evaluated establishment of four tree species and their capacity to degrade crude oil recently incorporated into the soil; the species were as follows: Cedrela odorata (tropical cedar), Haematoxylum campechianum (tinto bush), Swietenia macrophylla (mahogany), and Tabebuia rosea (macuilis). Three-month-old plants were planted in soil with three treatments of heavy petroleum and a control (C0 0 mg kg; C1 18,000 mg kg; C2 31,700 mg kg; C3 47,100 mg kg) with four repetitions per treatment and species; the experiment was carried out for 245 days. Height and biomass of all species significantly diminished as petroleum concentration increased, although plant survival was not affected. The quantity of colony-forming units (CFU) of rhizospheric bacteria varied among tree species and treatments; petroleum stimulated bacterial CFU for S. macrophylla. The number of fungi CFU for S. macrophylla and T. rosea was significantly greater in C0 than in soil with petroleum, but among species and among different concentrations, no significant differences were found. The greatest percentage of total petroleum hydrocarbon (TPH) degradation was found in C1 for soil without plants (45 %). Differences from the remaining treatments (petroleum concentrations in soil and plant species) were not significant (P < 0.05). Among all trees, H. campechianum had the greatest TPH degradation (32.5 % in C2). T. rosea (C1) and H. campechianum (C2) resulted in petroleum degradation at levels ranging from 20.5 to 32.5 %. On the basis of this experiment, the tree species used did not improve TPH degradation. However, all of them showed high rates of survival and vigor. So, as tree species provide goods and services, experiments with inoculation of hydrocarbonclastic microorganisms, addition of fertilizers, and mixture of tree and grasses are recommended.

In this study were examined chemical, microbiological, and enzymatic changes at different depths of the soil and rhizosphere, produced by the burning of a commercial sugarcane crop in tropical areas of México. Samples of silty loamy soil and rhizosphere were collected at three times in the sugarcane production cycle: before burning (BB), after first burning (AFB), and after second burnings (ASB), with a general interval of 15 days between the first and the third collection date. Soil organic matter (SOM), soil organic carbon (SOC), total nitrogen (Nt), phosphorus available (Pav), pH, and the C/N ratio were determined in soil and rhizosphere, as well as the enzymatic activities of phosphatase and urease. Furthermore, microbial respiration, microbial biomass, and nitrogen-fixing bacteria (NFB) and phosphate solubilizing bacteria (PSB) densities were monitored during 84 days. The Pav and the pH increased significantly in soil samples affected by the second burning of the stubbles, but SOM, SOC, Nt, the C/N ratio, phosphatase, and urease activities decreased as a result of the first and second burnings. This decrease was more pronounced in non-rhizospheric soil. The densities of NFB and PSB increased with the burning, as well as microbial respiration. All the variables evaluated recorded higher values in the soil surface layer.